JIT: Stack allocate non-escaping strings - #129202

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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)

@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI review requested due to automatic review settings June 10, 2026 01:18
@github-actionsgithub-actionsBot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jun 10, 2026
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Pull request overview

This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.

Changes:

  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.

Comment threadsrc/coreclr/jit/objectalloc.cpp Outdated
@build-analysisbuild-analysisBot mentioned this pull request Jun 10, 2026
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@EgorBo
EgorBo marked this pull request as ready for review June 10, 2026 13:13
EgorBoand others added 2 commits June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
CopilotAI review requested due to automatic review settings June 10, 2026 18:20
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Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Comment threadsrc/coreclr/jit/objectalloc.cpp Outdated
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment on lines +190 to +191
if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
(m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)

@tannergoodingtannergoodingJun 10, 2026

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nit, is it worth just making this something like:

if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)

Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?

These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.

@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them

tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp
This was referenced Jun 11, 2026
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Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Comment threadsrc/coreclr/jit/objectalloc.cpp
@EgorBo

EgorBo commented Jun 26, 2026

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@AndyAyersMS you wanted to take a look on this too

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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.

Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others added 2 commits July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI review requested due to automatic review settings July 6, 2026 17:35
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Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Comment threadsrc/coreclr/jit/objectalloc.cpp Outdated
@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
CopilotAI review requested due to automatic review settings July 6, 2026 17:45
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Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Let's postpone this till we get some meaningful diffs

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JIT: Stack allocate non-escaping strings - #129202
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EgorBo:egorbo/jit-stackalloc-strings
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JIT: Stack allocate non-escaping strings#129202
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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)
@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
June 10, 2026 01:18
@github-actionsgithub-actionsBot
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CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI
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Pull request overview
This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.
Changes:
  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

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EgorBo
marked this pull request as ready for review
June 10, 2026 13:13
EgorBoand others
added 2 commits
June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Comment threadsrc/coreclr/jit/objectalloc.cpp

Comment on lines

+190
 to 
+191

 if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
 (m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)
@tannergoodingtannergoodingJun 10, 2026

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nit, is it worth just making this something like:
if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)
Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?
These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.
@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

This was referenced Jun 11, 2026

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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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@AndyAyersMS you wanted to take a look on this too
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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.
Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others
added 2 commits
July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
July 6, 2026 17:35
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Let's postpone this till we get some meaningful diffs
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JIT: Stack allocate non-escaping strings - #129202
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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)
@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
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Pull request overview
This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.
Changes:
  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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June 10, 2026 13:13
EgorBoand others
added 2 commits
June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

Comment on lines

+190
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+191

 if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
 (m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)
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nit, is it worth just making this something like:
if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)
Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?
These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.
@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

This was referenced Jun 11, 2026

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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.
Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others
added 2 commits
July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
(function() {
var ref = document.referrer;
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parts.forEach(function(part, i) {
if (i % 2 === 0) {
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span.className = 'userscript-highlight';
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node.parentNode.replaceChild(frag, node);
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var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];
if (!skipTags.includes(node.tagName)) {
Array.from(node.childNodes).forEach(highlight);
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highlight(document.body);
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mutations.forEach(function(m) {
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observer.observe(document.body, { childList: true, subtree: true });
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} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
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var __re = new RegExp('^' + ".*" + '

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JIT: Stack allocate non-escaping strings - #129202
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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)
@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
June 10, 2026 01:18
@github-actionsgithub-actionsBot
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Pull request overview
This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.
Changes:
  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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@EgorBo
EgorBo
marked this pull request as ready for review
June 10, 2026 13:13
EgorBoand others
added 2 commits
June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
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review requested due to automatic review settings
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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

Comment on lines

+190
 to 
+191

 if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
 (m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)
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nit, is it worth just making this something like:
if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)
Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?
These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.
@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

This was referenced Jun 11, 2026

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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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@AndyAyersMS you wanted to take a look on this too
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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.
Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others
added 2 commits
July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
July 6, 2026 17:35
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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review requested due to automatic review settings
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Let's postpone this till we get some meaningful diffs
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
(function() {
var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
'ref', 'ref_src', 'source', 'medium', 'campaign'];
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try {
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} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
JIT: Stack allocate non-escaping strings - #129202
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EgorBo wants to merge 8 commits into
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EgorBo:egorbo/jit-stackalloc-strings
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JIT: Stack allocate non-escaping strings#129202
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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)
@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
June 10, 2026 01:18
@github-actionsgithub-actionsBot
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Pull request overview
This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.
Changes:
  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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June 10, 2026 13:13
EgorBoand others
added 2 commits
June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Comment threadsrc/coreclr/jit/objectalloc.cpp

Comment on lines

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+191

 if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
 (m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)
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nit, is it worth just making this something like:
if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)
Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?
These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.
@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

This was referenced Jun 11, 2026

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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.
Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others
added 2 commits
July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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JIT: Stack allocate non-escaping strings - #129202
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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)
@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
June 10, 2026 01:18
@github-actionsgithub-actionsBot
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Pull request overview
This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.
Changes:
  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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@EgorBo
EgorBo
marked this pull request as ready for review
June 10, 2026 13:13
EgorBoand others
added 2 commits
June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

Comment on lines

+190
 to 
+191

 if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
 (m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)
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nit, is it worth just making this something like:
if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)
Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?
These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.
@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

This was referenced Jun 11, 2026

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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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@AndyAyersMS you wanted to take a look on this too
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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.
Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others
added 2 commits
July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Let's postpone this till we get some meaningful diffs
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
if (el.style.position === 'fixed' || el.style.position === 'sticky' || getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {
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Skip to content
JIT: Stack allocate non-escaping strings - #129202
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EgorBo:egorbo/jit-stackalloc-strings
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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)
@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
June 10, 2026 01:18
@github-actionsgithub-actionsBot
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Pull request overview
This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.
Changes:
  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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June 10, 2026 13:13
EgorBoand others
added 2 commits
June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Comment threadsrc/coreclr/jit/objectalloc.cpp

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+190
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+191

 if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
 (m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)
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nit, is it worth just making this something like:
if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)
Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?
These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.
@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.
Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others
added 2 commits
July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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JIT: Stack allocate non-escaping strings - #129202
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Now that we intrinsified String.FastAllocateString, I guess it wouldn't hurt to enable escape analysis for it by repeating what we do for normal arrays. Most strings leak into other string APIs which actually don't escape, but we don't do IPA today. As is, this PR enables 74 strings to be stack-allocated (mostly in libraries_tests_no_tiered_compilation.run)
@EgorBo
JIT: Stack allocate non-escaping strings
3198d9e
Stack allocate non-escaping System.String instances created via
String.FastAllocateString when the length is a compile-time constant,
reusing the existing array stack-allocation machinery.
At object-allocator time a string allocation is modelled as
FastAllocateString(pMT, length) and is treated like a char[] array via a
new OAT_NEWSTR allocation kind. The call is retyped to a native pointer,
gets a StackArrayLocal arg and the GTF_CALL_M_STACK_ARRAY flag, and is
later expanded in fgExpandStackArrayAllocations into method-table and
length stores (the length field offset is the only difference from
arrays). The stack object uses a non-GC block layout sized for the
header, chars and null terminator, capped by JitObjectStackAllocationSize.
Gated by the new DOTNET_JitObjectStackAllocationString config (on by
default). R2R is not yet supported.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
This PR extends CoreCLR JIT object stack allocation support to cover System.String.FastAllocateString call sites (in addition to newarr), enabling escape analysis to stack-allocate eligible constant-length strings and expanding them via the existing stack-array allocation expansion path.
Changes:
  • Teach ObjectAllocator to recognize String.FastAllocateString(pMT, length) as a stack-allocation candidate (OAT_NEWSTR) when length is a compile-time constant.
  • Reuse the stack-array allocation machinery to represent and expand stack-allocated strings (extra StackArrayLocal arg, retype to native pointer, and late expansion in fgExpandStackArrayAllocations).
  • Add config/metrics/method-flag plumbing for string stack allocation (JitObjectStackAllocationString, new JIT metrics, OMF_HAS_FASTSTRINGALLOC).
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
FileDescription
src/coreclr/jit/valuenum.cppAssign a fresh non-heap VN for stack-allocated FastAllocateString calls flagged with GTF_CALL_M_STACK_ARRAY.
src/coreclr/jit/objectalloc.hAdd OAT_NEWSTR and the MorphAllocObjNodeHelperStr helper declaration.
src/coreclr/jit/objectalloc.cppDetect FastAllocateString candidates, size-check strings, and morph eligible ones into the existing stack-array allocation path.
src/coreclr/jit/jitmetadatalist.hAdd new JIT metrics for string allocation and stack-allocated strings.
src/coreclr/jit/jitconfigvalues.hAdd JitObjectStackAllocationString config switch.
src/coreclr/jit/importercalls.cppFlag methods containing FastAllocateString so escape analysis runs; also mark ThrowIfNull as special (non-fallthrough).
src/coreclr/jit/helperexpansion.cppExtend stack-array expansion to also expand stack-allocated FastAllocateString by using the string length field offset.
src/coreclr/jit/compiler.hAdd OMF_HAS_FASTSTRINGALLOC method flag.
Comment threadsrc/coreclr/jit/objectalloc.cpp
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June 10, 2026 13:13
EgorBoand others
added 2 commits
June 10, 2026 15:29
@EgorBo
JIT: Merge string stack-alloc into the array path
ef02bad
Fold the near-duplicate MorphAllocObjNodeHelperStr into
MorphAllocObjNodeHelperArr (parameterized by isString) and unify the
array/string detection in AllocationKind, since a stack-allocated string
is just a char array with a different header. No functional change:
SuperPMI asmdiffs are identical to the previous commit across
libraries.pmi, coreclr_tests, libraries_tests and benchmarks.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
622f7fa
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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

Comment on lines

+190
 to 
+191

 if ((m_compiler->optMethodFlags & OMF_HAS_NEWOBJ) == 0 && (m_compiler->optMethodFlags & OMF_HAS_NEWARRAY) == 0 &&
 (m_compiler->optMethodFlags & OMF_HAS_FASTSTRINGALLOC) == 0)
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nit, is it worth just making this something like:
if ((m_compiler->optMethodFlags & OMF_OBJALLOC_MASK) == 0)
Where OMF_OBJALLOC_MASK = OMF_HAS_NEWOBJ | OMF_HAS_NEWARRAY | OMF_HAS_FASTSTRINGALLOC?
These lines aren't the worst, but every time we add one more check I think it gets a little less readable and having a way to check it all at once is nice.
@EgorBoEgorBoJun 26, 2026

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Good idea, although, we probably should hide these under getters, most OMF flags are already hidden behind them
tannergooding
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Comment threadsrc/coreclr/jit/objectalloc.cpp

This was referenced Jun 11, 2026

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Pull request overview
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
@EgorBo
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commented
Jun 26, 2026

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@AndyAyersMS you wanted to take a look on this too
@EgorBo
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Add some test cases? The existing test pattern is a bit annoying but AI can probably mimic it without problems.
Comment threadsrc/coreclr/jit/objectalloc.cpp
EgorBoand others
added 2 commits
July 6, 2026 18:58
@EgorBo
Merge branch 'main' into egorbo/jit-stackalloc-strings
f6bfc0b
@EgorBo
JIT: Add FileCheck tests for stack-allocated strings
79c4877
Verify that non-escaping, constant-length strings produced by char.ToString(),
char.ToString(char), string equality of a ToString() result, and Rune.ToString()
are stack-allocated (the String.FastAllocateString call is removed).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
CopilotAI
review requested due to automatic review settings
July 6, 2026 17:35
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Outdated

@EgorBo
Potential fix for pull request finding
0f922cb
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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review requested due to automatic review settings
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Pull request overview
Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
Comment threadsrc/coreclr/jit/objectalloc.cpp
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Let's postpone this till we get some meaningful diffs
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