[experiment] Fold String.Concat by EgorBo · Pull Request #129659 · dotnet/runtime

EgorBo · 2026-06-20T16:25:55Z

Folds string.Concat(...) for jit-time known constant strings (including static readonly fields holding non-FOH strings).

just to see diffs in MihuBot -nuget if it's any popular in 3rd parties..

Diffs are promising

EgorBo · 2026-06-20T16:26:16Z

@MihuBot -nuget

dotnet-policy-service · 2026-06-20T16:27:20Z

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
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MihaZupan · 2026-06-20T16:29:41Z

It'll blow up on diffing CoreLib atm, seeing if #129658 is enough to make it work again

EgorBo · 2026-06-20T16:33:31Z

It'll blow up on diffing CoreLib atm, seeing if #2022 is enough to make it work again

um.. #2022 ?

MihaZupan · 2026-06-20T16:36:26Z

Oops, fixed.
2022 is the runtime-utils issue :) MihuBot/runtime-utils#2022

Copilot

Pull request overview

This PR experiments with enabling JIT-time folding of fixed-arity stringString.Concat calls into a single frozen string object when the inputs are compile-time readable, by introducing a new JIT↔EE interface API to allocate such frozen strings.

Changes:

Marks String.Concat(string?, ...) overloads as [Intrinsic] and adds a JIT importer expansion to constant-fold eligible calls.
Adds a new JIT↔EE interface method tryCreateStringObject and wires it through CoreCLR, SuperPMI record/replay, and NativeAOT/R2R wrappers.
Adjusts inline-observation scanning to avoid treating String.Concat’s intrinsic marking as an inlining heuristic signal.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs	Marks fixed-arity `Concat(string?,...)` overloads as intrinsic.
src/coreclr/vm/jitinterface.cpp	Implements `CEEInfo::tryCreateStringObject` to allocate/fill a frozen string.
src/coreclr/tools/superpmi/superpmi/icorjitinfo.cpp	Plumbs `tryCreateStringObject` through SuperPMI replay interface.
src/coreclr/tools/superpmi/superpmi-shim-simple/icorjitinfo_generated.cpp	Adds shim passthrough for the new JIT↔EE API.
src/coreclr/tools/superpmi/superpmi-shim-counter/icorjitinfo_generated.cpp	Adds shim passthrough + call counting for the new API.
src/coreclr/tools/superpmi/superpmi-shim-collector/icorjitinfo.cpp	Records/replays `tryCreateStringObject` in the collector shim.
src/coreclr/tools/superpmi/superpmi-shared/methodcontext.h	Declares record/replay support + packet ID for the new API.
src/coreclr/tools/superpmi/superpmi-shared/methodcontext.cpp	Implements record/replay logic for `tryCreateStringObject`.
src/coreclr/tools/superpmi/superpmi-shared/lwmlist.h	Adds a new LightWeightMap slot for `TryCreateStringObject`.
src/coreclr/tools/Common/JitInterface/ThunkGenerator/ThunkInput.txt	Adds the new API to the thunk generator input.
src/coreclr/tools/Common/JitInterface/CorInfoImpl_generated.cs	Adds managed callback plumbing for `tryCreateStringObject`.
src/coreclr/tools/aot/jitinterface/jitinterface_generated.h	Updates NativeAOT JIT interface wrapper for the new API.
src/coreclr/tools/aot/ILCompiler.RyuJit/JitInterface/CorInfoImpl.RyuJit.cs	Returns null for `tryCreateStringObject` during AOT compilation.
src/coreclr/tools/aot/ILCompiler.ReadyToRun/JitInterface/CorInfoImpl.ReadyToRun.cs	Returns null for `tryCreateStringObject` during R2R compilation.
src/coreclr/jit/namedintrinsiclist.h	Adds `NI_System_String_Concat`.
src/coreclr/jit/importercalls.cpp	Implements `impStringConcat` folding and dispatches it from `impIntrinsic`.
src/coreclr/jit/ICorJitInfo_wrapper_generated.hpp	Adds wrapper logging/dispatch for `tryCreateStringObject`.
src/coreclr/jit/ICorJitInfo_names_generated.h	Adds the API name for wrapper logging.
src/coreclr/jit/fgbasic.cpp	Prevents `String.Concat` intrinsic marking from affecting inliner heuristics.
src/coreclr/jit/compiler.h	Declares `impStringConcat`.
src/coreclr/inc/jiteeversionguid.h	Bumps the JIT↔EE interface GUID.
src/coreclr/inc/icorjitinfoimpl_generated.h	Adds the new interface method declaration (generated).
src/coreclr/inc/corinfo.h	Adds the new `ICorStaticInfo::tryCreateStringObject` API.

MihaZupan · 2026-06-20T16:51:14Z

@MihuBot -nuget -dependsOn https://github.com/MihaZupan/runtime/tree/revert-test-3

Copilot

Pull request overview

Copilot reviewed 25 out of 25 changed files in this pull request and generated 3 comments.

EgorBo · 2026-06-20T19:54:46Z

@MihuBot -nuget -dependsOn https://github.com/MihaZupan/runtime/tree/revert-test-3

MihaZupan · 2026-06-20T22:23:05Z

@MihuBot -nuget

EgorBo · 2026-06-20T23:25:22Z

isRunRarely check swept quite a few hits. Anyway, I think it should be done differently and recognize FastAllocateStrings + memmoves in a late phase.

Copilot

Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 5 comments.

Copilot

Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 7 comments.

+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
 public static string Concat(string? str0, string? str1)


+ CONTRACTL {
+ THROWS;
+ GC_TRIGGERS;
+ MODE_PREEMPTIVE;
+ } CONTRACTL_END;


+ bool isFrozen = false;
+ STRINGREF strObj = AllocateString((DWORD)length, /* preferFrozenHeap */ true, &isFrozen);
+


+ virtual CORINFO_OBJECT_HANDLE tryCreateStringObject(
+ uint16_t* str,
+ int length
+ ) = 0;


+// The input is a transient JIT-side buffer, so we key the record on a content hash of the
+// requested string (length + characters) rather than on the pointer value.
+static DWORDLONG HashStringObjectRequest(uint16_t* str, int length)
+{
+ DWORDLONG hash = 0xCBF29CE484222325ULL ^ (DWORDLONG)(DWORD)length; // FNV-1a offset basis


+ // Ask the VM to materialize a frozen string object with the computed contents.
+ CORINFO_OBJECT_HANDLE strObj = info.compCompHnd->tryCreateStringObject(buffer, (int)strLength);
+ if (strObj == NO_OBJECT_HANDLE)
+ {
+ JITDUMP("...VM could not create a frozen string object - bail out.\n");


+ // Success - remove the now-redundant initialization statements. Their only observable effect is
+ // writing into the string we are about to replace with a constant, and all the written data is
+ // constant, so they are safe to drop.
+ while (initStmts.Height() > 0)
+ {


Add a JIT-EE API `tryCreateStringObject` and assertion-prop optimizations that fold compile-time-constant strings into a single frozen string object: * String.Concat(string, ...) fixed-arity overloads are marked [Intrinsic] and recognized as NI_System_String_Concat. The JIT keeps them as calls (impMarkInlineCandidate skips this intrinsic) instead of inlining their bodies, and folds the call directly when all arguments are constant strings. * The same frozen-string materialization also folds a FastAllocateString followed by a sequence of constant memmoves (as produced by String.Substring). The VM only creates the string on the frozen object heap and never for collectible/unloadable contexts (and never returns a non-frozen instance); NativeAOT/crossgen return null. NI_System_String_Concat is also excluded from the inliner's CALLEE_INTRINSIC heuristic so marking Concat [Intrinsic] does not perturb inlining decisions. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

+ virtual CORINFO_OBJECT_HANDLE tryCreateStringObject(
+ uint16_t* str,
+ int length
+ ) = 0;


+ totalLength += strLen;
+ }
+
+ GenTree* con = optCreateConstStringNode(buffer, totalLength, call);


+ // String.Concat is marked [Intrinsic] only so that it can be constant-folded during assertion
+ // propagation (NI_System_String_Concat). Keep it as a call rather than inlining its body so the
+ // fold has something to recognize, and to avoid the size bloat of inlining Concat everywhere.
+ if (call->IsSpecialIntrinsic(this, NI_System_String_Concat))
+ {
+ return;
+ }


+ CORINFO_OBJECT_HANDLE strObj = info.compCompHnd->tryCreateStringObject(buffer, length);
+ if (strObj == NO_OBJECT_HANDLE)
+ {
+ JITDUMP("...VM could not create a frozen string object - bail out.\n");
+ return nullptr;
+ }
+
+ GenTree* con = gtNewIconEmbObjHndNode(strObj);
+ con->gtVNPair.SetBoth(vnStore->VNForHandle((ssize_t)strObj, GTF_ICON_OBJ_HDL));


CopilotAI review requested due to automatic review settings June 20, 2026 16:25

github-actionsBot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jun 20, 2026

Copilotstarted reviewing on behalf of EgorBo June 20, 2026 16:26 View session

dotnet-policy-serviceBot assigned EgorBo Jun 20, 2026

MihuBot mentioned this pull request Jun 20, 2026
[JitDiff X64] [EgorBo] [experiment] Fold String.Concat MihuBot/runtime-utils#2021
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Comment threadsrc/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs Outdated
Comment threadsrc/coreclr/tools/Common/JitInterface/ThunkGenerator/ThunkInput.txt
Comment threadsrc/coreclr/inc/corinfo.h Outdated
Comment threadsrc/coreclr/vm/jitinterface.cpp Outdated

This was referenced Jun 20, 2026
[JitDiff X64] [EgorBo] [experiment] Fold String.Concat MihuBot/runtime-utils#2023
Open
[JitDiff X64] [EgorBo] [experiment] Fold String.Concat MihuBot/runtime-utils#2026
Open

CopilotAI review requested due to automatic review settings June 20, 2026 18:01

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Comment threadsrc/coreclr/inc/corinfo.h Outdated
Comment threadsrc/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs Outdated
Comment threadsrc/coreclr/tools/superpmi/superpmi-shared/methodcontext.cpp Outdated

MihuBot mentioned this pull request Jun 20, 2026
[JitDiff X64] [EgorBo] [experiment] Fold String.Concat MihuBot/runtime-utils#2027
Open

MihuBot mentioned this pull request Jun 20, 2026
[JitDiff X64] [EgorBo] [experiment] Fold String.Concat MihuBot/runtime-utils#2028
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CopilotAI review requested due to automatic review settings June 21, 2026 02:21

EgorBo force-pushed the fold-string-concat-3 branch from af94797 to 83cbce6Compare June 21, 2026 02:21

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Comment threadsrc/coreclr/vm/jitinterface.cpp Outdated
Comment threadsrc/coreclr/tools/superpmi/superpmi-shared/methodcontext.cpp
Comment threadsrc/coreclr/inc/corinfo.h
Comment threadsrc/coreclr/jit/assertionprop.cpp
Comment threadsrc/coreclr/jit/assertionprop.cpp

EgorBo force-pushed the fold-string-concat-3 branch from 83cbce6 to c494901Compare June 21, 2026 02:42

MihuBot mentioned this pull request Jun 21, 2026
[JitDiff X64] [EgorBo] [experiment] Fold String.Concat MihuBot/runtime-utils#2029
Open

CopilotAI review requested due to automatic review settings June 21, 2026 04:16

EgorBo force-pushed the fold-string-concat-3 branch from c494901 to c900896Compare June 21, 2026 04:16

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EgorBo force-pushed the fold-string-concat-3 branch from c900896 to 81196bbCompare June 21, 2026 05:06

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EgorBo force-pushed the fold-string-concat-3 branch from 81196bb to 1a557d2Compare June 21, 2026 05:16

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Unable to pull image from mcr.microsoft.com #117164
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[JitDiff X64] [EgorBo] [experiment] Fold String.Concat MihuBot/runtime-utils#2031
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EgorBo closed this Jun 21, 2026

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		[MethodImpl(MethodImplOptions.AggressiveInlining)]
		public static string Concat(string? str0, string? str1)

		bool isFrozen = false;
		STRINGREF strObj = AllocateString((DWORD)length, /* preferFrozenHeap */ true, &isFrozen);

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