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Set metadata for vtable-related loads - #39995

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Aatch:vtable-ptr-metadata
Feb 25, 2017
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Set metadata for vtable-related loads#39995
bors merged 2 commits into
rust-lang:masterfrom
Aatch:vtable-ptr-metadata

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@Aatch

@AatchAatch commented Feb 21, 2017

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Give LLVM much more information about vtable pointers. Without the extra
information, LLVM has to be rather pessimistic about vtables, preventing
a number of obvious optimisations.

  • Makes the vtable pointer argument noalias and readonly.
  • Marks loads of the vtable pointer as nonnull.
  • Marks load from the vtable with !invariant.load metadata.

Fixes#39992

@Aatch
Set metadata for vtable-related loads
7af3406
Give LLVM much more information about vtable pointers. Without the extra
information, LLVM has to be rather pessimistic about vtables, preventing
a number of obvious optimisations.
* Makes the vtable pointer argument noalias and readonly.
* Marks loads of the vtable pointer as nonnull.
* Marks load from the vtable with `!invariant.load` metadata.
Fixesrust-lang#39992
@rust-highfive

rust-highfive commented Feb 21, 2017

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r? @eddyb

(rust_highfive has picked a reviewer for you, use r? to override)

@Aatch

Aatch commented Feb 21, 2017

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LLVM IR before and after this patch: https://gist.github.com/Aatch/fe555f251de910d7e765a11cbb5d78d4

@arielb1

arielb1 commented Feb 21, 2017

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@bors r+

@bors

bors commented Feb 21, 2017

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📌 Commit 7af3406 has been approved by arielb1

@bors

bors commented Feb 21, 2017

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⌛ Testing commit 7af3406 with merge 9436c01...

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bors commented Feb 21, 2017

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💔 Test failed - status-appveyor

@arielb1

arielb1 commented Feb 21, 2017

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unchanged test:

C:\projects\rust\src/test\codegen\function-arguments.rs:124:11: error: expected string not found in input
// CHECK: @trait_borrow(i8* nonnull, void (i8*)** nonnull)
^
C:\projects\rust\build\x86_64-pc-windows-gnu\test\codegen\function-arguments.ll:266:62: note: scanning from here
define internal void @str(i8* noalias nonnull readonly, i64) unnamed_addr #1 {
^
C:\projects\rust\build\x86_64-pc-windows-gnu\test\codegen\function-arguments.ll:276:22: note: possible intended match here
define internal void @trait_borrow(i8* nonnull, void (i8*)** noalias nonnull readonly) unnamed_addr #1 {

@Aatch

Aatch commented Feb 21, 2017

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@bors r=arielb1

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bors commented Feb 21, 2017

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📌 Commit d80cf80 has been approved by arielb1

@blussbluss added the relnotes Marks issues that should be documented in the release notes of the next release. label Feb 22, 2017
eddyb added a commit to eddyb/rust that referenced this pull request Feb 25, 2017
@eddyb
Rollup merge ofrust-lang#39995- Aatch:vtable-ptr-metadata, r=arielb1
e71db58
Set metadata for vtable-related loads
Give LLVM much more information about vtable pointers. Without the extra
information, LLVM has to be rather pessimistic about vtables, preventing
a number of obvious optimisations.
* Makes the vtable pointer argument noalias and readonly.
* Marks loads of the vtable pointer as nonnull.
* Marks load from the vtable with `!invariant.load` metadata.
Fixesrust-lang#39992
@eddybeddyb mentioned this pull request Feb 25, 2017
Closed
eddyb added a commit to eddyb/rust that referenced this pull request Feb 25, 2017
@eddyb
Rollup merge ofrust-lang#39995- Aatch:vtable-ptr-metadata, r=arielb1
5c0b4b3
Set metadata for vtable-related loads
Give LLVM much more information about vtable pointers. Without the extra
information, LLVM has to be rather pessimistic about vtables, preventing
a number of obvious optimisations.
* Makes the vtable pointer argument noalias and readonly.
* Marks loads of the vtable pointer as nonnull.
* Marks load from the vtable with `!invariant.load` metadata.
Fixesrust-lang#39992
@eddybeddyb mentioned this pull request Feb 25, 2017
Merged
bors added a commit that referenced this pull request Feb 25, 2017
@bors
Auto merge of#40091- eddyb:rollup, r=eddyb
1572bf1
Rollup of 28 pull requests
- Successful merges: #39859, #39864, #39888, #39903, #39905, #39914, #39945, #39950, #39953, #39961, #39980, #39988, #39993, #39995, #40019, #40020, #40022, #40024, #40025, #40026, #40027, #40031, #40035, #40037, #40038, #40064, #40069, #40086
- Failed merges: #39927, #40008, #40047
@bors
bors merged commit d80cf80 into rust-lang:masterFeb 25, 2017
@gnzlbg

gnzlbg commented Mar 1, 2017

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@Aatch do you have a benchmark/examples for this somewhere? Like to be able to see the "before" and "after" on the generated IR? LLVM devirtualization has improved a lot in the last year (still pretty basic though), but it would be nice to have a couple of "benchmarks" for cases in which we expect a backend to be able to devirtualize calls. That might also help filling LLVM bugs for the cases in which this is not the case, or discover situations in which we are not passing meta-data correctly.

bjorn3
bjorn3 reviewed Mar 1, 2017
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Comment threadsrc/librustc_trans/base.rs
let meta = if meta_ty.element_type().kind() == llvm::TypeKind::Pointer {
b.load_nonnull(meta, None)
} else {
b.load(meta, None)

@bjorn3bjorn3Mar 1, 2017

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Missing alignment?

@arielb1arielb1Mar 1, 2017

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None = ABI alignment. Vtables are ABI aligned.

nikic
nikic reviewed Mar 1, 2017
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Comment threadsrc/librustc_trans/abi.rs
// vtables can be safely marked non-null, readonly
// and noalias.
info.attrs.set(ArgAttribute::NonNull);
info.attrs.set(ArgAttribute::ReadOnly);

@nikicnikicMar 1, 2017

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Isn't readonly a function attribute? What does this mean when applied to arguments?

@AatchAatchMar 2, 2017

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When applied to a pointer argument, it means that the function will only read from it. That allows LLVM to assume that the value behind the pointer won't change across a call to that function.

@dpc

dpc commented Mar 4, 2017

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Please take a look at my last comment in #39992 about tight loops and this.

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