nontemporal_store: make sure that the intrinsic is truly just a hint #128149

RalfJung · 2024-07-24T18:59:23Z

The !nontemporal flag for stores in LLVM sounds like it is just a hint, but actually, it is not -- at least on x86, non-temporal stores need very special treatment by the programmer or else the Rust memory model breaks down. LLVM still treats these stores as-if they were normal stores for optimizations, which is highly dubious. Let's avoid all that dubiousness by making our own non-temporal stores be truly just a hint, which is possible on some targets (e.g. ARM). On all other targets, non-temporal stores become regular stores.

~~Blocked on rust-lang/stdarch#1541 propagating to the rustc repo, to make sure the _mm_stream intrinsics are unaffected by this change.~~

Fixes #114582
Cc @Amanieu @workingjubilee

rustbot · 2024-07-24T18:59:31Z

r? @fmease

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rustbot · 2024-07-24T18:59:33Z

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

compiler/rustc_codegen_gcc/src/builder.rs

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

Kobzol · 2024-07-24T21:22:38Z

It's a bit sad if this becomes an actual no-op on x86, nontemporal stores can be (very rarely) very useful. Is this usable through anything else than mm_stream_* and friends? These functions already contain a safety note that a fence has to be used after them.

workingjubilee · 2024-07-24T21:26:26Z

It's a bit sad if this becomes an actual no-op on x86, nontemporal stores can be (very rarely) very useful. Is this usable through anything else than mm_stream_* and friends? These functions already contain a safety note that a fence has to be used after them.

Those intrinsic functions are, in the latest commits to std::arch, internally assembly. They should behave identically to how they did before, please file a bug if they emit notably worse codegen. The distinction is that, in general, LLVM does not try to perform store deduplication into assembly code.

Kobzol · 2024-07-25T05:56:28Z

So mm_stream will still emit nontemporal instructions on x86? Thanks for clarifying!

RalfJung · 2024-07-25T06:18:15Z

Yes it will, that's why rust-lang/stdarch#1541 needs to propagate first.

Kobzol · 2024-07-25T06:20:32Z

Ok, great, then I misunderstood this change, sorry!

workingjubilee · 2024-07-25T06:51:10Z

Thank you, @RalfJung.

compiler/rustc_codegen_llvm/src/builder.rs

RalfJung · 2024-08-05T08:58:12Z

rust-lang/stdarch#1541 has propagated, so this should be good to go. We haven't heard from @fmease though so let's re-roll the reviewer dice.

r? compiler

jieyouxu

Thanks! The changes themselves look good based on the prior discussions. Just a question about the codegen test with regards to target coverage.

jieyouxu · 2024-08-11T09:42:02Z

compiler/rustc_codegen_llvm/src/builder.rs

+                const WELL_BEHAVED_NONTEMPORAL_ARCHS: &[&str] =
+                    &["aarch64", "arm", "riscv32", "riscv64"];


Question [REV1 (2/2)]: If I understand it correctly, the codegen test is modified such that it only codegens for aarch64-unknown-linux-gnu target, but here we have 4 "well-behaved" architectures that we do emit !nontemporal for.

tests/codegen/intrinsics/nontemporal.rs

jieyouxu · 2024-08-11T09:50:46Z

r? jieyouxu

tests/codegen/intrinsics/nontemporal.rs

jieyouxu

Thanks, feel free to r=me,Amanieu,Jubilee after maybe fixing the nit :3

tests/codegen/intrinsics/nontemporal.rs

RalfJung · 2024-08-12T09:36:14Z

@bors r=jieyouxu,Amanieu,Jubilee

bors · 2024-08-12T09:36:16Z

📌 Commit 5be6c7d has been approved by jieyouxu,Amanieu,Jubilee

It is now in the queue for this repository.

tests/codegen/intrinsics/nontemporal.rs

RalfJung · 2024-08-12T09:43:08Z

@bors r=jieyouxu,Amanieu,Jubilee

bors · 2024-08-12T09:43:11Z

📌 Commit 75743dc has been approved by jieyouxu,Amanieu,Jubilee

It is now in the queue for this repository.

…llaumeGomez Rollup of 10 pull requests Successful merges: - rust-lang#128149 (nontemporal_store: make sure that the intrinsic is truly just a hint) - rust-lang#128394 (Unify run button display with "copy code" button and with mdbook buttons) - rust-lang#128537 (const vector passed through to codegen) - rust-lang#128632 (std: do not overwrite style in `get_backtrace_style`) - rust-lang#128878 (Slightly refactor `Flags` in bootstrap) - rust-lang#128886 (Get rid of some `#[allow(rustc::untranslatable_diagnostic)]`) - rust-lang#128929 (Fix codegen-units tests that were disabled 8 years ago) - rust-lang#128937 (Fix warnings in rmake tests on `x86_64-unknown-linux-gnu`) - rust-lang#128978 (Use `assert_matches` around the compiler more) - rust-lang#128994 (Fix bug in `Parser::look_ahead`.) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#128149 - RalfJung:nontemporal_store, r=jieyouxu,Amanieu,Jubilee nontemporal_store: make sure that the intrinsic is truly just a hint The `!nontemporal` flag for stores in LLVM *sounds* like it is just a hint, but actually, it is not -- at least on x86, non-temporal stores need very special treatment by the programmer or else the Rust memory model breaks down. LLVM still treats these stores as-if they were normal stores for optimizations, which is [highly dubious](llvm/llvm-project#64521). Let's avoid all that dubiousness by making our own non-temporal stores be truly just a hint, which is possible on some targets (e.g. ARM). On all other targets, non-temporal stores become regular stores. ~~Blocked on rust-lang/stdarch#1541 propagating to the rustc repo, to make sure the `_mm_stream` intrinsics are unaffected by this change.~~ Fixes rust-lang#114582 Cc `@Amanieu` `@workingjubilee`

…ouxu,Amanieu,Jubilee nontemporal_store: make sure that the intrinsic is truly just a hint The `!nontemporal` flag for stores in LLVM *sounds* like it is just a hint, but actually, it is not -- at least on x86, non-temporal stores need very special treatment by the programmer or else the Rust memory model breaks down. LLVM still treats these stores as-if they were normal stores for optimizations, which is [highly dubious](llvm/llvm-project#64521). Let's avoid all that dubiousness by making our own non-temporal stores be truly just a hint, which is possible on some targets (e.g. ARM). On all other targets, non-temporal stores become regular stores. ~~Blocked on rust-lang/stdarch#1541 propagating to the rustc repo, to make sure the `_mm_stream` intrinsics are unaffected by this change.~~ Fixes rust-lang#114582 Cc `@Amanieu` `@workingjubilee`

rustbot assigned fmease Jul 24, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 24, 2024

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RalfJung force-pushed the nontemporal_store branch from 05e50ef to 8d0d26e Compare July 24, 2024 19:40

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workingjubilee mentioned this pull request Aug 3, 2024

feat(rust): Add NontemporalArithmeticChunked pola-rs/polars#17708

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RalfJung added 2 commits August 5, 2024 10:57

nontemporal_store: make sure that the intrinsic is truly just a hint

28e0907

RISC-V also has sane nontemporal stores

697787a

RalfJung force-pushed the nontemporal_store branch from 0135bb1 to 697787a Compare August 5, 2024 08:57

rustbot assigned TaKO8Ki and unassigned fmease Aug 5, 2024

jieyouxu approved these changes Aug 11, 2024

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 12, 2024

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make the codegen test also cover an ill-behaved arch, and add links

75743dc

RalfJung force-pushed the nontemporal_store branch from 5be6c7d to 75743dc Compare August 12, 2024 09:42

GuillaumeGomez mentioned this pull request Aug 12, 2024

Rollup of 10 pull requests #129008

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bors merged commit 095ca33 into rust-lang:master Aug 12, 2024
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rustbot added this to the 1.82.0 milestone Aug 12, 2024

RalfJung deleted the nontemporal_store branch August 14, 2024 12:15

		const WELL_BEHAVED_NONTEMPORAL_ARCHS: &[&str] =
		&["aarch64", "arm", "riscv32", "riscv64"];

nontemporal_store: make sure that the intrinsic is truly just a hint #128149

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