miri: optimize zeroed alloc #136035

SpecificProtagonist · 2025-01-25T01:27:11Z

When allocating zero-initialized memory in MIR interpretation, rustc allocates zeroed memory, marks it as initialized and then re-zeroes it. Remove the last step.

I don't expect this to have much of an effect on performance normally, but in my case in which I'm creating a large allocation via mmap it gets in the way.

rustbot · 2025-01-25T01:27:20Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @wesleywiser (or someone else) some time within the next two weeks.

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rustbot · 2025-01-25T01:27:22Z

Some changes occurred to the CTFE machinery

cc @rust-lang/wg-const-eval

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @rust-lang/wg-const-eval

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

jieyouxu · 2025-01-25T07:17:49Z

r? miri

SpecificProtagonist · 2025-01-25T09:51:34Z

Sorry, I'm not sure how I closed this – misclick?

Kobzol · 2025-01-25T11:36:51Z

@bors try @rust-timer queue

bors · 2025-01-25T11:38:02Z

⌛ Trying commit bd28faf with merge 837b710...

…c, r=<try> miri: optimize zeroed alloc When allocating zero-initialized memory in MIR interpretation, rustc allocates zeroed memory, marks it as initialized and then re-zeroes it. Remove the last step. I don't expect this to have much of an effect on performance normally, but in my case in which I'm creating a large allocation via mmap miri is unusable without this. There's probably a better way – with less code duplication – to implement this. Maybe adding a zero_init flag to the relevant methods, but then `Allocation::uninit` & co need a new name :)

bors · 2025-01-25T13:29:27Z

☀️ Try build successful - checks-actions
Build commit: 837b710 (837b710e5dd54b53b888f1ab109a3b93efc9a144)

oli-obk · 2025-01-25T14:11:32Z

This is not gonna show up in perf. No code path outside miri is changed

RalfJung · 2025-01-25T14:21:48Z

We should definitely explore ways to do this with less code duplication. :)
Adding a flag sounds like a good idea. The name of the method could just be Allocation::new etc?

rust-timer · 2025-01-25T15:26:43Z

Finished benchmarking commit (837b710): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.2%, secondary 2.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	2
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.2%	[-2.2%, -2.2%]	1

Cycles

Results (primary -1.6%, secondary -0.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.3%	[1.3%, 1.3%]	1
Improvements ✅ (primary)	-1.6%	[-1.6%, -1.6%]	1
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	-1.6%	[-1.6%, -1.6%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 771.261s -> 771.165s (-0.01%)
Artifact size: 325.82 MiB -> 325.82 MiB (0.00%)

SpecificProtagonist · 2025-01-25T18:34:42Z

Adding a flag sounds like a good idea. The name of the method could just be Allocation::new etc?

Changed 👍

compiler/rustc_const_eval/src/interpret/memory.rs

compiler/rustc_middle/src/mir/interpret/allocation.rs

compiler/rustc_const_eval/src/interpret/memory.rs

RalfJung · 2025-01-28T10:53:12Z

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -277,6 +278,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
        new_size: Size,
        new_align: Align,
        kind: MemoryKind<M::MemoryKind>,
+        init: AllocInit,


Might be good to explain in a doc comment for the fn that this only refers to the new, grown part of the alloc; the carried-over part is as (un)initialized as the old allocation.

oli-obk · 2025-01-28T11:04:08Z

Please squash the commits once the final doc comment has been added

Co-authored-by: Ralf Jung <[email protected]>

oli-obk · 2025-01-28T11:55:27Z

@bors r+

bors · 2025-01-28T11:55:30Z

📌 Commit eee9df4 has been approved by oli-obk

It is now in the queue for this repository.

RalfJung · 2025-01-28T12:18:42Z

Thanks for the PR, this is a nice optimization :)

bors · 2025-01-30T01:27:24Z

⌛ Testing commit eee9df4 with merge 5e55679...

bors · 2025-01-30T04:11:52Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 5e55679 to master...

rust-timer · 2025-01-30T05:58:20Z

Finished benchmarking commit (5e55679): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.6%, secondary 2.9%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Cycles

Results (secondary 0.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 775.772s -> 775.404s (-0.05%)
Artifact size: 328.49 MiB -> 328.40 MiB (-0.03%)

rustbot assigned wesleywiser Jan 25, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 25, 2025

oli-obk reviewed Jan 25, 2025

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miri: optimize zeroed alloc

eee9df4

Co-authored-by: Ralf Jung <[email protected]>

SpecificProtagonist force-pushed the miri-zeroed-alloc branch from ff38a95 to eee9df4 Compare January 28, 2025 11:50

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 Jan 28, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Jan 30, 2025

bors merged commit 5e55679 into rust-lang:master Jan 30, 2025
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rustbot added this to the 1.86.0 milestone Jan 30, 2025

This was referenced Jan 30, 2025

Mangle rustc_std_internal_symbols functions #127173

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miri: optimize zeroed alloc #136035

miri: optimize zeroed alloc #136035

SpecificProtagonist commented Jan 25, 2025 •

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miri: optimize zeroed alloc #136035

miri: optimize zeroed alloc #136035

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Overall result: ✅ improvements - no action needed

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Overall result: ✅ improvements - no action needed

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