Turn copy into moves during DSE. #113758

cjgillot · 2023-07-16T13:48:35Z

Dead store elimination computes whether removing a direct store to an unborrowed place is allowed.
Where removing a store is allowed, writing uninit is too.

This means that we can use this pass to transform copy operands into move operands. This is only interesting in call terminators, so we only handle those.

Special care is taken for the use_both(_1, _1) case:

moving the second argument is ok, as _1 is not live after the call;
moving the first argument is not, as the second argument reads _1.

Fixes #75993
Fixes #108068

r? @RalfJung
cc @JakobDegen

rustbot · 2023-07-16T13:48:43Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

RalfJung · 2023-07-16T13:54:21Z

Sorry, I can't review MIR transforms. I can help judge their soundness given a high-level description of what they do with representative examples, but reverse engineering that from a rustc implementation of a MIR transform is a lot of work that I don't have the time for.

r? mir-opt

cjgillot · 2023-07-16T14:13:38Z

...-opt/dead-store-elimination/call_arg_copy.move_simple.DeadStoreElimination.panic-unwind.diff

+      bb0: {
+          StorageLive(_2);
+-         _2 = use_both(_1, _1) -> [return: bb1, unwind continue];
+         _2 = use_both(_1, move _1) -> [return: bb1, unwind continue];


@RalfJung IMO, the thorny question is here. It this transformation ok?

That's a very good question. It is okay under the move semantics I implemented in Miri recently (assuming _1 is not read from again later), because there we do left-to-right evaluation so the first _1 is copied before the move action. But I don't know if there's a risk of codegen backends doing things that would cause problems here. Also move semantics are far from clear; I don't think we already have the semantics we want here -- we just have something that justifies what codegen does.

I think, similar to how we avoid making assumptions about the aliasing model, we should be conservative and avoid this if any of the other arguments (or the return value) could alias the newly moved argument.

Cc @bjorn3

Relevant UCG issue: rust-lang/unsafe-code-guidelines#416

I think this is fine given that the first _1 is copied before the call passes the address of the second argument. It may even be ok to do use_both(move _1, _1) with current backends, but I don't think whatever semantics we give to move arguments should allow that as semantically the move of the first arg happens before the copy of the second arg.

oli-obk · 2023-07-17T11:33:56Z

@bors r=JakobDegen,oli-obk

bors · 2023-07-17T11:33:58Z

📌 Commit 3af8610f5a91c003a7fdb5df5353774c8ca0431a has been approved by JakobDegen,oli-obk

It is now in the queue for this repository.

cjgillot · 2023-07-17T20:51:50Z

Just in case there is a perf effect (I doubt it)
@bors rollup=never

bors · 2023-07-17T23:38:58Z

⌛ Testing commit 3af8610f5a91c003a7fdb5df5353774c8ca0431a with merge 32322a067c781aef41fc5ea61450820258d956c4...

bors · 2023-07-18T00:19:10Z

💔 Test failed - checks-actions

cjgillot · 2023-07-19T07:39:56Z

Changed the codegen test to run with mir opts.
@bors r=JakobDegen,oli-obk

bors · 2023-07-19T07:39:58Z

📌 Commit 6d9c12730114034367fa73c394823285515c508e has been approved by JakobDegen,oli-obk

It is now in the queue for this repository.

bors · 2023-07-19T09:13:42Z

⌛ Testing commit 6d9c12730114034367fa73c394823285515c508e with merge a751a70cc37827cad61f1c2b5558990838050cc0...

bors · 2023-07-19T09:39:57Z

💔 Test failed - checks-actions

bors · 2023-07-20T09:28:14Z

📌 Commit 254bf60 has been approved by JakobDegen,oli-obk

It is now in the queue for this repository.

bors · 2023-07-20T10:48:34Z

⌛ Testing commit 254bf60 with merge 002a2388e0bd2a9ed50f460984b0c3523bd1ff71...

rust-log-analyzer · 2023-07-20T12:17:34Z

The job dist-aarch64-apple failed! Check out the build log: (web) (plain)

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bors · 2023-07-20T12:17:56Z

💔 Test failed - checks-actions

lqd · 2023-07-20T12:59:22Z

@bors retry rust-lang/crates.io#6850

bors · 2023-07-20T15:05:42Z

⌛ Testing commit 254bf60 with merge 06a53dd...

bors · 2023-07-20T16:56:58Z

☀️ Test successful - checks-actions
Approved by: JakobDegen,oli-obk
Pushing 06a53dd to master...

rust-timer · 2023-07-20T18:15:36Z

Finished benchmarking commit (06a53dd): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	9.4%	[9.4%, 9.4%]	1
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	1
Improvements ✅ (primary)	-1.0%	[-2.1%, -0.2%]	14
Improvements ✅ (secondary)	-0.8%	[-1.3%, -0.2%]	2
All ❌✅ (primary)	-0.3%	[-2.1%, 9.4%]	15

Max RSS (memory usage)

Results

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)	1.9%	[1.3%, 2.2%]	7
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-6.3%	[-10.5%, -3.8%]	6
Improvements ✅ (secondary)	-1.9%	[-2.7%, -1.1%]	2
All ❌✅ (primary)	-1.9%	[-10.5%, 2.2%]	13

Cycles

Results

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)	8.8%	[8.8%, 8.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.8%	[-0.8%, -0.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.0%	[-0.8%, 8.8%]	2

Binary size

Results

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)	0.1%	[0.0%, 0.2%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.5%, 0.2%]	12

Bootstrap: 648.618s -> 649.387s (0.12%)

…-errors Do not convert copies of packed projections to moves. This code path was introduced in rust-lang#113758 After seeing https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Packed.20fields.20and.20in-place.20function.20argument.2Freturn.20passing, this may be UB, so should be disallowed. This should not appear in normally-built MIR, which introduces temporary copies for packed projections.

Do not convert copies of packed projections to moves. This code path was introduced in rust-lang/rust#113758 After seeing https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Packed.20fields.20and.20in-place.20function.20argument.2Freturn.20passing, this may be UB, so should be disallowed. This should not appear in normally-built MIR, which introduces temporary copies for packed projections.

rustbot assigned RalfJung Jul 16, 2023

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. labels Jul 16, 2023

rustbot assigned oli-obk and unassigned RalfJung Jul 16, 2023

cjgillot commented Jul 16, 2023

View reviewed changes

cjgillot added the A-mir-opt Area: MIR optimizations label Jul 16, 2023

JakobDegen approved these changes Jul 17, 2023

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bors added the merged-by-bors This PR was explicitly merged by bors. label Jul 20, 2023

bors merged commit 06a53dd into rust-lang:master Jul 20, 2023

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cjgillot deleted the move-dse branch July 20, 2023 21:26

cjgillot mentioned this pull request Aug 23, 2023

Do not convert copies of packed projections to moves. #115138

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Turn copy into moves during DSE. #113758

Turn copy into moves during DSE. #113758

cjgillot commented Jul 16, 2023 •

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Turn copy into moves during DSE. #113758

Turn copy into moves during DSE. #113758

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Overall result: ❌✅ regressions and improvements - ACTION NEEDED

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