Enable DestinationPropagation by default #142915

cjgillot · 2025-06-23T16:49:29Z

This PR proposes to perform destination propagation on MIR. Most of the pass was fully rewritten by @JakobDegen in #96451.

This pass is quite heavy, as it needs to perform and save the results of a full liveness dataflow analysis. This accounts for ~50% of the pass' runtime.

Perf sees a few decent savings in later llvm passes, but also sizeable régressions when there are no savings to balance this pass' runtime.

cjgillot · 2025-06-23T16:49:42Z

@bors try @rust-timer queue

Enable DestinationPropagation by default r? `@ghost`

bors · 2025-06-23T16:50:54Z

⌛ Trying commit b835228 with merge 2989fd8...

bors · 2025-06-23T19:21:53Z

☀️ Try build successful - checks-actions
Build commit: 2989fd8 (2989fd868755417871c65b8864528f20125af5bc)

rust-timer · 2025-06-24T06:32:17Z

Finished benchmarking commit (2989fd8): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.2%, 3.1%]	27
Regressions ❌ (secondary)	0.6%	[0.2%, 2.5%]	36
Improvements ✅ (primary)	-0.4%	[-1.7%, -0.1%]	60
Improvements ✅ (secondary)	-0.7%	[-2.9%, -0.2%]	71
All ❌✅ (primary)	-0.0%	[-1.7%, 3.1%]	87

Max RSS (memory usage)

Results (primary 0.7%, secondary 1.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	5.6%	[1.7%, 10.5%]	5
Regressions ❌ (secondary)	1.6%	[1.4%, 2.2%]	9
Improvements ✅ (primary)	-4.3%	[-5.9%, -2.5%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[-5.9%, 10.5%]	10

Cycles

Results (primary 0.3%, secondary -1.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.0%	[0.5%, 1.4%]	5
Regressions ❌ (secondary)	3.1%	[1.8%, 4.4%]	3
Improvements ✅ (primary)	-1.5%	[-1.7%, -1.3%]	2
Improvements ✅ (secondary)	-2.6%	[-3.5%, -1.5%]	8
All ❌✅ (primary)	0.3%	[-1.7%, 1.4%]	7

Binary size

Results (primary -0.0%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.4%]	22
Regressions ❌ (secondary)	0.4%	[0.0%, 2.0%]	94
Improvements ✅ (primary)	-0.1%	[-1.0%, -0.0%]	54
Improvements ✅ (secondary)	-1.0%	[-13.0%, -0.0%]	28
All ❌✅ (primary)	-0.0%	[-1.0%, 0.4%]	76

Bootstrap: 688.878s -> 687.306s (-0.23%)
Artifact size: 371.92 MiB -> 371.62 MiB (-0.08%)

rustbot · 2025-06-25T19:10:42Z

r? @petrochenkov

rustbot has assigned @petrochenkov.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

rustbot · 2025-06-25T19:10:43Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

petrochenkov · 2025-06-25T20:36:20Z

r? mir

oli-obk

r=me

oli-obk · 2025-06-30T13:33:16Z

compiler/rustc_mir_transform/src/dest_prop.rs

-        sess.mir_opt_level() >= 3
+        sess.mir_opt_level() >= 2


The comments above can now be removed

cjgillot · 2025-07-01T14:14:27Z

There is a pathological behaviour with cranelift-codegen I'd like to fix first. There are many locals that can be merged with _0. The current version needs to recompute all candidates and all conflicts each time it merges _0 with something.

cjgillot · 2025-07-02T10:10:38Z

@bors try @rust-timer queue

bors · 2025-07-02T10:11:55Z

⌛ Trying commit 9298dfa with merge 00ffd2b...

Enable DestinationPropagation by default This PR proposes to perform destination propagation on MIR. Most of the pass was fully rewritten by `@JakobDegen` in #96451. This pass is quite heavy, as it needs to perform and save the results of a full liveness dataflow analysis. This accounts for ~50% of the pass' runtime. Perf sees a few decent savings in later llvm passes, but also sizeable régressions when there are no savings to balance this pass' runtime.

bors · 2025-07-02T12:27:32Z

☀️ Try build successful - checks-actions
Build commit: 00ffd2b (00ffd2bf2b816d1f0df05611fccdd7dad0a8db22)

rust-timer · 2025-07-03T06:23:59Z

Finished benchmarking commit (00ffd2b): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.1%, 2.3%]	31
Regressions ❌ (secondary)	0.6%	[0.1%, 4.6%]	47
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.2%]	40
Improvements ✅ (secondary)	-0.8%	[-2.9%, -0.1%]	69
All ❌✅ (primary)	-0.0%	[-0.8%, 2.3%]	71

Max RSS (memory usage)

Results (primary -1.8%, secondary -2.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	5.8%	[5.8%, 5.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.1%	[-4.7%, -1.9%]	6
Improvements ✅ (secondary)	-2.6%	[-4.1%, -1.1%]	2
All ❌✅ (primary)	-1.8%	[-4.7%, 5.8%]	7

Cycles

Results (primary 1.3%, secondary -2.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.7%	[1.0%, 2.3%]	8
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-1.8%	[-1.8%, -1.8%]	1
Improvements ✅ (secondary)	-3.2%	[-5.0%, -0.7%]	9
All ❌✅ (primary)	1.3%	[-1.8%, 2.3%]	9

Binary size

Results (primary -0.0%, secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.5%]	21
Regressions ❌ (secondary)	0.4%	[0.0%, 2.0%]	93
Improvements ✅ (primary)	-0.1%	[-1.0%, -0.0%]	59
Improvements ✅ (secondary)	-1.0%	[-12.8%, -0.0%]	28
All ❌✅ (primary)	-0.0%	[-1.0%, 0.5%]	80

Bootstrap: 462.08s -> 461.558s (-0.11%)
Artifact size: 372.26 MiB -> 371.46 MiB (-0.22%)

cjgillot · 2025-07-03T10:05:35Z

@bors try @rust-timer queue

rust-timer · 2025-07-03T10:05:40Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

bors · 2025-07-03T10:06:53Z

⌛ Trying commit 8a9442c with merge 4eb4056...

Enable DestinationPropagation by default This PR proposes to perform destination propagation on MIR. Most of the pass was fully rewritten by `@JakobDegen` in #96451. This pass is quite heavy, as it needs to perform and save the results of a full liveness dataflow analysis. This accounts for ~50% of the pass' runtime. Perf sees a few decent savings in later llvm passes, but also sizeable régressions when there are no savings to balance this pass' runtime.

bors · 2025-07-03T12:14:47Z

☀️ Try build successful - checks-actions
Build commit: 4eb4056 (4eb40562038e65257378a63c0919a7427a3f6b6c)

rust-timer · 2025-07-03T12:14:51Z

Queued 4eb4056 with parent 6268d0a, future comparison URL.
There are currently 9 preceding artifacts in the queue.
It will probably take at least ~11.2 hours until the benchmark run finishes.

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jun 23, 2025

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 23, 2025

bors added a commit that referenced this pull request Jun 23, 2025

Auto merge of #142915 - cjgillot:dest-prop-default, r=<try>

2989fd8

Enable DestinationPropagation by default r? `@ghost`

cjgillot added the A-mir-opt Area: MIR optimizations label Jun 23, 2025

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 24, 2025

cjgillot marked this pull request as ready for review June 25, 2025 19:10

rustbot assigned petrochenkov Jun 25, 2025

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 25, 2025

rustbot assigned oli-obk and unassigned petrochenkov Jun 25, 2025

oli-obk approved these changes Jun 30, 2025

View reviewed changes

cjgillot added 3 commits July 1, 2025 12:34

Enable DestinationPropagation by default.

184894f

Introduce fast insertion at extremities to IntervalSet.

905d2e0

Remove obsolete comment.

011314d

cjgillot added 3 commits July 2, 2025 08:42

Precompute full conflicts matrix.

b4970e4

Only record conflicts between relevant locals.

ffcc38a

Early exit if no candidates.

9298dfa

cjgillot force-pushed the dest-prop-default branch from b835228 to 9298dfa Compare July 2, 2025 10:10

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 2, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 3, 2025

Only save liveness for relevant locals.

8a9442c

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 3, 2025

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Enable DestinationPropagation by default #142915

Enable DestinationPropagation by default #142915

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Enable DestinationPropagation by default #142915

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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