Simplify trivial constants in SimplifyConstCondition #147654

dianqk · 2025-10-14T00:25:34Z

After InstSimplify-after-simplifycfg with -Zub_checks=false, there are many of the following patterns.

_13 = const false;
assume(copy _13);
_12 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];

Simplifying them to unreachable early should make CFG simpler.

dianqk · 2025-10-14T00:54:20Z

@bors try @rust-timer queue

Simplify trivial constants in SimplifyConstCondition

rust-bors · 2025-10-14T03:06:20Z

☀️ Try build successful (CI)
Build commit: 2d02757 (2d0275706b38d9e50cb9131ae9dc3ae769529a20, parent: 4b94758d2ba7d0ef71ccf5fde29ce4bc5d6fe2a4)

rust-timer · 2025-10-14T04:17:58Z

Finished benchmarking commit (2d02757): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.1%]	9
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.7%)

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

	mean	range	count
Regressions ❌ (primary)	4.9%	[4.9%, 4.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[-3.6%, 4.9%]	2

Cycles

Results (secondary 3.0%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[2.5%, 3.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	0.1%	[0.0%, 0.2%]	7
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	8
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.1%, 0.2%]	15

Bootstrap: 475.312s -> 475.552s (0.05%)
Artifact size: 388.16 MiB -> 388.14 MiB (-0.00%)

dianqk · 2025-10-14T13:44:27Z

@bors2 try parent=last @rust-timer queue

Simplify trivial constants in SimplifyConstCondition

rustbot · 2025-10-14T13:49:52Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2025-10-14T13:49:55Z

r? @davidtwco

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

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rust-bors · 2025-10-14T15:55:26Z

☀️ Try build successful (CI)
Build commit: 2b77d79 (2b77d79571afb4b51d2e238b5007f06b01912318, parent: 4b94758d2ba7d0ef71ccf5fde29ce4bc5d6fe2a4)

rust-timer · 2025-10-14T18:03:34Z

Finished benchmarking commit (2b77d79): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	9
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.9%)

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

	mean	range	count
Regressions ❌ (primary)	4.7%	[4.7%, 4.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.9%	[-3.0%, 4.7%]	2

Cycles

Results (secondary 3.5%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.5%	[3.5%, 3.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	0.1%	[0.0%, 0.2%]	7
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	8
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.1%, 0.2%]	15

Bootstrap: 475.312s -> 474.926s (-0.08%)
Artifact size: 388.16 MiB -> 388.14 MiB (-0.00%)

compiler/rustc_mir_transform/src/simplify_branches.rs

cjgillot · 2025-10-15T03:17:47Z

compiler/rustc_mir_transform/src/lib.rs

            // optimizations. This invalidates CFG caches, so avoid putting between
            // `ReferencePropagation` and `GVN` which both use the dominator tree.
            &instsimplify::InstSimplify::AfterSimplifyCfg,
+            &o1(simplify_branches::SimplifyConstCondition::AfterInstSimplify),


Could you add a comment to explain why you put this pass here?

compiler/rustc_mir_transform/src/gvn.rs

cjgillot

One nit. r=me once CI is fixed.

View changes since this review

cjgillot · 2025-10-15T18:25:43Z

compiler/rustc_mir_transform/src/gvn.rs

+            TerminatorKind::UnwindResume
+            | TerminatorKind::UnwindTerminate(_)
+            | TerminatorKind::Drop { .. }
+            | TerminatorKind::Call { .. }
+            | TerminatorKind::TailCall { .. }
+            | TerminatorKind::Assert { .. }
+            | TerminatorKind::Yield { .. }
+            | TerminatorKind::CoroutineDrop
+            | TerminatorKind::FalseEdge { .. }
+            | TerminatorKind::FalseUnwind { .. }
+            | TerminatorKind::InlineAsm { .. } => false,


Should we move all terminators that don't write to memory to true?

I haven't seen that method, so I intend to check all terminators and add maybe_write_memory in #147681.

rustbot · 2025-10-16T13:30:28Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 14, 2025

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Auto merge of #147654 - dianqk:simplify-const-condition, r=<try>

2d02757

Simplify trivial constants in SimplifyConstCondition

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 14, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 14, 2025

dianqk force-pushed the simplify-const-condition branch from 3877315 to dcd9af0 Compare October 14, 2025 13:36

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Auto merge of #147654 - dianqk:simplify-const-condition, r=<try>

2b77d79

Simplify trivial constants in SimplifyConstCondition

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 14, 2025

dianqk marked this pull request as ready for review October 14, 2025 13:49

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 14, 2025

rustbot assigned davidtwco Oct 14, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 14, 2025

cjgillot reviewed Oct 15, 2025

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cjgillot self-assigned this Oct 15, 2025

davidtwco removed their assignment Oct 15, 2025

dianqk force-pushed the simplify-const-condition branch 3 times, most recently from 64c5566 to 8f854f8 Compare October 15, 2025 15:05

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dianqk force-pushed the simplify-const-condition branch from 8f854f8 to bdc9444 Compare October 16, 2025 00:24

dianqk added 3 commits October 16, 2025 21:30

mir-opt: Simplify trivial constants in SimplifyConstCondition

a673575

GVN: Preserve derefs at unreachable and return

e881061

Bless collect-in-promoted-const.rs

0db7401

dianqk force-pushed the simplify-const-condition branch from bdc9444 to 0db7401 Compare October 16, 2025 13:30

Simplify trivial constants in SimplifyConstCondition #147654

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Simplify trivial constants in SimplifyConstCondition #147654

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Overall result: ✅ improvements - no action needed

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