Simplify the canonical enum clone branches to a copy statement #148034

dianqk · 2025-10-23T15:13:29Z

I have overhauled MatchBranchSimplification in this PR. This pass is tested merge cases one by one, which is more readable and extensible.

This PR also merges the following pattern that is mostly generated by GVN into one basic block that contains the copy statement:

match a {
    Foo::A(_) => *a,
    Foo::B => Foo::B
}

Fixes #128081.

dianqk · 2025-10-23T15:19:29Z

@bors try @rust-timer queue

[WIP] Simplify the canonical enum clone branches to a copy statement

rust-bors · 2025-10-23T15:43:28Z

💔 Test for b4269e3 failed: CI. Failed jobs:

try - dist-x86_64-linux (web logs, extended logs)

dianqk · 2025-10-23T23:51:23Z

@bors try @rust-timer queue

[WIP] Simplify the canonical enum clone branches to a copy statement

rust-bors · 2025-10-24T02:05:14Z

☀️ Try build successful (CI)
Build commit: 673e3f6 (673e3f61f8b218d304147e821d485edf0ebf3b09, parent: 6501e64fcb02d22b49d6e59d10a7692ec8095619)

rust-timer · 2025-10-24T03:16:55Z

Finished benchmarking commit (673e3f6): 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%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.3%	[0.3%, 0.4%]	6
Improvements ✅ (primary)	-1.0%	[-2.4%, -0.1%]	6
Improvements ✅ (secondary)	-0.2%	[-1.3%, -0.1%]	17
All ❌✅ (primary)	-0.8%	[-2.4%, 0.7%]	7

Max RSS (memory usage)

Results (primary 2.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[1.9%, 4.3%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[1.9%, 4.3%]	3

Cycles

Results (primary -3.4%, secondary -2.0%)

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)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-3.4%	[-3.4%, -3.4%]	1
Improvements ✅ (secondary)	-2.7%	[-4.5%, -2.1%]	6
All ❌✅ (primary)	-3.4%	[-3.4%, -3.4%]	1

Binary size

Results (primary -0.1%, 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)	0.1%	[0.0%, 0.2%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.4%, -0.0%]	20
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	41
All ❌✅ (primary)	-0.1%	[-0.4%, 0.2%]	23

Bootstrap: 476.496s -> 474.095s (-0.50%)
Artifact size: 390.49 MiB -> 390.42 MiB (-0.02%)

dianqk · 2025-10-26T06:53:24Z

@bors try @rust-timer queue

Simplify the canonical enum clone branches to a copy statement

rust-bors · 2025-10-26T09:05:46Z

☀️ Try build successful (CI)
Build commit: da8af3c (da8af3c91a895fd9646d0e6ef1477767ea0b6076, parent: e5177985a5493c783d6ba20960d2723d9d2e9214)

rust-timer · 2025-10-26T10:14:57Z

Finished benchmarking commit (da8af3c): 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%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.9%	[-2.5%, -0.1%]	6
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.1%]	6
All ❌✅ (primary)	-0.7%	[-2.5%, 0.7%]	7

Max RSS (memory usage)

Results (primary 3.0%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[2.1%, 4.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.0%	[2.1%, 4.0%]	2

Cycles

Results (secondary -2.1%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.1%, 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)	0.1%	[0.1%, 0.2%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.5%, -0.0%]	17
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	43
All ❌✅ (primary)	-0.1%	[-0.5%, 0.2%]	19

Bootstrap: 475.08s -> 474.068s (-0.21%)
Artifact size: 390.51 MiB -> 390.49 MiB (-0.00%)

rustbot · 2025-10-26T11:05:30Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2025-10-26T11:05:33Z

r? @madsmtm

rustbot has assigned @madsmtm.
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

dianqk · 2025-10-30T13:11:25Z

r? cjgillot

madsmtm · 2025-10-30T13:49:14Z

r? cjgillot

Yeah, that's probably a better choice, I'm not that familiar with MIR.

compiler/rustc_mir_transform/src/patch.rs

cjgillot · 2025-11-02T02:53:48Z

compiler/rustc_mir_transform/src/match_branches.rs

        let typing_env = body.typing_env(tcx);
        let mut apply_patch = false;
        let mut patch = MirPatch::new(body);
        for (bb, bb_data) in body.basic_blocks.iter_enumerated() {


Pre-existing: we examine terminators. Should we traverse the body in post-order? And have a patch for each iteration?

compiler/rustc_mir_transform/src/match_branches.rs

cjgillot · 2025-11-11T16:07:05Z

compiler/rustc_mir_transform/src/match_branches.rs

+            let new_stmts = if body.basic_blocks[first_case].statements.is_empty() {
+                Vec::new()
+            } else if let Some(else_target) = as_else_target
+                && let Some(new_stmts) = simplify_if(


Can this be made a method on SimplifyMatch? Or keep simplify_match a simple function that constructs a SimplifyMatch helper?

I adopt a more general approach.

compiler/rustc_mir_transform/src/match_branches.rs

cjgillot · 2025-11-11T16:35:18Z

compiler/rustc_mir_transform/src/match_branches.rs

+            (*first, others)
+        }
+        _ => return None,
+    };


Should we return None if other_cases is empty?

The new changes should be able to treat this as a general case.

cjgillot · 2025-11-11T16:42:12Z

compiler/rustc_mir_transform/src/match_branches.rs

+        let bbs = &self.body.basic_blocks;
+        // Check if the copy source matches the following pattern.
+        // _2 = discriminant(*_1); // "*_1" is the expected the copy source.
+        // switchInt(move _2) -> [0: bb3, 1: bb2, otherwise: bb1];


How do we check aliasing? To ensure that the enum *_1 is not changed between the switch and the current statement.

Oops, I missed. I checked whether the statement is the first statement.

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 23, 2025