No longer need `alloca`s for consuming `Result<!, i32>` and similar #144347

scottmcm · 2025-07-23T06:32:21Z

In optimized builds GVN gets rid of these already, but in opt-level=0 we actually make allocas for this, which particularly impacts ?-style things that use actually-only-one-variant types like this.

While doing so, rewrite LocalAnalyzer::process_place to be non-recursive, solving a 6+ year old FIXME.

r? codegen

rustbot · 2025-07-23T06:32:26Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

scottmcm · 2025-07-23T06:39:57Z

tests/codegen-llvm/enum/enum-transparent-extract.rs

+#[no_mangle]
+pub fn make_unmake_result_never(x: i32) -> i32 {
+    // CHECK-LABEL: define i32 @make_unmake_result_never(i32 %x)
+    // CHECK: start:
+    // CHECK-NEXT: ret i32 %x
+
+    let y: Result<i32, Never> = Ok(x);
+    let Ok(z) = y;
+    z
+}
+
+#[no_mangle]
+pub fn extract_control_flow_never(x: ControlFlow<&str, Never>) -> &str {
+    // CHECK-LABEL: define { ptr, i64 } @extract_control_flow_never(ptr align 1 %x.0, i64 %x.1)
+    // CHECK: start:
+    // CHECK-NEXT: %[[P0:.+]] = insertvalue { ptr, i64 } poison, ptr %x.0, 0
+    // CHECK-NEXT: %[[P1:.+]] = insertvalue { ptr, i64 } %[[P0]], i64 %x.1, 1
+    // CHECK-NEXT: ret { ptr, i64 } %[[P1]]
+
+    let ControlFlow::Break(s) = x;
+    s
+}


Compare today: https://rust.godbolt.org/z/9dezqPq6n

define i32 @make_unmake_result_never(i32 %x) unnamed_addr { start: %y = alloca [4 x i8], align 4 store i32 %x, ptr %y, align 4 %z = load i32, ptr %y, align 4 ret i32 %z } define { ptr, i64 } @extract_control_flow_never(ptr align 1 %0, i64 %1) unnamed_addr { start: %x = alloca [16 x i8], align 8 store ptr %0, ptr %x, align 8 %2 = getelementptr inbounds i8, ptr %x, i64 8 store i64 %1, ptr %2, align 8 %s.0 = load ptr, ptr %x, align 8 %3 = getelementptr inbounds i8, ptr %x, i64 8 %s.1 = load i64, ptr %3, align 8 %4 = insertvalue { ptr, i64 } poison, ptr %s.0, 0 %5 = insertvalue { ptr, i64 } %4, i64 %s.1, 1 ret { ptr, i64 } %5 }

In optimized builds GVN gets rid of these already, but in `opt-level=0` we actually make `alloca`s for this, which particularly impacts `?`-style things that use actually-only-one-variant types like this.

scottmcm · 2025-07-23T07:48:40Z

@bors try @rust-timer queue

rust-bors · 2025-07-23T07:48:44Z

⌛ Trying commit 6a5c7e0 with merge 88eefbd…

To cancel the try build, run the command @bors try cancel.

No longer need `alloca`s for consuming `Result<!, i32>` and similar

rust-bors · 2025-07-23T10:02:43Z

☀️ Try build successful (CI)
Build commit: 88eefbd (88eefbd05a766727d674a8f4a54e38670ce81d3b, parent: a7a1618e6c835f1f00940ad72203d05808209a0d)

rust-timer · 2025-07-23T11:13:47Z

Finished benchmarking commit (88eefbd): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (primary -5.4%, 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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-5.4%	[-5.4%, -5.4%]	1
Improvements ✅ (secondary)	-2.6%	[-2.6%, -2.6%]	1
All ❌✅ (primary)	-5.4%	[-5.4%, -5.4%]	1

Cycles

Results (primary -0.4%, secondary -0.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	3.3%	[2.6%, 4.1%]	3
Improvements ✅ (primary)	-3.2%	[-3.2%, -3.2%]	1
Improvements ✅ (secondary)	-4.4%	[-7.2%, -2.8%]	3
All ❌✅ (primary)	-0.4%	[-3.2%, 2.3%]	2

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)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.0%, 0.0%]	5

Bootstrap: 463.694s -> 465.155s (0.32%)
Artifact size: 374.58 MiB -> 374.70 MiB (0.03%)

scottmcm · 2025-07-23T16:13:23Z

Those html5ever binary size regressions are 336 bytes, 328 bytes, 328 bytes, and 328 bytes, which I'm not worried about.

Thus I would consider this very perf-neutral and thus good to go.

(I was hoping for a bit better than that, but for neutral I think it's fine, and we can try things like rustc_always_inline on Result::branch and such in a future PR to make this more applicable. Similarly this restructuring will hopefully make it easier to pull in some changes from #138582 and thus make it applicable to even more things later as well.)

With no icount changes I'll demote it to
@bors rollup=iffy
but feel free to pick a different level when reviewing if you feel differently.

WaffleLapkin · 2025-07-25T12:51:30Z

@bors r+

bors · 2025-07-25T12:51:33Z

📌 Commit 6a5c7e0 has been approved by WaffleLapkin

It is now in the queue for this repository.

bors · 2025-07-27T06:37:58Z

⌛ Testing commit 6a5c7e0 with merge 86ef320...

bors · 2025-07-27T09:37:20Z

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing 86ef320 to master...

github-actions · 2025-07-27T09:40:29Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 052114f (parent) -> 86ef320 (this PR)

Test differences

Show 7 test diffs

Stage 1

[codegen] tests/codegen-llvm/enum/enum-transparent-extract.rs: [missing] -> pass (J2)

Stage 2

[codegen] tests/codegen-llvm/enum/enum-transparent-extract.rs: [missing] -> pass (J0)
[codegen] tests/codegen-llvm/enum/enum-transparent-extract.rs: [missing] -> ignore (only executed when the pointer width is 64bit) (J1)

Additionally, 4 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 86ef32029427cfc4161a3fd7a51992302f7c5552 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-aarch64-linux: 5818.3s -> 7981.8s (37.2%)
x86_64-apple-1: 7188.0s -> 9432.7s (31.2%)
aarch64-apple: 5664.2s -> 4750.2s (-16.1%)
x86_64-apple-2: 6009.0s -> 5349.6s (-11.0%)
pr-check-1: 1666.6s -> 1530.5s (-8.2%)
dist-aarch64-apple: 5778.0s -> 6225.2s (7.7%)
x86_64-msvc-ext3: 5985.4s -> 6429.8s (7.4%)
x86_64-gnu-llvm-19-1: 3537.6s -> 3292.0s (-6.9%)
dist-ohos-armv7: 4172.3s -> 3894.7s (-6.7%)
dist-s390x-linux: 4811.2s -> 5085.5s (5.7%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-07-27T10:48:52Z

Finished benchmarking commit (86ef320): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (secondary 3.6%)

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)	4.4%	[2.0%, 8.0%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.5%	[-1.5%, -1.5%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -1.1%, secondary -5.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.6%	[1.6%, 1.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.5%	[-2.8%, -2.2%]	2
Improvements ✅ (secondary)	-5.2%	[-9.7%, -2.5%]	15
All ❌✅ (primary)	-1.1%	[-2.8%, 1.6%]	3

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)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.0%, 0.0%]	5

Bootstrap: 467.835s -> 467.671s (-0.04%)
Artifact size: 376.68 MiB -> 376.75 MiB (0.02%)

scottmcm · 2025-07-27T16:33:56Z

(Those cycle changes look fake in the couple I checked.)

rustbot assigned saethlin Jul 23, 2025

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 23, 2025

scottmcm commented Jul 23, 2025

View reviewed changes

This comment has been minimized.

Sign in to view

scottmcm force-pushed the ssa-enums-v0 branch from 6611fa9 to 98078d8 Compare July 23, 2025 07:02

This comment has been minimized.

Sign in to view

No longer need allocas for consuming Result<!, i32> and similar

6a5c7e0

In optimized builds GVN gets rid of these already, but in `opt-level=0` we actually make `alloca`s for this, which particularly impacts `?`-style things that use actually-only-one-variant types like this.

scottmcm force-pushed the ssa-enums-v0 branch from 98078d8 to 6a5c7e0 Compare July 23, 2025 07:09

This comment has been minimized.

Sign in to view

rust-bors bot added a commit that referenced this pull request Jul 23, 2025

Auto merge of #144347 - scottmcm:ssa-enums-v0, r=<try>

88eefbd

No longer need `alloca`s for consuming `Result<!, i32>` and similar

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

This comment has been minimized.

Sign in to view

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

WaffleLapkin approved these changes Jul 25, 2025

View reviewed changes

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 Jul 25, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Jul 27, 2025

bors merged commit 86ef320 into rust-lang:master Jul 27, 2025
13 checks passed

rustbot added this to the 1.90.0 milestone Jul 27, 2025

scottmcm deleted the ssa-enums-v0 branch July 27, 2025 16:33

No longer need allocas for consuming Result<!, i32> and similar #144347

No longer need allocas for consuming Result<!, i32> and similar #144347

Uh oh!

Conversation

scottmcm commented Jul 23, 2025

Uh oh!

rustbot commented Jul 23, 2025

Uh oh!

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

This comment has been minimized.

scottmcm commented Jul 23, 2025

Uh oh!

This comment has been minimized.

rust-bors bot commented Jul 23, 2025

Uh oh!

rust-bors bot commented Jul 23, 2025

Uh oh!

This comment has been minimized.

rust-timer commented Jul 23, 2025

Overall result: no relevant changes - no action needed

Uh oh!

scottmcm commented Jul 23, 2025

Uh oh!

WaffleLapkin commented Jul 25, 2025

Uh oh!

bors commented Jul 25, 2025

Uh oh!

bors commented Jul 27, 2025

Uh oh!

bors commented Jul 27, 2025

Uh oh!

Uh oh!

github-actions bot commented Jul 27, 2025

Test differences

Stage 1

Stage 2

Job duration changes

Uh oh!

rust-timer commented Jul 27, 2025

Overall result: no relevant changes - no action needed

Uh oh!

scottmcm commented Jul 27, 2025

Uh oh!

Uh oh!

No longer need `alloca`s for consuming `Result<!, i32>` and similar #144347

No longer need `alloca`s for consuming `Result<!, i32>` and similar #144347