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.

Use r? to explicitly pick a reviewer

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

bors · 2025-10-17T13:08:58Z

💔 Test failed - checks-actions

samueltardieu · 2025-10-17T13:11:17Z

Probably a spurious failure. See https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Repeated.20spurious.20failure.20of.20.60x86_64-mingw-1.60/with/545487472.

@bors retry

Simplify trivial constants in SimplifyConstCondition 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.

bors · 2025-10-17T14:31:12Z

⌛ Testing commit 7af5775 with merge ea28252...

rust-log-analyzer · 2025-10-17T15:40:42Z

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[WARNING] `tests/rustdoc-gui/search-result-display.goml` line 39: Delta is 0 for "x", maybe try to use `compare-elements-position` instead?

======== tests/rustdoc-gui/sidebar-macro-reexport.goml ========

[ERROR] `tests/rustdoc-gui/sidebar-macro-reexport.goml` line 3: Error: Navigating frame was detached: for command `go-to: "file://" + |DOC_PATH| + "/test_docs/macro.repro.html"`
`tests/rustdoc-gui/sidebar-macro-reexport.goml` sidebar-macro-reexport output:
Protocol error: Connection closed. Most likely the page has been closed.
stack: Error: Protocol error: Connection closed. Most likely the page has been closed.
    at assert (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/util/assert.js:18:15)
    at CdpPage.close (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/Page.js:814:36)
    at async runAllCommands (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:433:13)
    at async innerRunTestCode (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:696:21)
    at async innerRunTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:633:17)
    at async runTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:824:27)


======== tests/rustdoc-gui/sidebar-macro-reexport.goml ========

[ERROR] `tests/rustdoc-gui/sidebar-macro-reexport.goml` line 3: Error: Navigating frame was detached: for command `go-to: "file://" + |DOC_PATH| + "/test_docs/macro.repro.html"`
`tests/rustdoc-gui/sidebar-macro-reexport.goml` sidebar-macro-reexport output:
Protocol error: Connection closed. Most likely the page has been closed.
stack: Error: Protocol error: Connection closed. Most likely the page has been closed.
    at assert (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/util/assert.js:18:15)
    at CdpPage.close (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/Page.js:814:36)
    at async runAllCommands (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:433:13)
    at async innerRunTestCode (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:696:21)
    at async innerRunTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:633:17)
    at async runTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:824:27)



<= doc-ui tests done: 142 succeeded, 1 failed, 0 filtered out

bors · 2025-10-17T15:41:26Z

💔 Test failed - checks-actions

cjgillot · 2025-10-17T16:35:39Z

@bors retry

bors · 2025-10-18T09:29:54Z

⌛ Testing commit 7af5775 with merge c8a31b7...

bors · 2025-10-18T12:36:38Z

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing c8a31b7 to master...

github-actions · 2025-10-18T12:39:16Z

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 ab1d244 (parent) -> c8a31b7 (this PR)

Test differences

Show 7 test diffs

Stage 1

[mir-opt] tests/mir-opt/const_prop/trivial_const.rs: [missing] -> pass (J0)
[mir-opt] tests/mir-opt/pre-codegen/two_unwrap_unchecked.rs: [missing] -> pass (J0)

Stage 2

[mir-opt] tests/mir-opt/const_prop/trivial_const.rs: [missing] -> pass (J1)
[mir-opt] tests/mir-opt/pre-codegen/two_unwrap_unchecked.rs: [missing] -> pass (J1)

Additionally, 3 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 c8a31b780d5415358566a20b94912620a3f27067 --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-apple: 5920.4s -> 6655.3s (12.4%)
pr-check-1: 1449.9s -> 1618.9s (11.7%)
aarch64-gnu-llvm-20-2: 2463.8s -> 2180.8s (-11.5%)
x86_64-gnu-aux: 6969.0s -> 6195.5s (-11.1%)
dist-sparcv9-solaris: 5502.5s -> 5003.9s (-9.1%)
x86_64-gnu: 6972.0s -> 6432.8s (-7.7%)
x86_64-gnu-tools: 3501.4s -> 3761.3s (7.4%)
i686-msvc-1: 10000.1s -> 9274.8s (-7.3%)
dist-aarch64-linux: 6157.1s -> 6591.2s (7.1%)
x86_64-gnu-miri: 4413.2s -> 4686.3s (6.2%)

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 · 2025-10-18T13:46:51Z

Finished benchmarking commit (c8a31b7): comparison URL.

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

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

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

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%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	0.6%	[0.2%, 1.1%]	4
Improvements ✅ (primary)	-0.5%	[-0.9%, -0.2%]	13
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	8
All ❌✅ (primary)	-0.4%	[-0.9%, 0.5%]	14

Max RSS (memory usage)

Results (primary -1.0%, secondary -1.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.0%, 2.4%]	2
Regressions ❌ (secondary)	2.7%	[2.1%, 3.6%]	6
Improvements ✅ (primary)	-7.3%	[-7.3%, -7.3%]	1
Improvements ✅ (secondary)	-4.3%	[-5.5%, -3.2%]	11
All ❌✅ (primary)	-1.0%	[-7.3%, 2.4%]	3

Cycles

Results (secondary 1.3%)

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)	5.0%	[1.6%, 9.8%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.0%	[-8.8%, -2.4%]	5
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.1%, 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%, 0.3%]	11
Regressions ❌ (secondary)	0.0%	[0.0%, 0.1%]	4
Improvements ✅ (primary)	-0.2%	[-0.8%, -0.0%]	48
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-0.1%	[-0.8%, 0.3%]	59

Bootstrap: 473.727s -> 473.714s (-0.00%)
Artifact size: 390.45 MiB -> 390.54 MiB (0.02%)

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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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

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bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Oct 17, 2025

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bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Oct 17, 2025

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 Oct 17, 2025

bors mentioned this pull request Oct 18, 2025

Warn on unused_attributes in uitests #147813

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bors added the merged-by-bors This PR was explicitly merged by bors. label Oct 18, 2025

bors merged commit c8a31b7 into rust-lang:master Oct 18, 2025
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rustbot added this to the 1.92.0 milestone Oct 18, 2025

dianqk deleted the simplify-const-condition branch October 18, 2025 13:01

bors mentioned this pull request Oct 18, 2025

Extend GVN to perform local value numbering. #143333

Draft

rustbot added the perf-regression Performance regression. label Oct 18, 2025

Simplify trivial constants in SimplifyConstCondition #147654

Simplify trivial constants in SimplifyConstCondition #147654

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Overall result: ✅ improvements - no action needed

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Overall result: ✅ improvements - no action needed

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Test differences

Stage 1

Stage 2

Job duration changes

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

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dianqk commented Oct 14, 2025 •

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