coverage: Enlarge empty spans during MIR instrumentation, not codegen #144298

Zalathar · 2025-07-22T10:26:31Z

This re-lands the part of #140847 that was (hopefully) not responsible for the coverage-instrumentation regressions that caused that PR to be reverted.

Enlarging empty spans was historically performed during MIR instrumentation, but had to be moved to codegen as part of larger changes in #134497, leading to the status quo. But now there should be no reason not to move that step back to its more logical home in instrumentaion.

This allows us to assume that coverage spans will only be discarded during codegen in very unusual situations.

rustbot · 2025-07-22T10:26:35Z

r? @compiler-errors

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rustbot · 2025-07-22T10:26:38Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

oli-obk · 2025-07-22T13:45:04Z

@bors try @rust-timer queue

rust-bors · 2025-07-22T13:45:08Z

⌛ Trying commit f877aa7 with merge 67f76fa…

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

coverage: Enlarge empty spans during MIR instrumentation, not codegen

rust-bors · 2025-07-22T16:00:58Z

☀️ Try build successful (CI)
Build commit: 67f76fa (67f76fa0a9c98ec4db372ba9faa18b2946e252ad, parent: c0b282f0ccdab7523cdb8dfa41b23bed5573da76)

rust-timer · 2025-07-22T17:44:54Z

Finished benchmarking commit (67f76fa): 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

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Results (primary 2.2%, secondary 3.6%)

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	mean	range	count
Regressions ❌ (primary)	2.2%	[2.0%, 2.4%]	2
Regressions ❌ (secondary)	3.6%	[3.6%, 3.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[2.0%, 2.4%]	2

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Bootstrap: 463.984s -> 466.318s (0.50%)
Artifact size: 374.67 MiB -> 374.59 MiB (-0.02%)

wesleywiser · 2025-07-22T18:27:17Z

@bors r+ rollup

bors · 2025-07-22T18:27:20Z

📌 Commit f877aa7 has been approved by wesleywiser

It is now in the queue for this repository.

coverage: Enlarge empty spans during MIR instrumentation, not codegen This re-lands the part of rust-lang#140847 that was (hopefully) not responsible for the coverage-instrumentation regressions that caused that PR to be reverted. --- Enlarging empty spans was historically performed during MIR instrumentation, but had to be moved to codegen as part of larger changes in rust-lang#134497, leading to the status quo. But now there should be no reason not to move that step back to its more logical home in instrumentaion.

Rollup of 10 pull requests Successful merges: - #144173 (Remove tidy checks for `tests/ui/issues/`) - #144234 (Fix broken TLS destructors on 32-bit win7) - #144239 (Clean `rustc/parse/src/lexer` to improve maintainability) - #144247 (coretests/num: use ldexp instead of hard-coding a power of 2) - #144256 (Don't ICE on non-TypeId metadata within TypeId) - #144290 (update tests/ui/SUMMARY.md) - #144292 (mbe: Use concrete type for `get_unused_rule`) - #144298 (coverage: Enlarge empty spans during MIR instrumentation, not codegen) - #144311 (Add powerpc64le-unknown-linux-musl to CI rustc targets) - #144315 (bootstrap: add package.json and package-lock.json to dist tarball) r? `@ghost` `@rustbot` modify labels: rollup

coverage: Enlarge empty spans during MIR instrumentation, not codegen This re-lands the part of rust-lang#140847 that was (hopefully) not responsible for the coverage-instrumentation regressions that caused that PR to be reverted. --- Enlarging empty spans was historically performed during MIR instrumentation, but had to be moved to codegen as part of larger changes in rust-lang#134497, leading to the status quo. But now there should be no reason not to move that step back to its more logical home in instrumentaion.

Rollup of 10 pull requests Successful merges: - #144173 (Remove tidy checks for `tests/ui/issues/`) - #144234 (Fix broken TLS destructors on 32-bit win7) - #144239 (Clean `rustc/parse/src/lexer` to improve maintainability) - #144247 (coretests/num: use ldexp instead of hard-coding a power of 2) - #144256 (Don't ICE on non-TypeId metadata within TypeId) - #144290 (update tests/ui/SUMMARY.md) - #144292 (mbe: Use concrete type for `get_unused_rule`) - #144298 (coverage: Enlarge empty spans during MIR instrumentation, not codegen) - #144311 (Add powerpc64le-unknown-linux-musl to CI rustc targets) - #144315 (bootstrap: add package.json and package-lock.json to dist tarball) r? `@ghost` `@rustbot` modify labels: rollup try-job: x86_64-gnu-aux

Rollup of 9 pull requests Successful merges: - #144173 (Remove tidy checks for `tests/ui/issues/`) - #144234 (Fix broken TLS destructors on 32-bit win7) - #144239 (Clean `rustc/parse/src/lexer` to improve maintainability) - #144256 (Don't ICE on non-TypeId metadata within TypeId) - #144290 (update tests/ui/SUMMARY.md) - #144292 (mbe: Use concrete type for `get_unused_rule`) - #144298 (coverage: Enlarge empty spans during MIR instrumentation, not codegen) - #144311 (Add powerpc64le-unknown-linux-musl to CI rustc targets) - #144315 (bootstrap: add package.json and package-lock.json to dist tarball) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144298 - Zalathar:empty-span, r=wesleywiser coverage: Enlarge empty spans during MIR instrumentation, not codegen This re-lands the part of #140847 that was (hopefully) not responsible for the coverage-instrumentation regressions that caused that PR to be reverted. --- Enlarging empty spans was historically performed during MIR instrumentation, but had to be moved to codegen as part of larger changes in #134497, leading to the status quo. But now there should be no reason not to move that step back to its more logical home in instrumentaion.

folkertdev · 2025-07-26T00:16:32Z

compiler/rustc_mir_transform/src/coverage/spans.rs

+    // For all empty spans, either enlarge them to be non-empty, or discard them.
+    let source_map = tcx.sess.source_map();
+    covspans.retain_mut(|covspan| {
+        let Some(span) = ensure_non_empty_span(source_map, covspan.span) else { return false };


After some local testing, it is the return false here that causes the issues of

malformed instrumentation profile data: function name is empty

So what spans are kept/discarded is changed by this PR. Perhaps due to some of the modifications to the spans that happen right above here?

The spans that are modified or discarded would have met the same fate later anyway, during codegen. This PR just makes that happen earlier.

So there's something mysterious going on, where discarding these spans early somehow causes us to emit malformed or incomplete instrprof metadata.

…thar Revert "coverage: Enlarge empty spans during MIR instrumentation, not codegen" Surprisingly, rust-lang#144298 alone (extracted from rust-lang#140847) was enough to re-trigger the failures observed in rust-lang#141577 (comment). --- This reverts commit f877aa7. --- r? ghost

Rollup merge of #144480 - Zalathar:revert-empty-span, r=Zalathar Revert "coverage: Enlarge empty spans during MIR instrumentation, not codegen" Surprisingly, #144298 alone (extracted from #140847) was enough to re-trigger the failures observed in #141577 (comment). --- This reverts commit f877aa7. --- r? ghost

coverage: Infer `instances_used` from `pgo_func_name_var_map` In obscure circumstances involving macro-expanded spans, we would sometimes emit a covfun record for a function with no physical coverage counters, and therefore no corresponding entry in the “PGO names” section of the binary. The absence of that name entry causes `llvm-cov` to fail with the cryptic error message: ```text malformed instrumentation profile data: function name is empty ``` We can eliminate this mismatch by removing `instances_used` entirely, and instead inferring its contents from the keys of `pgo_func_name_var_map`. This makes it impossible for a "used" function to lack a PGO name entry. --- This is an attempt to eliminate the cause of rust-lang#141577 when re-landing changes like rust-lang#144298 in the future. I haven't been able to reproduce the underlying issue in an in-tree test, because the only known repro involves a non-trivial derive proc-macro that relies on `syn` and `proc-macro2`. But I have manually verified in a separate branch that this change would have prevented the reoccurrence of rust-lang#141577 (comment).

Rollup merge of #144530 - Zalathar:instances-used, r=lqd coverage: Infer `instances_used` from `pgo_func_name_var_map` In obscure circumstances involving macro-expanded spans, we would sometimes emit a covfun record for a function with no physical coverage counters, and therefore no corresponding entry in the “PGO names” section of the binary. The absence of that name entry causes `llvm-cov` to fail with the cryptic error message: ```text malformed instrumentation profile data: function name is empty ``` We can eliminate this mismatch by removing `instances_used` entirely, and instead inferring its contents from the keys of `pgo_func_name_var_map`. This makes it impossible for a "used" function to lack a PGO name entry. --- This is an attempt to eliminate the cause of #141577 when re-landing changes like #144298 in the future. I haven't been able to reproduce the underlying issue in an in-tree test, because the only known repro involves a non-trivial derive proc-macro that relies on `syn` and `proc-macro2`. But I have manually verified in a separate branch that this change would have prevented the reoccurrence of #141577 (comment).

coverage: Enlarge empty spans during MIR instrumentation, not codegen

f877aa7

This allows us to assume that coverage spans will only be discarded during codegen in very unusual situations.

Zalathar added the A-code-coverage Area: Source-based code coverage (-Cinstrument-coverage) label Jul 22, 2025

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Auto merge of #144298 - Zalathar:empty-span, r=<try>

67f76fa

coverage: Enlarge empty spans during MIR instrumentation, not codegen

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 22, 2025

wesleywiser approved these changes Jul 22, 2025

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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 22, 2025

jhpratt mentioned this pull request Jul 23, 2025

Rollup of 12 pull requests #144343

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jhpratt mentioned this pull request Jul 23, 2025

Rollup of 12 pull requests #144345

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matthiaskrgr mentioned this pull request Jul 23, 2025

Rollup of 11 pull requests #144346

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matthiaskrgr mentioned this pull request Jul 23, 2025

Rollup of 10 pull requests #144348

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matthiaskrgr mentioned this pull request Jul 23, 2025

Rollup of 10 pull requests #144349

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GuillaumeGomez mentioned this pull request Jul 23, 2025

Rollup of 12 pull requests #144355

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matthiaskrgr mentioned this pull request Jul 23, 2025

Rollup of 9 pull requests #144360

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bors merged commit 7ec557c into rust-lang:master Jul 23, 2025
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rustbot added this to the 1.90.0 milestone Jul 23, 2025

Zalathar deleted the empty-span branch July 23, 2025 22:45

Zalathar mentioned this pull request Jul 25, 2025

Instrumentation profile data contains empty function name (?) as of nightly-2025-05-19 #141577

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folkertdev reviewed Jul 26, 2025

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Zalathar mentioned this pull request Jul 26, 2025

Revert "coverage: Enlarge empty spans during MIR instrumentation, not codegen" #144480

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Zalathar mentioned this pull request Jul 27, 2025

coverage: Infer instances_used from pgo_func_name_var_map #144530

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Zalathar mentioned this pull request Jul 28, 2025

coverage: Run-make regression test for #[derive(arbitrary::Arbitrary)] #144571

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