coverage: Emit mappings for unused functions without generating stubs

[Zalathar]

For a while I've been annoyed by the fact that generating coverage maps for unused functions involves generating a stub function at the LLVM level.

As I suspected, generating that stub function isn't actually necessary, as long as we specifically tell LLVM about the symbol names of all the functions that have coverage mappings but weren't codegenned (due to being unused).

---

There is some helper code that gets moved around in the follow-up patches, so look at the first patch to see the most important functional changes.

---

@rustbot label +A-code-coverage

[rustbot]

r? @davidtwco

(rustbot has picked a reviewer for you, use r? to override)

[Zalathar]

(I'm happy to note that this has been made much easier by being able to use #116046 as a base.)

[cjgillot]

The third commit may have a perf effect.
@bors try @rust-timer queue

[bors]

⌛ Trying commit d29e54a with merge 16e6908...

[bors]

☀️ Try build successful - checks-actions
Build commit: 16e6908 (16e690814eddd2ffafbf2ef883d9caf99da40ff0)

[rust-timer]

Finished benchmarking commit (16e6908): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to 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)

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

Cycles

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

Binary size

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

Bootstrap: 630.346s -> 629.734s (-0.10%)
Artifact size: 304.04 MiB -> 304.05 MiB (0.00%)

[Zalathar]

(Rebased, no changes.)

[Zalathar]

I'm sort of on the fence about the query follow-up stuff.

I want to do something, because right now the query really shouldn't be used from non-coverage code, and is confusing from the coverage side.

But the other option would just be to rename it and put something like for_coverage in the name.

[cjgillot]

For the query stuff: there is no visible effect in the perf build, but probably because it doesn't do instrumented builds.
The simplification is nice.
If you change your mind later, I can definitely review another PR.

Which test verifies the handling of unused functions?

[Zalathar]

Which test verifies the handling of unused functions?

The main ones are:

• tests/run-coverage/unused.rs
• tests/run-coverage/uses_crate.rs
• tests/run-coverage/uses_inline_crate.rs
• tests/coverage-map/status-quo/unused.rs

It also gets checked incidentally by some of the other coverage-map and run-coverage tests that happen to contain unused functions.

[cjgillot]

Thanks.
@bors r+

[bors]

📌 Commit 6f1ca8d has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 6f1ca8d with merge 0d16646...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 0d16646 to master...

[rust-timer]

Finished benchmarking commit (0d16646): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	2.4%	[2.1%, 2.7%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.0%	[-1.0%, -1.0%]	1
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

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

Binary size

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

Bootstrap: 632.219s -> 631.606s (-0.10%)
Artifact size: 301.96 MiB -> 301.98 MiB (0.00%)