Stop llvm.expecting assert terminators

[scottmcm]

We're putting llvm.expect calls before the https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.TerminatorKind.html#variant.Assert terminators.

But we don't need them. One of the arms is always to a panic function that's marked #[cold], which is cold https://llvm.org/docs/LangRef.html#function-attributes in LLVM, which

When computing edge weights, basic blocks post-dominated by a cold function call are also considered to be cold; and, thus, given low weight.

So even without us emitting the extra intrinsic call, LLVM knows what to expect for the br. Thus we can save the (small) effort of emitting it and then LLVM optimizing it out.

r? compiler

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit c04b955 with merge b1efbc0...

[bors]

☀️ Try build successful - checks-actions
Build commit: b1efbc0 (b1efbc052906efbb73aab7a10cf61b07ec67a80b)

[rust-timer]

Finished benchmarking commit (b1efbc0): comparison URL.

Overall result: ✅ improvements - 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 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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.1%	[-1.1%, -1.1%]	1
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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.1%, -0.1%]	1

Cycles

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

Binary size

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)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	30
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	3
All ❌✅ (primary)	-0.0%	[-0.1%, 0.0%]	34

Bootstrap: 671.059s -> 675.174s (0.61%)
Artifact size: 315.99 MiB -> 315.88 MiB (-0.04%)

[scottmcm]

Interesting, dropping the expect gave a "1611442.25x Significance Factor" instruction improvement on the svg-render-1 runtime benchmark. I wish we had a graph to know if that's real...

[Kobzol]

I wish we had a graph to know if that's real...

Yeah, me too :( Didn't have a lot of time recently to work on the rustc-perf UI, sadly. However, we now have a GSoC 2024 project to improve the rustc-perf frontend, so hopefully this will improve in the future :)

[cjgillot]

@bors r+

[bors]

📌 Commit c04b955 has been approved by cjgillot

It is now in the queue for this repository.

[saethlin]

The binary size improvements here looks interesting, except that the artifact sizes reports a smaller libLLVM.so, which doesn't make much sense to me. Strange...

[Kobzol]

That's just a small fluctuation that happens on pretty much every commit, even no-op ones. Possibly caused by PGO/BOLT.

[scottmcm]

@saethlin Probably PGO in LLVM -- this makes a pretty big difference to how hot the LLVM code is for this, since it used to be called for every arithmetic operation in debug, for example, and now isn't. So it'll definitely churn the LLVM build a bit.

[bors]

⌛ Testing commit c04b955 with merge 02f7806...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 02f7806 to master...

[rust-timer]

Finished benchmarking commit (02f7806): comparison URL.

Overall result: ✅ improvements - 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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.6%	[-3.6%, -3.6%]	1

Cycles

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

Binary size

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)	0.1%	[0.0%, 0.3%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	27
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.1%, 0.3%]	31

Bootstrap: 675.389s -> 676.151s (0.11%)
Artifact size: 315.94 MiB -> 315.84 MiB (-0.03%)