Don't re-assume in transmutes that don't change niches

[scottmcm]

I noticed in nightly 2025-02-21 that transmute is emitting way more assumes than necessary for newtypes.

For example, the three transmutes in https://rust.godbolt.org/z/fW1KaTc4o emits

define noundef range(i32 1, 0) i32 @repeatedly_transparent_transmute(i32 noundef range(i32 1, 0) %_1) unnamed_addr {
start:
  %0 = sub i32 %_1, 1
  %1 = icmp ule i32 %0, -2
  call void @llvm.assume(i1 %1)
  %2 = sub i32 %_1, 1
  %3 = icmp ule i32 %2, -2
  call void @llvm.assume(i1 %3)
  %4 = sub i32 %_1, 1
  %5 = icmp ule i32 %4, -2
  call void @llvm.assume(i1 %5)
  %6 = sub i32 %_1, 1
  %7 = icmp ule i32 %6, -2
  call void @llvm.assume(i1 %7)
  %8 = sub i32 %_1, 1
  %9 = icmp ule i32 %8, -2
  call void @llvm.assume(i1 %9)
  %10 = sub i32 %_1, 1
  %11 = icmp ule i32 %10, -2
  call void @llvm.assume(i1 %11)
  ret i32 %_1
}

But those are all just newtypes that don't change size or niches, so none of it's needed.

After this PR it's down to just

define noundef range(i32 1, 0) i32 @repeatedly_transparent_transmute(i32 noundef range(i32 1, 0) %_1) unnamed_addr {
start:
  ret i32 %_1
}

because none of those assumes in the original actually did anything.

(Transmuting to something with a difference niche, though, still has the assumes -- the other tests continue to pass checking that.)

[rustbot]

r? @lcnr

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[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 23c6b93 with merge 8a03975...

[lcnr]

r? compiler

[bors]

☀️ Try build successful - checks-actions
Build commit: 8a03975 (8a03975caf81e62affabd75ed6e5ad991b8a45b3)

[rust-timer]

Finished benchmarking commit (8a03975): 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 the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-0.7%	[-1.3%, -0.4%]	16
All ❌✅ (primary)	-0.1%	[-0.1%, -0.1%]	1

Max RSS (memory usage)

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

Cycles

Results (secondary 2.5%)

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)	2.5%	[2.2%, 2.9%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 770.178s -> 768.923s (-0.16%)
Artifact size: 359.64 MiB -> 359.62 MiB (-0.00%)

[BoxyUwU]

r? @saethlin

[saethlin]

Love to see us emitting less assumes but not regressing codegen.

@bors r+

[bors]

📌 Commit 23c6b93 has been approved by saethlin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 23c6b93 with merge f6733d7...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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

[scottmcm]

The hosted runner encountered an error while running your job. (Error Type: Disconnect).

@bors retry

[bors]

⌛ Testing commit 23c6b93 with merge 446649d...

[bors]

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing 446649d to master...

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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

gh pr comment ${HEAD_PR} -F output.log
shell: /usr/bin/bash -e {0}
##[endgroup]
fatal: ambiguous argument 'HEAD^1': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
##[error]Process completed with exit code 128.

[rust-timer]

Finished benchmarking commit (446649d): 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)

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

Cycles

Results (secondary -2.0%)

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)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 765.603s -> 764.414s (-0.16%)
Artifact size: 361.95 MiB -> 362.00 MiB (0.01%)