compiler-builtins CI fails on powerpc64

[Amanieu]

Some memset tests fail on both powerpc64-unknown-linux-gnu and powerpc64le-unknown-linux-gnu, but only with --release. Bisecting show #91743 as the culprit, but I think it just exposes an existing LLVM bug rather than being the root cause.

powerpc64le

---- memset_backward_aligned stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 28, 29, 30, 31]`', testcrate/tests/mem.rs:265:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- memset_backward_misaligned_aligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 0, 0, 0, 0, 204, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:253:9

---- memset_backward_misaligned_nonaligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 204, 204, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:241:9

powerpc64

---- memset_backward_aligned stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 204, 204, 204, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 28, 29, 30, 31]`', testcrate/tests/mem.rs:265:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- memset_backward_misaligned_aligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 204, 204, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 6, 7, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:253:9

---- memset_backward_misaligned_nonaligned_start stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 1, 2, 3, 4, 5, 204, 204, 0, 0, 0, 0, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`,
 right: `[0, 1, 2, 3, 4, 5, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 204, 23, 24, 25, 26, 27, 28, 29, 30, 31]`', testcrate/tests/mem.rs:241:9

[Amanieu]

#88520 might be related: we previously had some test failures due to the LLVM 13 upgrade, which we never got around to fixing (we just disable those tests on PPC64).

[apiraino]

removing I-prioritize since this topic will be handled in a T-compiler meeting

@rustbot label -I-prioritize

[RalfJung]

removing I-prioritize since this topic will be handled in a T-compiler meeting

That was 2 years ago; did anything come out of that meeting?

Is the bug even still present?

[apiraino]

@RalfJung do you have a specific question about this issue or was your question just a "pulse check"?

I did some serious backtracking. The issue was discussed 3 years ago (on Zulip) but the meeting mentioned above was only recently scheduled (in meeting proposal) and it's about disabling tests for components we don't control (such as LLVM), not specifically about this issue.

I can't say if this bug is still present. @Amanieu do you know more?