u128::count_ones is broken on aarch64-unknown-none-softfloat

[AaronKutch]

#![no_std]

pub fn u128_count_ones(x: u128) -> u32 {
    x.count_ones()
}

when compiled with cargo build --target aarch64-unknown-none-softfloat it gives:

error: could not compile `testlib`

Caused by:
  process didn't exit successfully: `rustc --crate-name testlib --edition=2018 src\lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C metadata=ec790c6e5e4455bc -C extra-filename=-ec790c6e5e4455bc --out-dir C:\Users\aaron\Documents\github\testlib\target\aarch64-unknown-none-softfloat\debug\deps --target aarch64-unknown-none-softfloat -C incremental=C:\Users\aaron\Documents\github\testlib\target\aarch64-unknown-none-softfloat\debug\incremental -L dependency=C:\Users\aaron\Documents\github\testlib\target\aarch64-unknown-none-softfloat\debug\deps -L dependency=C:\Users\aaron\Documents\github\testlib\target\debug\deps` (exit code: 0xc0000005, STATUS_ACCESS_VIOLATION)

This is preventing upgrading the compiler-builtins used by rustc. See rust-lang/compiler-builtins#398 and #79863.

[LeSeulArtichaut]

From the backtrace in rust-lang/compiler-builtins#398 (comment), this appears to be a LLVM issue?

[nikic]

IR test case (still reproduces on LLVM master):

; RUN: llc -mtriple=aarch64 -mattr=-neon < %s
define i128 @test(i128 %i) {
  %c = call i128 @llvm.ctpop(i128 %i)
  ret i128 %c
}

declare i128 @llvm.ctpop(i128)

[nikic]

The problem is that https://github.com/llvm/llvm-project/blob/b21840751278128ef6942074ae89ea63c9c6ac58/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp#L15907-L15909 does not check whether the LowerCTPOP() call is successful and pushes an empty SDValue as the result.

[AaronKutch]

I was able to remove the reference to u128::count_ones in compiler-builtins, so this isn't a severe issue. But, it should definitely be fixed quickly so that future versions of LLVM are fixed.

[nikic]

I've put up https://reviews.llvm.org/D93825.

[nikic]

This should be fixed on nightly, as a result of the LLVM 12 upgrade.

[est31]

Can this be closed? Should one revert rust-lang/compiler-builtins#399 to make sure the bug is fixed?

[AaronKutch]

We don't have to revert anything, count_ones was actually unused in compiler-builtins. It didn't produce an unused warning because it was part of a trait

[nikic]

Closing this as the upstream issue is fixed and we have long since updated to LLVM 12.