Remove __multc3

[XrXr]

This C file doesn't seem to be compiled correctly, as it creates a reference to the x86 specific __builtin_copysignq on ARM. This intrinsic is unused and unavailable on Windows because it's for complex numbers Rust doesn't support, so it should be fine to remove.

The inclusion of the file seems to be the cause of some downstream link errors: rust-lang/rust#125619

[Amanieu]

This is actually a bug in upstream LLVM that should be fixed there:

#if __has_builtin(__builtin_copysignf128)
#define crt_copysignf128(x, y) __builtin_copysignf128((x), (y))
#elif __has_builtin(__builtin_copysignq) || (defined(__GNUC__) && __GNUC__ >= 7)
#define crt_copysignf128(x, y) __builtin_copysignq((x), (y))
#endif

The condition is incorrect: on GCC 7+, we should be using either __builtin_copysignq or __builtin_copysignf128 depending on the target. From the glibc source I find that x86, ia64 and powerpc use __builtin_copysignq while all other targets use __builtin_copysignf128.

GCC 6 and before simply don't properly support f128.

[XrXr]

Thanks, I was hoping we could fix this just by removing it. I'm surprised that this one slipped through the cracks since any attempt to actually use __multc3 on aarach64 should trigger the link error. I guess upstream LLVM doesn't test the !__has_builtin(__builtin_copysignf128) case, and rust releases build this crate without __builtin_copysignf128 (maybe because of cross compilation?).

But if __builtin_copysignf128 is unavailable, we can't use it in the upstream implementation. 🤔

[Amanieu]

The issue only happens when compiler-rt is built with GCC, which happens in our build.rs. This is why it was not caught earlier: compiler-rt is normally only built as part of Clang toolchains and clang will always return true for __has_builtin(__builtin_copysignf128).

The best way to fix this would be to send a PR to llvm/llvm-project, and then backport the fix to rust's LLVM fork (rust-lang/llvm-project).

[XrXr]

I'm having trouble recreating the bad object with GCC to make a patch for compiler-rt. I have GCC 13.2, and __has_builtin(__builtin_copysignf128) has been available since GCC 7.1 (1 2). When I make a crate that depends on compiler-builtins with the c feature, it links and runs fine. I'm able to single step into my local copy of __multc3 and everything.
I suspect rust binary releases are built with an old GCC. If they are, a patch for compiler-rt might need to include an implementation of copysignf128 without using the builtin. But also, using a newer GCC might solve this issue without any code change.
I'll poke around to figure out the build environment of binary releases.

[XrXr]

Closing, as I was able to piece enough together to send llvm/llvm-project#93890. Hopefully I get a response. Thanks for the guidance!