[compiler-rt] Stop using x86 builtin on AArch64 with GCC

[XrXr]

Previously, building multc3.c on A64 with GCC 7 or up but 9 and lower will attempt to reference __builtin_copysignq, an x86-specific intrinsic:

$ gcc -c multc3.c
In file included from fp_lib.h:24,
                 from multc3.c:14:
multc3.c: In function '__multc3':
int_math.h:71:32: warning: implicit declaration of function '__builtin_copysignq'; did you mean '__builtin_copysign'? [-Wimplicit-function-declaration]
 #define crt_copysignf128(x, y) __builtin_copysignq((x), (y))
                                ^~~~~~~~~~~~~~~~~~~

This is because __has_builtin is from GCC 10, and defined to 0 at the top of int_math.h for affected GCC versions, so the fallback definition is used. But __builtin_copysignq is unavailable on A64.

Use version detection to find __builtin_copysignf128 instead. It's available since GCC 7 and available on both x86 and A64, given this macro is only used when CRT_HAS_IEEE_TF.

---

I realize this is fixing a problem for an out-of-tree build configuration, but help would be greatly appreciated. Rust builds multc3.c with GCC 8 and this mis-selection is causing build issues way downstream.

ref: d2ce3e9

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[XrXr]

Ping

[epilys]

Friendly ping

[statham-arm]

This certainly seems like a thing that should be fixed. But can you explain why the right decision is not to make both conditions just #if __has_builtin(__builtin_copysignXXX)? Why is it ever sensible to add the extra clause that overrides __has_builtin returning false, based on the compiler version?

If the answer is (for example) that a particular version of gcc or clang lies about not having a builtin, then should there be a comment explaining that?

[statham-arm]

Ah, someone offline has pointed out to me that gcc only gained __has_builtin in gcc 10, so before that, you have to check based on the gcc version.

Elsewhere in the file there's a very similar check for __builtin_fabs128 and __builtin_fabsq. Do those have the same availability in older gcc versions? Should that check be fixed in the same way as this one?

[XrXr]

Yes, there are other similar cases. Maybe instead of sprinkling the conditions backfilling for __has_builtin ad-hoc at each usage site, we should centralize the backfills with something like:

#ifndef __has_builtin
// Simulate __has_builtin using version detection for older compilers
#define __has_builtin(x) COMPILER_RT_HAS_BUILTIN_ ##x

#if defined(__GNUC__) && __GNUC__ >= 7
#define COMPILER_RT_HAS_BUILTIN___builtin_copysignf128 1
#else
#define COMPILER_RT_HAS_BUILTIN___builtin_copysignf128 0
#endif

#endif

(Compiler Explorer link)

This way usages can simply check __has_builtin(x) and look nicer.

The downside of this approach is that all the checked attributes need to be backfilled in the simulated definition.

Alternatively, I can backfill and comment at each site as suggested. What do you think? @statham-arm

[statham-arm]

I enjoy a good piece of C macro trickery 🙂 but in this case I agree that it doesn't really save anything, because you've got to make sure all the affected builtins are listed at the top of the file.

Also, I don't want to make you go and fix every case of this in the whole of LLVM just to get the one immediate problem fixed!

I think it's a reasonable stopping point to just fix the two similar cases in this file in the same way. Otherwise someone seeing them right next to each other might think they were deliberately opposite ways round, and wonder why.

[XrXr]

Thanks! How does f0b52f1 look?

[statham-arm]

LGTM, thanks!

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