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AK: Prefer __builtin_{subc,addc} intrinsics to x86-specific ones

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GCC 14 (https://gcc.gnu.org/g:2b4e0415ad664cdb3ce87d1f7eee5ca26911a05b)
has added support for the previously Clang-specific add/subtract with
borrow builtins. Let's use `__has_builtin` to detect them instead of
assuming that only Clang has them. We should prefer these to the
x86-specific ones as architecture-independent middle-end optimizations
might deal with them better.

As an added bonus, this significantly improves codegen on AArch64
compared to the fallback implementation that uses
`__builtin_{add,sub}_overflow`.

For now, the code path with the x86-specific intrinsics stays so as to
avoid regressing the performance of builds with GCC 12 and 13.
This commit is contained in:
Daniel Bertalan 2023-06-23 08:47:19 +00:00 committed by Andreas Kling
parent e7b7d1b347
commit 7dadc9ff33
Notes: sideshowbarker 2024-07-16 23:08:48 +09:00 
Author: https://github.com/BertalanD
Commit: https://github.com/SerenityOS/serenity/commit/7dadc9ff33
Pull-request: https://github.com/SerenityOS/serenity/pull/19578
Reviewed-by: https://github.com/DanShaders ✅
1 changed files with 11 additions and 24 deletions
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35
AK/BigIntBase.h
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@ -159,20 +159,11 @@ ALWAYS_INLINE constexpr NativeWord extend_sign(bool sign)
return sign ? max_word : 0;
}
// FIXME: 1) If available, we might try to use AVX2 and AVX512.
// 2) We should use something similar to __builtin_ia32_addcarryx_u64 for GCC on !x86_64 or
// get https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173 fixed to use
// architecture-agnostic __builtin_addc function family unconditionally. The same applies
// to `sub_words`.
// FIXME: If available, we might try to use AVX2 and AVX512.
ALWAYS_INLINE constexpr NativeWord add_words(NativeWord word1, NativeWord word2, bool& carry)
{
if (!is_constant_evaluated()) {
#if ARCH(X86_64)
unsigned long long output;
carry = __builtin_ia32_addcarryx_u64(carry, word1, word2, &output);
return static_cast<NativeWord>(output);
#else
# if defined(AK_COMPILER_CLANG)
#if __has_builtin(__builtin_addc)
NativeWord ncarry, output;
if constexpr (SameAs<NativeWord, unsigned int>)
output = __builtin_addc(word1, word2, carry, reinterpret_cast<unsigned int*>(&ncarry));
@ -184,7 +175,10 @@ ALWAYS_INLINE constexpr NativeWord add_words(NativeWord word1, NativeWord word2,
VERIFY_NOT_REACHED();
carry = ncarry;
return output;
# endif
#elif ARCH(X86_64)
unsigned long long output;
carry = __builtin_ia32_addcarryx_u64(carry, word1, word2, &output);
return static_cast<NativeWord>(output);
#endif
}
// Note: This is usually too confusing for both GCC and Clang.
@ -202,17 +196,7 @@ ALWAYS_INLINE constexpr NativeWord add_words(NativeWord word1, NativeWord word2,
ALWAYS_INLINE constexpr NativeWord sub_words(NativeWord word1, NativeWord word2, bool& carry)
{
if (!is_constant_evaluated()) {
#if ARCH(X86_64)
unsigned long long output;
# if defined(AK_COMPILER_CLANG)
// This is named __builtin_ia32_sbb_u64 in G++. *facepalm*
carry = __builtin_ia32_subborrow_u64(carry, word1, word2, &output);
# else
carry = __builtin_ia32_sbb_u64(carry, word1, word2, &output);
# endif
return static_cast<NativeWord>(output);
#else
# if defined(AK_COMPILER_CLANG)
#if __has_builtin(__builtin_subc)
NativeWord ncarry, output;
if constexpr (SameAs<NativeWord, unsigned int>)
output = __builtin_subc(word1, word2, carry, reinterpret_cast<unsigned int*>(&ncarry));
@ -224,7 +208,10 @@ ALWAYS_INLINE constexpr NativeWord sub_words(NativeWord word1, NativeWord word2,
VERIFY_NOT_REACHED();
carry = ncarry;
return output;
# endif
#elif ARCH(X86_64) && defined(AK_COMPILER_GCC)
unsigned long long output;
carry = __builtin_ia32_sbb_u64(carry, word1, word2, &output);
return static_cast<NativeWord>(output);
#endif
}
NativeWord output;