[PATCH v2] libstdc++: simd: x86: accept 64-bit long double as double [PR124657]
Jonathan Wakely
jwakely@redhat.com
Wed Apr 22 09:26:38 GMT 2026
On Wed, 22 Apr 2026 at 07:20, Alexandre Oliva <oliva@adacore.com> wrote:
>
> On Apr 21, 2026, Matthias Kretz <MatthiasKretz@gmx.net> wrote:
>
> >> This would be ever so slightly more efficient as:
>
> >> because it wouldn't instantiate the is_same_v variable templates
> >> unless required, and wouldn't care about long double for the common
> >> case of 80-bit long double.
> >>
> >> But the patch seems reasonable to me. Matthias?
>
> > Yes. It looks like a clean solution to me. I like Jonathan's suggestion.
> > std::experimental::simd is too expensive to compile already.
>
> Here's a patch that implements the suggested change, and that, along
> with https://gcc.gnu.org/pipermail/gcc-patches/2026-April/712616.html
> (Ping?) enables pr109261_constexpr_simd.cc to succeed with
> -mlong-double-64 on x86_64 -msse. Regstrapped on x86_64-linux-gnu and
> powerpc64le-linux-gnu. Ok for stage1?
>
> Jonathan, should I credit you for the preprocessor micro-optimization?
No need.
> I didn't quite follow it to the letter, because I thought it looked
> cleaner this way, and IIUC it would not bring any notable inefficiency.
Your version always instantiates both variable templates, even if the
first one is true, but that only affects the -mlong-double-64 case
which I don't care about micro-optimizing :-)
OK for stage 1, thanks.
>
>
> Various simd_x86 functions that handle double need to be adjusted to
> match 64-bit long double as well.
>
> Introduce __is_x86_ps<_Tp>() and __is_x86_pd<_Tp>() and use them
> instead of is_same_v<_Tp, float> and is_same_v<_Tp, double>,
> respectively.
>
>
> for libstdc++-v3/ChangeLog
>
> PR libstdc++/124657
> * include/experimental/bits/simd_x86.h
> (__is_x86_ps<_Tp>): New. Replace is_same_v<_Tp, float> with it.
> (__is_x86_pd<_Tp>): New. Replace is_same_v<_Tp, double> with it.
> ---
> libstdc++-v3/include/experimental/bits/simd_x86.h | 82 +++++++++++++--------
> 1 file changed, 50 insertions(+), 32 deletions(-)
>
> diff --git a/libstdc++-v3/include/experimental/bits/simd_x86.h b/libstdc++-v3/include/experimental/bits/simd_x86.h
> index 74c7a61998e0d..5d4ed5ad58c8e 100644
> --- a/libstdc++-v3/include/experimental/bits/simd_x86.h
> +++ b/libstdc++-v3/include/experimental/bits/simd_x86.h
> @@ -415,13 +415,31 @@ template <size_t _Np, typename _Tp, typename _Kp>
> #endif
>
> // ISA & type detection {{{
> +template <typename _Tp>
> + constexpr bool
> + __is_x86_ps()
> + {
> + return is_same_v<_Tp, float>;
> + }
> +
> +template <typename _Tp>
> + constexpr bool
> + __is_x86_pd()
> + {
> +#if __LDBL_MANT_DIG == __DBL_MANT_DIG
> + if constexpr (is_same_v<_Tp, long double>)
> + return true;
> +#endif
> + return is_same_v<_Tp, double>;
> + }
> +
> template <typename _Tp, size_t _Np>
> constexpr bool
> __is_sse_ps()
> {
> return __have_sse
> - && is_same_v<_Tp,
> - float> && sizeof(__intrinsic_type_t<_Tp, _Np>) == 16;
> + && __is_x86_ps<_Tp>()
> + && sizeof(__intrinsic_type_t<_Tp, _Np>) == 16;
> }
>
> template <typename _Tp, size_t _Np>
> @@ -429,8 +447,8 @@ template <typename _Tp, size_t _Np>
> __is_sse_pd()
> {
> return __have_sse2
> - && is_same_v<_Tp,
> - double> && sizeof(__intrinsic_type_t<_Tp, _Np>) == 16;
> + && __is_x86_pd<_Tp>()
> + && sizeof(__intrinsic_type_t<_Tp, _Np>) == 16;
> }
>
> template <typename _Tp, size_t _Np>
> @@ -438,8 +456,8 @@ template <typename _Tp, size_t _Np>
> __is_avx_ps()
> {
> return __have_avx
> - && is_same_v<_Tp,
> - float> && sizeof(__intrinsic_type_t<_Tp, _Np>) == 32;
> + && __is_x86_ps<_Tp>()
> + && sizeof(__intrinsic_type_t<_Tp, _Np>) == 32;
> }
>
> template <typename _Tp, size_t _Np>
> @@ -447,8 +465,8 @@ template <typename _Tp, size_t _Np>
> __is_avx_pd()
> {
> return __have_avx
> - && is_same_v<_Tp,
> - double> && sizeof(__intrinsic_type_t<_Tp, _Np>) == 32;
> + && __is_x86_pd<_Tp>()
> + && sizeof(__intrinsic_type_t<_Tp, _Np>) == 32;
> }
>
> template <typename _Tp, size_t _Np>
> @@ -456,8 +474,8 @@ template <typename _Tp, size_t _Np>
> __is_avx512_ps()
> {
> return __have_avx512f
> - && is_same_v<_Tp,
> - float> && sizeof(__intrinsic_type_t<_Tp, _Np>) == 64;
> + && __is_x86_ps<_Tp>()
> + && sizeof(__intrinsic_type_t<_Tp, _Np>) == 64;
> }
>
> template <typename _Tp, size_t _Np>
> @@ -465,8 +483,8 @@ template <typename _Tp, size_t _Np>
> __is_avx512_pd()
> {
> return __have_avx512f
> - && is_same_v<_Tp,
> - double> && sizeof(__intrinsic_type_t<_Tp, _Np>) == 64;
> + && __is_x86_pd<_Tp>()
> + && sizeof(__intrinsic_type_t<_Tp, _Np>) == 64;
> }
>
> // }}}
> @@ -2397,9 +2415,9 @@ template <typename _Abi, typename>
> [[maybe_unused]] const auto __yi = __to_intrin(__y);
> if constexpr (sizeof(__xi) == 64)
> {
> - if constexpr (is_same_v<_Tp, float>)
> + if constexpr (__is_x86_ps<_Tp> ())
> return _mm512_mask_cmp_ps_mask(__k1, __xi, __yi, _CMP_LT_OS);
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return _mm512_mask_cmp_pd_mask(__k1, __xi, __yi, _CMP_LT_OS);
> else if constexpr (is_signed_v<_Tp> && sizeof(_Tp) == 1)
> return _mm512_mask_cmplt_epi8_mask(__k1, __xi, __yi);
> @@ -2422,9 +2440,9 @@ template <typename _Abi, typename>
> }
> else if constexpr (sizeof(__xi) == 32)
> {
> - if constexpr (is_same_v<_Tp, float>)
> + if constexpr (__is_x86_ps<_Tp> ())
> return _mm256_mask_cmp_ps_mask(__k1, __xi, __yi, _CMP_LT_OS);
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return _mm256_mask_cmp_pd_mask(__k1, __xi, __yi, _CMP_LT_OS);
> else if constexpr (is_signed_v<_Tp> && sizeof(_Tp) == 1)
> return _mm256_mask_cmplt_epi8_mask(__k1, __xi, __yi);
> @@ -2447,9 +2465,9 @@ template <typename _Abi, typename>
> }
> else if constexpr (sizeof(__xi) == 16)
> {
> - if constexpr (is_same_v<_Tp, float>)
> + if constexpr (__is_x86_ps<_Tp> ())
> return _mm_mask_cmp_ps_mask(__k1, __xi, __yi, _CMP_LT_OS);
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return _mm_mask_cmp_pd_mask(__k1, __xi, __yi, _CMP_LT_OS);
> else if constexpr (is_signed_v<_Tp> && sizeof(_Tp) == 1)
> return _mm_mask_cmplt_epi8_mask(__k1, __xi, __yi);
> @@ -2505,9 +2523,9 @@ template <typename _Abi, typename>
> [[maybe_unused]] const auto __yi = __to_intrin(__y);
> if constexpr (sizeof(__xi) == 64)
> {
> - if constexpr (is_same_v<_Tp, float>)
> + if constexpr (__is_x86_ps<_Tp> ())
> return _mm512_mask_cmp_ps_mask(__k1, __xi, __yi, _CMP_LE_OS);
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return _mm512_mask_cmp_pd_mask(__k1, __xi, __yi, _CMP_LE_OS);
> else if constexpr (is_signed_v<_Tp> && sizeof(_Tp) == 1)
> return _mm512_mask_cmple_epi8_mask(__k1, __xi, __yi);
> @@ -2530,9 +2548,9 @@ template <typename _Abi, typename>
> }
> else if constexpr (sizeof(__xi) == 32)
> {
> - if constexpr (is_same_v<_Tp, float>)
> + if constexpr (__is_x86_ps<_Tp> ())
> return _mm256_mask_cmp_ps_mask(__k1, __xi, __yi, _CMP_LE_OS);
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return _mm256_mask_cmp_pd_mask(__k1, __xi, __yi, _CMP_LE_OS);
> else if constexpr (is_signed_v<_Tp> && sizeof(_Tp) == 1)
> return _mm256_mask_cmple_epi8_mask(__k1, __xi, __yi);
> @@ -2555,9 +2573,9 @@ template <typename _Abi, typename>
> }
> else if constexpr (sizeof(__xi) == 16)
> {
> - if constexpr (is_same_v<_Tp, float>)
> + if constexpr (__is_x86_ps<_Tp> ())
> return _mm_mask_cmp_ps_mask(__k1, __xi, __yi, _CMP_LE_OS);
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return _mm_mask_cmp_pd_mask(__k1, __xi, __yi, _CMP_LE_OS);
> else if constexpr (is_signed_v<_Tp> && sizeof(_Tp) == 1)
> return _mm_mask_cmple_epi8_mask(__k1, __xi, __yi);
> @@ -5021,10 +5039,10 @@ template <typename _Abi, typename>
> = _Abi::template _S_implicit_mask_intrin<_Tp>();
> return 0 != __testc(__a, __b);
> }
> - else if constexpr (is_same_v<_Tp, float>)
> + else if constexpr (__is_x86_ps<_Tp> ())
> return (_mm_movemask_ps(__a) & ((1 << _Np) - 1))
> == (1 << _Np) - 1;
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return (_mm_movemask_pd(__a) & ((1 << _Np) - 1))
> == (1 << _Np) - 1;
> else
> @@ -5084,9 +5102,9 @@ template <typename _Abi, typename>
> else
> return 0 == __testz(__a, __a);
> }
> - else if constexpr (is_same_v<_Tp, float>)
> + else if constexpr (__is_x86_ps<_Tp> ())
> return (_mm_movemask_ps(__a) & ((1 << _Np) - 1)) != 0;
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return (_mm_movemask_pd(__a) & ((1 << _Np) - 1)) != 0;
> else
> return (_mm_movemask_epi8(__a) & ((1 << (_Np * sizeof(_Tp))) - 1))
> @@ -5120,9 +5138,9 @@ template <typename _Abi, typename>
> else
> return 0 != __testz(__a, __a);
> }
> - else if constexpr (is_same_v<_Tp, float>)
> + else if constexpr (__is_x86_ps<_Tp> ())
> return (__movemask(__a) & ((1 << _Np) - 1)) == 0;
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> return (__movemask(__a) & ((1 << _Np) - 1)) == 0;
> else
> return (__movemask(__a) & int((1ull << (_Np * sizeof(_Tp))) - 1))
> @@ -5150,13 +5168,13 @@ template <typename _Abi, typename>
> = _Abi::template _S_implicit_mask_intrin<_Tp>();
> return 0 != __testnzc(__a, __b);
> }
> - else if constexpr (is_same_v<_Tp, float>)
> + else if constexpr (__is_x86_ps<_Tp> ())
> {
> constexpr int __allbits = (1 << _Np) - 1;
> const auto __tmp = _mm_movemask_ps(__a) & __allbits;
> return __tmp > 0 && __tmp < __allbits;
> }
> - else if constexpr (is_same_v<_Tp, double>)
> + else if constexpr (__is_x86_pd<_Tp> ())
> {
> constexpr int __allbits = (1 << _Np) - 1;
> const auto __tmp = _mm_movemask_pd(__a) & __allbits;
>
> --
> Alexandre Oliva, happy hacker https://blog.lx.oliva.nom.br/
> Free Software Activist FSFLA co-founder GNU Toolchain Engineer
> More tolerance and less prejudice are key for inclusion and diversity.
> Excluding neuro-others for not behaving ""normal"" is *not* inclusive!
>
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