[PATCH] libstdc++: simd: use preprocessor to test for 64-bit long double [PR124657]

[Alexandre Oliva]

On Apr 21, 2026, Jonathan Wakely <jwakely@redhat.com> wrote:

> #if  __LDBL_MANT_DIG__ == __DBL_MANT_DIG__

I tried to apply the same technique to simd.h, on top of
https://gcc.gnu.org/pipermail/gcc-patches/2026-April/712616.html, but it
doesn't save any instantiations, neither on x86 nor on ppc.  It seems to
make things generally uglier, but it might offer a little compile-time
benefit on both, so I left it as a separate patch...  WDYT?

Also regstrapped on x86_64-linux-gnu and powerpc64le-linux-gnu.  I also
tested pr109261_constexpr_simd.cc with -mlong-double-64 on both.  Ok for
stage1?


When long double is as wide as double on x86 or powerpc
(-mlong-double-64), we remap it to double in __intrinsic_type.

Use the preprocessor to compare __L?DBL_MANT_DIG instead of
floating-type type sizes on x86 SSE and PowerPC AltiVec.


for  libstdc++-v3/ChangeLog

	PR libstdc++/124657
	* include/experimental/bits/simd.h [_GLIBCXX_SIMD_HAVE_SSE]
	(__intrinsic_type::_S_is_ldouble): New.  Use all over.
	(__intrinsic_type::type): Use preprocessor tests.
	[__ALTIVEC__] (__intrinsic_type): Likewise.
---
 libstdc++-v3/include/experimental/bits/simd.h |   25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/experimental/bits/simd.h b/libstdc++-v3/include/experimental/bits/simd.h
index 8c30c8af26881..177d3598b54b5 100644
--- a/libstdc++-v3/include/experimental/bits/simd.h
+++ b/libstdc++-v3/include/experimental/bits/simd.h
@@ -2469,16 +2469,24 @@ template <>
 template <typename _Tp, size_t _Bytes>
   struct __intrinsic_type<_Tp, _Bytes, enable_if_t<__is_vectorizable_v<_Tp> && _Bytes <= 64>>
   {
+    static constexpr bool _S_is_ldouble = is_same_v<_Tp, long double>;
+
     // allow _Tp == long double with -mlong-double-64
-    static_assert(!(is_same_v<_Tp, long double>
-		    && sizeof(long double) > sizeof(double)),
+#if __LDBL_MANT_DIG != __DBL_MANT_DIG
+    static_assert(!_S_is_ldouble,
 		  "no __intrinsic_type support for long double on x86");
+#endif
 
     static constexpr size_t _S_VBytes = _Bytes <= 16 ? 16 : _Bytes <= 32 ? 32 : 64;
 
     using type [[__gnu__::__vector_size__(_S_VBytes)]]
       = conditional_t<is_integral_v<_Tp>, long long int,
-		      conditional_t<is_same_v<_Tp, long double>, double, _Tp> >;
+#if __LDBL_MANT_DIG != __DBL_MANT_DIG
+		      _Tp
+#else
+		      conditional_t<_S_is_ldouble, double, _Tp>
+#endif
+		      >;
   };
 #endif // _GLIBCXX_SIMD_HAVE_SSE
 
@@ -2582,12 +2590,17 @@ template <typename _Tp, size_t _Bytes>
     static constexpr bool _S_is_ldouble = is_same_v<_Tp, long double>;
 
     // allow _Tp == long double with -mlong-double-64
-    static_assert(!(_S_is_ldouble && sizeof(long double) > sizeof(double)),
+#if __LDBL_MANT_DIG != __DBL_MANT_DIG
+    static_assert(!_S_is_ldouble,
 		  "no __intrinsic_type support for 128-bit floating point on PowerPC");
+#endif
 
 #ifndef __VSX__
     static_assert(!(is_same_v<_Tp, double>
-		    || (_S_is_ldouble && sizeof(long double) == sizeof(double))),
+#if __LDBL_MANT_DIG == __DBL_MANT_DIG
+		    || _S_is_ldouble
+#endif
+		    ),
 		  "no __intrinsic_type support for 64-bit floating point on PowerPC w/o VSX");
 #endif
 
@@ -2595,9 +2608,11 @@ template <typename _Tp, size_t _Bytes>
     {
       if constexpr (is_floating_point_v<_Tp>)
 	{
+#if __LDBL_MANT_DIG == __DBL_MANT_DIG
 	  if constexpr (_S_is_ldouble)
 	    return double {};
 	  else
+#endif
 	    return _Tp {};
 	}
       else if constexpr (is_signed_v<_Tp>)


-- 
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!