Re: [PATCH] libstdc++: Add missing constexpr to simd

[Jonathan Wakely <jwakely@redhat.com>]

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On Mon, 22 May 2023 at 16:36, Matthias Kretz via Libstdc++ <
libstdc++@gcc.gnu.org> wrote:

> OK for trunk and backporting?
>
> regtested on x86_64-linux and aarch64-linux
>
> The constexpr API is only available with -std=gnu++XX (and proposed for
> C++26). The proposal is to have the complete simd API usable in constant
> expressions.
>
> This patch resolves several issues with using simd in constant
> expressions.
>
> Issues why constant_evaluated branches are necessary:
>

I note that using if (not __builtin_constant_evaluated()) will fail if
compiled with -fno-operator-names, which is why we don't use 'not', 'and',
etc. elsewhere in libstdc++. I don't know if (or why) anybody uses that
option though, so I don't think you need to hange anything in stdx::simd.




> * subscripting vector builtins is not allowed in constant expressions
>

Is that just because nobody made it work (yet)?


* if the implementation needs/uses memcpy
> * if the implementation would otherwise call SIMD intrinsics/builtins
>


The indentation looks off here and in the _M_set member function following
it:

     operator[](size_t __i) const noexcept
     {
       if constexpr (_S_tuple_size == 1)
  return _M_subscript_read(__i);
       else
- {
 #ifdef _GLIBCXX_SIMD_USE_ALIASING_LOADS
-  return reinterpret_cast<const __may_alias<_Tp>*>(this)[__i];
-#else
-  if constexpr (__is_scalar_abi<_Abi0>())
-    {
-      const _Tp* ptr = &first;
-      return ptr[__i];
-    }
-  else
-    return __i < simd_size_v<_Tp, _Abi0>
-     ? _M_subscript_read(__i)
-     : second[__i - simd_size_v<_Tp, _Abi0>];
+ if (not __builtin_is_constant_evaluated())
+ return reinterpret_cast<const __may_alias<_Tp>*>(this)[__i];
+      else
 #endif
+ if constexpr (__is_scalar_abi<_Abi0>())
+ {
+  const _Tp* ptr = &first;
+  return ptr[__i];
  }
+      else
+ return __i < simd_size_v<_Tp, _Abi0> ? _M_subscript_read(__i)
+     : second[__i - simd_size_v<_Tp, _Abi0>];
     }


Are the copyright years on
testsuite/experimental/simd/pr109261_constexpr_simd.cc correct, or just
copy&paste?