[PATCH] libstdc++: Fix simd<char> conversion for -fno-signed-char for Clang

[Jonathan Wakely]

On Mon, 3 Jun 2024 at 16:31, Matthias Kretz <m.kretz@gsi.de> wrote:
>
> Tested on x86_64-linux-gnu (also -m32 and -mx32), aarch64-linux-gnu, and arm-
> linux-gnueabi(hf).
>
> OK for trunk and backports?

OK for all.


>
> ----------------------- 8< -----------------------
>
> The special case for Clang in the trait producing a signed integer type
> lead to the trait returning 'char' where it should have been 'signed
> char'. This workaround was introduced because on Clang the return type
> of vector compares was not convertible to '_SimdWrapper<
> __int_for_sizeof_t<...' unless '__int_for_sizeof_t<char>' was an alias
> for 'char'. In order to not rewrite the complete mask type code (there
> is code scattered around the implementation assuming signed integers),
> this needs to be 'signed char'; so the special case for Clang needs to
> be removed.
> The conversion issue is now solved in _SimdWrapper, which now
> additionally allows conversion from vector types with compatible
> integral type.
>
> Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/115308
>         * include/experimental/bits/simd.h (__int_for_sizeof): Remove
>         special cases for __clang__.
>         (_SimdWrapper): Change constructor overload set to allow
>         conversion from vector types with integral conversions via bit
>         reinterpretation.
> ---
>  libstdc++-v3/include/experimental/bits/simd.h | 45 +++++++++++--------
>  1 file changed, 27 insertions(+), 18 deletions(-)
>
>
> --
> ──────────────────────────────────────────────────────────────────────────
>  Dr. Matthias Kretz                           https://mattkretz.github.io
>  GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
>  stdₓ::simd
> ──────────────────────────────────────────────────────────────────────────