[PATCH] libstdc++: Fix find_last_set(simd_mask) to ignore padding bits

[Jonathan Wakely]

On Tue, 18 Jun 2024 at 07:22, 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, to go for GCC 11.5 as early as possible?

OK for all branches, thanks.


>
> ----------------------- 8< -----------------------
>
> With the change to the AVX512 find_last_set implementation, the change
> to AVX512 operator!= is unnecessary. However, the latter was not
> producing optimal code and unnecessarily set the padding bits. In
> theory, the compiler could determine that with the new !=
> implementation, the bit operation for clearing the padding bits is a
> no-op and can be elided.
>
> Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/115454
>         * include/experimental/bits/simd_x86.h (_S_not_equal_to): Use
>         neq comparison instead of bitwise negation after eq.
>         (_S_find_last_set): Clear unused high bits before computing
>         bit_width.
>         * testsuite/experimental/simd/pr115454_find_last_set.cc: New
>         test.
> ---
>  .../include/experimental/bits/simd_x86.h      | 26 +++++-----
>  .../simd/pr115454_find_last_set.cc            | 49 +++++++++++++++++++
>  2 files changed, 62 insertions(+), 13 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/experimental/simd/
> pr115454_find_last_set.cc
>
>
> --
> ──────────────────────────────────────────────────────────────────────────
>  Dr. Matthias Kretz                           https://mattkretz.github.io
>  GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
>  stdₓ::simd
> ──────────────────────────────────────────────────────────────────────────