My mailer clipped off the top of the commit message:

Simplifies (x << c) >> c where x is a signed integral type of
width >= int and c = precision(type) - 1 into -(x & 1). Tested successfully
on x86_64 and x86 targets.

        PR middle-end/101955

gcc/ChangeLog:

        * match.pd (x << c) >> c -> -(x & 1): New simplification.

gcc/testsuite/ChangeLog:

        * gcc.dg/pr101955.c: New test.

On Thu, Jul 20, 2023 at 10:47 AM Drew Ross <drross@redhat.com> wrote:

>         PR middle-end/101955
>
> gcc/ChangeLog:
>
>         * match.pd (x << c) >> c -> -(x & 1): New simplification.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/pr101955.c: New test.
> ---
>  gcc/match.pd                    |  9 +++++
>  gcc/testsuite/gcc.dg/pr101955.c | 62 +++++++++++++++++++++++++++++++++
>  2 files changed, 71 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/pr101955.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 8543f777a28..bf63652e80f 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -3766,6 +3766,15 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>        && (wi::ltu_p (wi::to_wide (@1), element_precision (type))))
>    (bit_and @0 (rshift { build_minus_one_cst (type); } @1))))
>
> +/* Optimize (X << C) >> C where C = precision(type) - 1 and X is signed
> +   into -(X & 1).  */
> +(simplify
> + (rshift (lshift @0 INTEGER_CST@1) @@1)
> + (if (ANY_INTEGRAL_TYPE_P (type)
> +      && !TYPE_UNSIGNED (type)
> +      && wi::eq_p (wi::to_wide (@1), element_precision (type) - 1))
> +  (negate (bit_and @0 { build_one_cst (type); }))))
> +
>  /* Optimize x >> x into 0 */
>  (simplify
>   (rshift @0 @0)
> diff --git a/gcc/testsuite/gcc.dg/pr101955.c
> b/gcc/testsuite/gcc.dg/pr101955.c
> new file mode 100644
> index 00000000000..0e233269e21
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr101955.c
> @@ -0,0 +1,62 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-dse1 -Wno-psabi" } */
> +
> +typedef int v4si __attribute__((vector_size(16)));
> +
> +__attribute__((noipa)) int
> +t1 (int x)
> +{
> +  return (x << 31) >> 31;
> +}
> +
> +__attribute__((noipa)) int
> +t2 (int x)
> +{
> +  int y = x << 31;
> +  int z = y >> 31;
> +  return z;
> +}
> +
> +__attribute__((noipa)) int
> +t3 (int x)
> +{
> +  int w = 31;
> +  int y = x << w;
> +  int z = y >> w;
> +  return z;
> +}
> +
> +__attribute__((noipa)) long long
> +t4 (long long x)
> +{
> +  return (x << 63) >> 63;
> +}
> +
> +__attribute__((noipa)) long long
> +t5 (long long x)
> +{
> +  long long y = x << 63;
> +  long long z = y >> 63;
> +  return z;
> +}
> +
> +__attribute__((noipa)) long long
> +t6 (long long x)
> +{
> +  int w = 63;
> +  long long y = x << w;
> +  long long z = y >> w;
> +  return z;
> +}
> +
> +__attribute__((noipa)) v4si
> +t7 (v4si x)
> +{
> +  return (x << 31) >> 31;
> +}
> +
> +/* { dg-final { scan-tree-dump-not " >> " "dse1" } } */
> +/* { dg-final { scan-tree-dump-not " << " "dse1" } } */
> +/* { dg-final { scan-tree-dump-times " -" 7 "dse1" } } */
> +/* { dg-final { scan-tree-dump-times " & " 7 "dse1" } } */
> +
> --
> 2.39.3
>
>