On Fri, Jul 21, 2023 at 8:09 AM Drew Ross via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> 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.
Thinking about this some more, I think this should be handled in
expand rather than on the gimple level.
It is very much related to PR 110717 even. We are basically truncating
to a signed one bit integer and then sign extending that across the
whole code.
Thanks,
Andrew
>
> 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 | 10 +++++
> gcc/testsuite/gcc.dg/pr101955.c | 69 +++++++++++++++++++++++++++++++++
> 2 files changed, 79 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/pr101955.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 8543f777a28..820fc890e8e 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -3766,6 +3766,16 @@ 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 (nop_convert? (lshift @0 uniform_integer_cst_p@1)) @@1)
> + (with { tree cst = uniform_integer_cst_p (@1); }
> + (if (ANY_INTEGRAL_TYPE_P (type)
> + && !TYPE_UNSIGNED (type)
> + && wi::eq_p (wi::to_wide (cst), element_precision (type) - 1))
> + (negate (bit_and (convert @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..386154911c5
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr101955.c
> @@ -0,0 +1,69 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-dse1 -Wno-psabi" } */
> +
> +typedef int v4si __attribute__((vector_size(4 * sizeof(int))));
> +
> +__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;
> +}
> +
> +__attribute__((noipa)) v4si
> +t8 (v4si x)
> +{
> + v4si t = {31,31,31,31};
> + return (x << t) >> t;
> +}
> +
> +/* { dg-final { scan-tree-dump-not " >> " "dse1" } } */
> +/* { dg-final { scan-tree-dump-not " << " "dse1" } } */
> +/* { dg-final { scan-tree-dump-times " -" 8 "dse1" } } */
> +/* { dg-final { scan-tree-dump-times " & " 8 "dse1" } } */
> +
> --
> 2.39.3
>
