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From: "steinar+gcc at gunderson dot no" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101200] New: Unneeded AND after shift
Date: Thu, 24 Jun 2021 20:19:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101200

            Bug ID: 101200
           Summary: Unneeded AND after shift
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: steinar+gcc at gunderson dot no
  Target Milestone: ---

The code after reduction is:

struct {
  int b[6];
} c;
unsigned char d;
void e() {
  unsigned char a =3D d >> 4, f =3D d & 15;
  c.b[a] =3D c.b[f];
}

with g++-11 -O2, this produces

        movzbl  d(%rip), %eax
        movq    %rax, %rdx
        shrq    $4, %rax
        andl    $15, %edx
        andl    $15, %eax
        movl    c(,%rdx,4), %edx
        movl    %edx, c(,%rax,4)
        ret

The second AND with 15 is unneeded and should have been optimized away by V=
RP
as I understand it. I can't reproduce it with ARM, though, so maybe there's
something x86-specific?

Compiler is

  gcc version 11.1.0 (Debian 11.1.0-3)=20

The same code is generated back to at least 4.9. Also present in

  gcc version 12.0.0 20210527 (experimental) [master revision
262e75d22c3:7bb6b9b2f47:9d3a953ec4d2695e9a6bfa5f22655e2aea47a973] (Debian
20210527-1)=