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From: "koenigni at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/98425] New: Superfluous sign-extend for
 constrained integer
Date: Wed, 23 Dec 2020 10:19:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98425

            Bug ID: 98425
           Summary: Superfluous sign-extend for constrained integer
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: koenigni at gcc dot gnu.org
  Target Milestone: ---

Hello everyone,

A small missed optimization I noticed while toying around with the differen=
ce
between signed and unsigned integers. The following code

int
baz(int *p, int i) {
  int j;
  if (i >=3D 0) {
    j =3D i + 4;
    return p[j];
  } else
    __builtin_unreachable();
}

is compiled with `gcc -O3 -S` to

baz:
  addl  $4, %esi
  movslq  %esi, %rsi
  movl  (%rdi,%rsi,4), %eax
  ret

The movslq instruction is unnecessary since i is constrained to never be
negative and therefore no sign extension is needed. This probably also prev=
ents
the addl instructions to be removed and the offset being put into the movl.=
=20

For comparison, clang (with the same options) compiles the code to=20

baz:
  movl %esi, %eax
  movl 16(%rdi, %rax, 4), %eax
  ret

Optimal would probably be

baz:
  movl 16(%rdi, %rsi, 4), %eax
  ret=