[Bug rtl-optimization/98425] Superfluous sign-extend for constrained integer

[koenigni at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98425

--- Comment #5 from Nicolas Koenig  ---
The advantage of using mov over movs for known-positive integers would be that
32bit moves are also move-eliminated, while movs always has to be executed.

[Bug rtl-optimization/98425] New: Superfluous sign-extend for constrained integer

[koenigni at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98425

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 difference
between signed and unsigned integers. The following code

int
baz(int *p, int i) {
  int j;
  if (i >= 0) {
j = 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 prevents
the addl instructions to be removed and the offset being put into the movl. 

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

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

Optimal would probably be

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