[Bug target/94643] New: [x86_64] gratuitous sign extension of nonnegative value from 32 to 64 bits

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94643

            Bug ID: 94643
           Summary: [x86_64] gratuitous sign extension of nonnegative
                    value from 32 to 64 bits
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugdal at aerifal dot cx
  Target Milestone: ---

Test case:

#include <stdint.h>
uint16_t a[];
uint64_t f(int i)
{
        return a[i]*16;
}

Produces:

        movslq  %edi, %rdi
        movzwl  a(%rdi,%rdi), %eax
        sall    $4, %eax
        cltq
        ret

The value is necessarily in the range [0,1M) (in particular, nonnegative) and
operation on eax has already cleared the upper bits of rax, so cltq is
completely gratuitous. I've observed the same in nontrivial examples where
movslq gets used.