[Bug target/64691] New: Suboptimal register allocation for bytes comparison on i386

[enkovich.gnu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64691

            Bug ID: 64691
           Summary: Suboptimal register allocation for bytes comparison on
                    i386
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: enkovich.gnu at gmail dot com

This problem was actually found in 256.bzip2 benchmark codes compiled by GCC
5.0 on -O2.  There is a small loop with bytes comparison which appeared to be
ineffective because compared values were not allocated on registers allowing
byte access.  That caused additional copies and as a result significant loop
slow down.

Situation may be simulated on a small test if we restrict registers usage.

>cat test.c
void test (unsigned char *p, unsigned char val)
{
  unsigned char tmp1, tmp2;
  int i;

  i = 0;
  tmp1 = p[0];
  while (val != tmp1)
    {
      i++;
      tmp2 = tmp1;
      tmp1  = p[i];
      p[i] = tmp2;
    }
  p[0]= tmp1;
}
>gcc -O2 -m32 -ffixed-ebx test.c -S

Here is a loop:

.L3:
        movzbl  (%eax), %ebp
        movl    %esi, %ecx
        movb    %dl, (%eax)
        addl    $1, %eax
        movl    %ebp, %edx
        cmpb    %dl, %cl
        jne     .L3

We have an extra register copy esi->ecx to perform comparison.

Suppose the easiest way to get better register allocation here would be to
transform QI comparison into SI one to relax register constraints.