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.
