https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64753
Bug ID: 64753
Summary: Redundant cmp instruction on x86_64
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: rv at rasmusvillemoes dot dk
Host: x86_64
Target: x86_64
The linux kernel's library strncmp is this:
int strncmp(const char *cs, const char *ct, size_t count)
{
unsigned char c1, c2;
while (count) {
c1 = *cs++;
c2 = *ct++;
if (c1 != c2)
return c1 < c2 ? -1 : 1;
if (!c1)
break;
count--;
}
return 0;
}
Compiling with gcc -O2 -S I get this:
strncmp:
.LFB0:
.cfi_startproc
testq %rdx, %rdx
je .L10
movzbl (%rdi), %ecx
movzbl (%rsi), %r8d
# cmpb %r8b, %cl
# jne .L3
testb %cl, %cl
je .L10
subq $1, %rdx
xorl %eax, %eax
jmp .L4
.p2align 4,,10
.p2align 3
.L6:
movzbl 1(%rdi,%rax), %ecx
movzbl 1(%rsi,%rax), %r8d
# cmpb %r8b, %cl
# jne .L3
addq $1, %rax
testb %cl, %cl
je .L10
.L4:
cmpq %rdx, %rax
jne .L6
.L10:
xorl %eax, %eax
ret
.p2align 4,,10
.p2align 3
.L3:
cmpb %r8b, %cl
sbbl %eax, %eax
orl $1, %eax
ret
.cfi_endproc
At the two places marked # we do a cmp and a conditional jump to .L3, where for
good measure the same cmp is done again... there's no other path to .L3, so it
would seem that simply omitting that extra cmp should be ok.
This is with gcc-5.0 (GCC) 5.0.0 20150112 (experimental), but I see the same
with gcc (Debian 4.7.2-5) 4.7.2.
