https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102793
Bug ID: 102793
Summary: AArch64: sequential comparisons with equal conditional
blocks don't use ccmp
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: christophm30 at gmail dot com
Target Milestone: ---
The following code:
int ccmp(uint64_t* s1, uint64_t* s2, int(*foo)(void))
{
uint64_t d1, d2, bar;
d1 = *s1++;
d2 = *s2++;
bar = (d1 ^ d2) & 0xabcd;
if (bar == 0 || d1 != d2)
return foo();
return 0;
}
int noccmp(uint64_t* s1, uint64_t* s2, int(*foo)(void))
{
uint64_t d1, d2, bar;
d1 = *s1++;
d2 = *s2++;
bar = (d1 ^ d2) & 0xabcd;
if (bar == 0)
return foo();
if (d1 != d2)
return foo();
return 0;
}
...produces (GCC master or earlier, ARM64, with -O3):
ccmp:
ldr x3, [x0]
mov x4, 43981
ldr x0, [x1]
eor x1, x3, x0
tst x1, x4
ccmp x3, x0, 0, ne
bne .L5
mov w0, 0
ret
.L5:
mov x16, x2
br x16
noccmp:
ldr x3, [x0]
mov x4, 43981
ldr x0, [x1]
eor x1, x3, x0
tst x1, x4
beq .L8
cmp x3, x0
beq .L9
.L8:
mov x16, x2
br x16
.L9:
mov w0, 0
ret
Since both conditional blocks do exactly the same (they call foo()), both
functions could use the ccmp instruction.
I just observed this and have not analysed this any further.
