https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110791
Bug ID: 110791
Summary: [12/13/14 Regression] arm: Wrong code with -Os
-march=armv8.1-m.main
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: acoplan at gcc dot gnu.org
Target Milestone: ---
The following testcase is miscompiled since at least GCC 12 with -Os
-march=armv8.1-m.main:
void __attribute__ ((noinline))
foo (char *path, int *result)
{
char *p = path;
while (p >= path && *p != '/')
p--;
while (p > path && p[-1] == '/')
p--;
if (p < path)
*result = 1;
}
int main(void)
{
char path[4] = "usr";
int x = 0;
foo (path + 2, &x);
if (!x)
__builtin_abort ();
}
Below is the assembly we currently generate together with comments showing how
this goes wrong at runtime:
foo:
mov r3, r0 @ r3 <- (p = path)
ldrb r2, [r3], #-1 @ r2 <- *p; p--;
cmp r2, #47
it eq
moveq r3, r0 @ if (r2 == 47) p <- path
subs r2, r3, r0
cmp r0, r3
add r2, r2, #1
bhi .L9 @ if (path > p) goto .L9 [taken]
adds r0, r0, #1
bne .L6
.L9:
movs r2, #1
.L6:
subs r2, r2, #1 @ r2 <- 0 [fall through from above]
bne .L3 @ [not taken, r2 was #1]
bcc .L4 @ [not taken]
bx lr @ [return without setting *result = 1]
.L3:
ldrb r0, [r3, #-1]!
cmp r0, #47
beq .L6
bx lr
.L4:
movs r3, #1
str r3, [r1]
bx lr
At -O2 the code is both correct and much better quality:
foo:
ldrb r3, [r0] @ zero_extendqisi2
cmp r3, #47
itt ne
movne r3, #1
strne r3, [r1]
bx lr
