https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110228
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|[14 Regression] llvm-16 |[13/14 Regression] llvm-16
|miscompiled due to an maybe |miscompiled due to an maybe
|uninitialized and |uninitialized and
|optimizations saying that |optimizations saying that
|the uninitialized has a |the uninitialized has a
|nonzero bits of 1. |nonzero bits of 1.
Target Milestone|14.0 |13.2
--- Comment #16 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
here is a (x86_64 as it requires the inline-asm to force an undefined value
inside LookupFlags but LookupFlags is not used until the next iteration)
program which we have been miscompiling -O2 since r13-4459-g6508d5e5a1a8:
```
unsigned a[4] = {1,1,1,1};
unsigned tt1 = 0;
__attribute__((noipa))
static void bug(unsigned & p, unsigned *t, int n, int t2) {
for(int i = 0; i < n; i++) {
bool LookupFlags ;
unsigned v = t[i];
unsigned tt = tt1;
if (v == 0)
LookupFlags = 0;
else if (v == 1)
LookupFlags = 1;
if (LookupFlags) {
tt|=3u;
LookupFlags = 0;
}
asm("movq $-1, %q1":"+a"(LookupFlags));
p = tt;
}
}
int main() {
unsigned r = 42;
bug(r,a, sizeof(a)/sizeof(a[0]), 1);
__builtin_printf("%u\n", r);
if (r != 3) __builtin_abort();
}
```
Yes it is a bit odd but that was the only way I could get the code added in
r13-4459-g6508d5e5a1a8 to be invoked and even have LookupFlags|=v==1 happening
and get what could get undefined behavior in the loop.
