https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101676
Bug ID: 101676
Summary: ^ not changed to | if the non-zero don't overlap
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pinskia at gcc dot gnu.org
Target Milestone: ---
Take:
int f(int a, int b)
{
if (a & ~8) __builtin_unreachable();
if (b & 8) __builtin_unreachable();
return a ^ b;
}
----- CUT -----
The xor here can converted into ior as a and b non-zero bits don't overlap.
Note clang does this optimization.
But clang does not do the following (which we should be able to do too).
Take:
int f(int a, int b)
{
if (a & 8) __builtin_unreachable();
if (b & ~1) __builtin_unreachable();
return (a) ^ (b<<3);
}
This should be able to turn into a bit insert of b into a.
Right now both GCC (and really clang) turn this on aarch64:
eor w0, w0, w1, lsl 3 ;;;clang uses orr
Both can handle the following though:
int f(int a, int b)
{
// if (a & 8) __builtin_unreachable();
// if (b & ~1) __builtin_unreachable();
return (a&~8) ^ ((b&1)<<3);
}
Unless you uncomment out the condtionals :).
