https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113072
Bug ID: 113072
Summary: `(a ^ CST0) & (~a ^ CST0)` is not optimized to 0 at
the gimple level
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Keywords: TREE
Severity: enhancement
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pinskia at gcc dot gnu.org
Target Milestone: ---
Take:
```
int foo(int a, int b) { return (a ^ 4) & (~a ^ 4); }
```
This should be optimized to 0 but is missed at the gimple level.
RTL Combine catches it:
```
Trying 8, 7 -> 9:
8: {r105:SI=r101:SI^0xfffffffffffffffb;clobber flags:CC;}
REG_DEAD r101:SI
REG_UNUSED flags:CC
7: {r104:SI=r101:SI^0x4;clobber flags:CC;}
REG_UNUSED flags:CC
9: {r103:SI=r104:SI&r105:SI;clobber flags:CC;}
REG_DEAD r105:SI
REG_DEAD r104:SI
REG_UNUSED flags:CC
Failed to match this instruction:
(parallel [
(set (reg:SI 103)
(const_int 0 [0]))
(clobber (reg:CC 17 flags))
])
Successfully matched this instruction:
(set (reg:SI 103)
(const_int 0 [0]))
allowing combination of insns 7, 8 and 9
original costs 4 + 4 + 4 = 12
replacement cost 4
```
via apply_distributive_law (I think) which does basically `(a ^ 4) & (a ^
(~4))` -> `(4 ^ (~4)) & (a ^ a)` which is 0 as `a ^ a` is 0.
