https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87009
Bug ID: 87009
Summary: Can't find XOR pattern applying De Morgan sequentially
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: mcccs at gmx dot com
Target Milestone: ---
Optimization: -Ofast
Suggested debugging layout: https://godbolt.org/g/ERWctt
int xor_int(int a, int b) {
...
}
Don't work:
int x = ~(a|b);
return ~(x|a)|~(x|b);
return ~(~(a|b)|a)|~(~(a|b)|b);
int x = ~a&~b;
return ~((x|a)&(x|b));
return ~(((~a&~b)|a)&((~a&~b)|b));
int x = ~a&~b;
return ~(x|a)|~(x|b);
return ~((~a&~b)|a)|~((~a&~b)|b);
int x = ~a&~b;
return (~x&~a)|(~x&~b);
int x = ~a&~b;
return ~x&(~a|~b);
Those two work:
return (~(~a&~b)&~a)|(~(~a&~b)&~b);
return ~(~a&~b)&(~a|~b);
A different calculation:
Don't work:
return ~(~(~a&b)&~(a&~b));
return ~(~(~a|~b)|~(a|b));
return ~((a&b)|~(a|b));
This works:
return ~(a&b)&(a|b);
Different calculation:
return ~((a|~b)&(~a|b));
return ~(a|~b)|~(~a|b);
return (~a&b)|~(~a|b);
This works:
return (~a&b)|(a&~b);
SUGGESTED SOLUTTION
At match.pd:774 There's the De Morgan law
for bitwise AND but there isn't one for OR.
https://github.com/gcc-mirror/gcc/blob/fe4311f/gcc/match.pd#L774
No one noticed this, because it can (somehow) transform ~(~a|b) to a&~b
sometimes, (for example, if BMI instructions is enabled, it can use the 'andn'
instruction which calculates a&~b) If I understand it correctly, it only works
if it's not simplified a different way until RTL optimization comes.
/* ~(~a | b) --> a & ~b */
(simplify
(bit_not (bit_ior:cs (bit_not @0) @1))
(bit_and @0 (bit_not @1)))
