Hm...

If we have this transformation:

define i8 @​src(i8 %0) {
%1:
%2 = xor i8 %0, 255
%3 = ctpop i8 %2
ret i8 %3
}
=>
define i8 @​tgt(i8 %0) {
%1:
%2 = ctpop i8 %0
%3 = sub i8 8, %2
ret i8 %3
}
Transformation seems to be correct!

Then

int tgt(unsigned int a)
{
return (32 -__builtin_popcount(a)) & 1;
}

is nicely optimised to

tgt(unsigned int): # @​tgt(unsigned int)
popcnt eax, edi
and eax, 1
ret

@​spatel, replace xor with sub, WDYT?

int A(unsigned int a)
{
return (32 -__builtin_popcount(a));
}

int B(unsigned int a)
{
return (__builtin_popcount(~a)) ;
}

A(unsigned int): # @​A(unsigned int)
popcnt ecx, edi
mov eax, 32
sub eax, ecx
ret
B(unsigned int): # @​B(unsigned int)
not edi
popcnt eax, edi
ret

int tgt(unsigned int a)
{
return (32 -__builtin_popcount(a)) & 1;
}

is nicely optimised to

tgt(unsigned int): # @​tgt(unsigned int)
popcnt eax, edi
and eax, 1
ret
@​spatel, replace xor with sub, WDYT?

We really should have this folding to "not" in IR:
https://alive2.llvm.org/ce/z/uuo8xa

...because a 'not' is better for analysis than a 'sub'.

If that causes a regression on the parity pattern, we should add another fold for that. I'll take a look.

https://reviews.llvm.org/rGe10d7d455d4e

Please check if that solves the motivating cases.

We probably still want the sub->not transform, but if it is not showing up, it may not be that important - can either keep this open to track that or open another report.

Thanks.

I will close this PR and create new one.

mentioned in issue llvm/llvm-bugzilla-archive#50104