Re: [PATCH] expand: Convert cst - x into cst xor x.

[Robin Dapp <rdapp@linux.ibm.com>]

> Do we have evidence that targets properly cost XOR vs SUB RTXen?
> 
> It might actually be a reload optimization - when the constant is
> available in a register use 'sub', when it needs to be reloaded
> use 'xor'?
> 
> That said, I wonder if the fallout of changing some SUB to XOR
> is bigger than the benefit when we do it early (missed combines, etc.)?

Regarding fallout I did a bootstrap and regtest for various backends
now.  No change on Power9, s390x and aarch64.  On x86 there is one
additional FAIL in pr78103-3.c:

unsigned long long
bar (unsigned int x)
{
  return __CHAR_BIT__ * sizeof (unsigned int) - 1 - __builtin_clz (x);
}

is supposed to become

        bsrl    %edi, %eax
        ret

but now is

        bsrl    %edi, %eax
        xorl    $31, %eax
        xorq    $31, %rax
        ret

The x86 backend has various splitters catching and simplifying something
like

 (xor (minus (const_int 63) (clz (match_operand))) (const_int 63))

to

 (bsr ...).

From a quick glance, there are several combinations of 31, 63, xor, clz
which would need to be duplicated(?) to match against the changed
patterns.  Perhaps xor is always cheaper on x86 and a simple change from
(minus (const_int 63) (...)) to (xor (const_int 63) (...)) would be
sufficient but this would still need to be reviewed separately.

Needing to keep both patterns (as neither minus nor xor can be
considered "more canonical" than the other) seems like an annoyance.

Regards
 Robin