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

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

> cost might also depend on the context in case flag setting
> behavior differs for xor vs sub (on x86 sub looks strictly more
> powerful here).  The same is probably true when looking for
> a combination with another bitwise operation.
> 
> Btw, why not perform the optimization in expand_binop?  That
> for example already does
> 
>   if (binoptab == sub_optab && CONST_INT_P (op1))
>     {
>       op1 = negate_rtx (mode, op1);
>       binoptab = add_optab;
>     }
> 
> alternatively a targets expander can do the selection as well.

I was under the impression optabs/expand_binops is only supposed to
"optimize" when it's clear that it is an optimization/canonicalization.
I didn't see other functions there trying two alternatives and also none
seems to use range information already.

Regarding the proper costing (including the surroundings): is it even
possible to encompass everything in such a localized decision? A
target's expander decision would also not take this into account when
deciding? If so, should we not perform this conversion generally and not
only target specifc?

Regards
 Robin