Re: match.pd: Optimize (x & y) ^ (x | y)

[Marek Polacek <polacek@redhat.com>]

On Fri, Jun 12, 2015 at 08:44:45AM +0200, Marc Glisse wrote:
> On Fri, 12 Jun 2015, Richard Biener wrote:
> 
> >>Not judging at all whether it is desirable or not, but you might have
> >>hit the issue that when you want several convert?, you need to use the
> >>spelling convert1?, convert2?, and it stops there, while here you would
> >>probably want at least 4 (maybe 6?) for this case. You might be able to
> >>work around it with a user-defined predicate, but I keep getting errors
> >>like
> >>generic-match.c:6655:16: error: redeclaration of ‘tree_node* o20_pops [2]’

I remember trying convert1?, convert2?, and so on.  But I gave up soon.

> >>If you want to reproduce the error (this is probably not good as is, it
> >>is only provided as a reproducer)
> >>
> >>(match (nopand @0 @1)
> >> (bit_and (convert1? @0) (convert2? @1))
> >> (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
> >>      && tree_nop_conversion_p (type, TREE_TYPE (@1)))))
> >>(match (nopior @0 @1)
> >> (bit_ior (convert1? @0) (convert2? @1))
> >> (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
> >>      && tree_nop_conversion_p (type, TREE_TYPE (@1)))))
> >>(simplify
> >> (bit_xor:c (convert1? (nopand@2 @0 @1))
> >>            (convert2? (nopior@3 @0 @1)))
> >> (if (tree_nop_conversion_p (type, TREE_TYPE (@2))
> >>      && tree_nop_conversion_p (type, TREE_TYPE (@3)))
> >>  (bit_xor (convert @0) (convert @1))))
> >>
> >>
> >>fold-const.c traditionally avoided the combinatorial explosion by using
> >>strip_nops.
> >
> >Yeah.  We can probably special case conditional conversions in code
> >generation instead of lowering it.  And then go the full way and special
> >case nop conversions so you can avoid writing the predicate as well.
> 
> Without special casing, I currently have:
> 
> (match (nopcvt @0)
>  (convert? @0)
>  (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))))
> (simplify
>  (bit_xor:c (convert1? (bit_and@2 (nopcvt @0) (nopcvt @1)))
>             (convert2? (bit_ior:c (nopcvt @0) (nopcvt @1))))
>  (if (tree_nop_conversion_p (type, TREE_TYPE (@2)))
>   (bit_xor (convert @0) (convert @1))))
> 
> which simplifies Jakub's testcase without exploding the size of *-match.c,
> but it is still not very satisfying.

Yeah, imagine if we'd have to change every pattern like that :-(.

	Marek