Re: [PATCH] Move ~X | X -> -1 folding

[Richard Biener <rguenther at suse dot de>]

On Tue, 30 Jun 2015, Marek Polacek wrote:

> On Tue, Jun 30, 2015 at 11:08:35AM +0200, Marc Glisse wrote:
> > On Tue, 30 Jun 2015, Marek Polacek wrote:
> > 
> > >This moves a simple optimization.  Here it's plain to see how :c
> > >removes the need to duplicate code to handle commutativity.
> > 
> > Note that the same transformation would work for plus and xor.
>  
> Sounds like a good follow-up.  I think moving from fold-const.c to
> match.pd ought to be 1:1 for clarity.

Yes, I agree fully here.

> I'll prepare a patch to also
> handle +/^.

Thanks.

> > >I put some more converts into the pattern, but then it's turned
> > >out that I also need the tree_nop_conversion_p (otherwise we'd
> > >regress binop-notor2.c that uses booleans).
> > 
> > I don't really see why removing tree_nop_conversion_p would regress anything
> > (though you would probably need to build the all_ones constant in TREE_TYPE
> > (@0) and convert that to type). For my curiosity, could you explain a bit
> > more?
> 
> This wasn't all that clear to me.  The testcase in question is 
> 
> int
> foo (_Bool a, _Bool b)
> {
>   return (a | (a == 0)) | ((b ^ 1) | b); 
> }
> 
> this ought to be simplified to "return 1".  Through various folding we
> arrive at
> 
> (int) ~b | (int) b
> 
> so we'd turn that into -1 (all_ones_cst of type int).  But for boolean b
> "~b | b" is always 1, right?

Actually our bools are signed ;)  Even for unsigned bools we'd
then simply build '1' via build_all_ones_cst.

Richard.