[Bug tree-optimization/56175] Issue with combine phase on x86.

[ysrumyan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56175

--- Comment #7 from Yuri Rumyantsev <ysrumyan at gmail dot com> 2013-02-12 13:05:16 UTC ---
(In reply to comment #6)
> (In reply to comment #5)
> > This pattern is already recognized by simplify_bitwise_binary but only for
> > usual int type, i.e. if we change all short types to the ordinary int (or
> > unsigned) this simplification takes place (dump after 1st forwprop):
> > 
> >   <bb 4>:
> >   x_8 = x_2(D) >> 1;
> >   y_9 = y_4(D) >> 1;
> >   _10 = x_8 & 1;
> >   _11 = y_9 & 1;
> >   _16 = x_8 ^ y_9;
> >   z_12 = _16 & 1;
> > 
> > i.e. the issue is redundant type conversions:
> > 
> >   <bb 3>:
> >   x_7 = x_2(D) >> 1;
> >   y_8 = y_4(D) >> 1;
> >   _13 = x_7 & 1;
> >   _9 = (signed char) _13;
> >   _14 = y_8 & 1;
> >   _10 = (signed char) _14;
> >   _11 = _9 ^ _10;
> > 
> > I assume that if we delete these redundant conversions the required
> > simplification will happen.
> 
> Ah, well.  The issue is that we transformed (unsigned char)y & 1
> to (unsigned char)(y & 1).

Hi Richard,

We'd like to fix this issue since we can get +10.5% speedup on Atom.
What is your opinion on how better to fix this issue with 1st pattern in
simplify_bitwise_binary?

I have no idea why gcc does such transformation and what gain we can get from
it - decrease size of constant or create more opportunities for cse?
I can propose the following possible changes:

1. Introduce a hook for doing such transformation.
2. Introduce a new forwprop pass that does not do such transformation.
3. Do not perform such transformation for small positive constant.
4. Do not performa such transformation if (type-x) c == c.
etc.

Any help will be appreciated.
Yuri.