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Subject: [Bug tree-optimization/56175] Issue with combine phase on x86.
Date: Tue, 12 Feb 2013 13:26:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56175

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> 2013-02-12 13:25:59 UTC ---
(In reply to comment #7)
> (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?

Well, you'd have to track down what is responsible for that transform.

Generally promoting operations (and automatic vars) to word-mode may
be beneficial on most targets.  But that should be done late.

> 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.

First track it down ;)

> Any help will be appreciated.
> Yuri.