[patch] Decompose gimplify_expr
Steven Bosscher
stevenb@suse.de
Wed Aug 3 09:39:00 GMT 2005
On Wednesday 03 August 2005 11:32, Zdenek Dvorak wrote:
> Hello,
>
> > > > > In any case, I don't intend to implement any complicated
> > > > > optimizations. I definitely will avoid any "special case handling";
> > > > > if the optimization is too complicated, it is hardly worth doing
> > > > > at the expansion time. I will be considering only simple
> > > > > optimizations like unreachable code removal and constant
> > > > > propagation that have a good chance to be very useful and save
> > > > > significant amount of memory and compile time (especially on code
> > > > > that uses macros a lot).\
> > > >
> > > > I will simply note that we have *already* run into issues because
> > > > things like "constant propagation" were added to cleanup_cfg. (It
> > > > ends up causing us to need to update ssa in some cases that it
> > > > doesn't do, and shouldn't have to, since it's a fricking cfg cleanup
> > > > pass)
> > >
> > > constant propagation is not performed anywhere in cfg cleanup.
> >
> > Constant propagation is performed in tree_merge_blocks when the PHI
> > argument is a constant. IIRC Richard Guenther had an example of this
> > actually happening in one of his test cases.
>
> OK, I forgot on this. Nevertheless, the constant propagation itself of
> course does not cause need to update SSA;
It does in Richi's case because ARRAY_REF<array, ssa_name> does not need
SFTs, but ARRAY_REF<array,constant> does if the constant is smaller than
the threshold he has for structure alias analysis for arrays.
(I guess Richard can explain it best himself, so he's in the CC: now.)
Gr.
Steven
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