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Re: [patch] Decompose gimplify_expr
- From: Steven Bosscher <stevenb at suse dot de>
- To: Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>
- Cc: gcc-patches at gcc dot gnu dot org, Daniel Berlin <dberlin at dberlin dot org>, Diego Novillo <dnovillo at redhat dot com>, Mark Mitchell <mark at codesourcery dot com>, Geoffrey Keating <geoffk at apple dot com>, Richard Guenther <rguenther at suse dot de>
- Date: Wed, 3 Aug 2005 11:38:58 +0200
- Subject: Re: [patch] Decompose gimplify_expr
- References: <20050722234109.GB23954@atrey.karlin.mff.cuni.cz> <200508031126.18667.stevenb@suse.de> <20050803093249.GC29641@atrey.karlin.mff.cuni.cz>
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