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Re: [tree-ssa] Maintaining/representing def-use/use-def information
- From: Chris Lattner <sabre at nondot dot org>
- To: Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>
- Cc: Andrew MacLeod <amacleod at redhat dot com>, Jeff Law <law at redhat dot com>,gcc mailing list <gcc at gcc dot gnu dot org>
- Date: Mon, 15 Dec 2003 15:18:43 -0600 (CST)
- Subject: Re: [tree-ssa] Maintaining/representing def-use/use-def information
On Mon, 15 Dec 2003, Zdenek Dvorak wrote:
> Hello,
>
> > No, we don't. We don't have any concept of "defining variable" or
> > "versions" at all. Though all SSA construction papers talk about building
> > versions of source variables, once you start doing transformations, that
> > idea becomes quickly meaningless.
>
> except for ssa-pre, that requires the original variable names. I still
> consider it an indication that there is something rotten with the
> optimization, but did not get the idea how to make it better so far.
Yes. SSA-PRE, as presented in the papers, only works on a program which
just been just newly converted into SSA form (things like overlapping
lifetimes for "versions" of variables break it severely).
There are ways around this, but the SSAPRE papers have several fairly
severe bugs and ommissions in them. This information is not really needed
for implementing PRE on SSA form.
-Chris
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