[Bug tree-optimization/35972] [4.4 Regression] load-PRE missed opportunities without SFTs

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from rguenther at suse dot de  2008-04-19 01:50 -------
Subject: Re:  [4.4 Regression] load-PRE missed
 opportunities without SFTs

On Fri, 18 Apr 2008, aldot at gcc dot gnu dot org wrote:

> ------- Comment #4 from aldot at gcc dot gnu dot org  2008-04-18 18:00 -------
> While not really useful, i dare to state that SCC(N)-VN unfortunately misses a
> lot of what it is ment to serve (IMHO), generally:
> 
> 1) PR11832
> testcase from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11832#c0
> Needs adjustment (looked into it -- AFAICR -- befor 4.3 was branched; didn't
> retry after you recently touching SCCVN, i admit)
> 
> 2) PR5738
> The testcase from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=5738#c0
> is completely undiagnosed (and _severely_ mishandled -- IMHO) by current SCCVN.
> This one is particularly mean since you may run into dominator related pitfalls
> very, very fast, from what i've seen (before you touched tree-ssa-pre,
> admitteldy).
> 
> A couple of sensible and nice improvements are mentioned in ¹) above.

These are both testcases for missed code-motion which neither PRE nor
FRE performs.  While SCCVN can figure out that two stmts compute the
same value it still needs a pass that makes use of it ;)

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35972