[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

["rakdver at atrey dot karlin dot mff dot cuni dot cz" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #24 from rakdver at atrey dot karlin dot mff dot cuni dot cz  2006-07-13 09:03 -------
Subject: Re:  poor optimization choices when iterating over a std::string
(probably not c++-specific)

> > > However, ch isn't just copying the loop header; it is also
> > > copying the *entire body of the loop*, which nothing can fix. I call
> > > that a clear bug.
> >
> > how do you define a loop header?
> 
> I was under the impression it was just the one basic block called out
> in the .ch dump, e.g.
> 
> ;; Loop 1
> ;;  header 6, latch 5
> ;;  depth 1, level 1, outer 0
> 
> -- basic block 6 happens to contain just the code from the syntactic
> loop condition.  Andrew informs me that this is wrong, and that in
> this case the header is the entire loop, but I will come back at that
> with 'ch should never be duplicating the entire loop; if the header is
> the entire loop, it should do something more sensible, like duplicate
> just the first basic block or something.'

currently, we stop once the copied region is too large.  This means that
on "normal" loops that have a body that does something, we won't copy
whole loop.  Of course, any heuristics will have cases when it won't
perform ideally.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364