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