> Hi Honza, Hi, > > I wasn't sure what to do here so I figured I'd ask. > > In adding support for multiple exits to the vectorizer I didn't know how to > update this bit: > > https://github.com/gcc-mirror/gcc/blob/master/gcc/tree-vect-loop-manip.cc#L3363 > > Essentially, if skip_vector (i.e. not enough iteration to enter the vector > loop) then the > previous code would update the new probability to be the same as that of the > exit edge. This made sense because that's the only edge which could bring > you to > the next loop preheader. > > With multiple exits this is no longer the case since any exit can bring you > to the > Preaheader node. I figured the new counts should simply be the sum of all > exit > edges. But that gives quite large count values compared to the rest of the > loop. The sum of all exit counts (not probabilities) relative to header count should give you estimated probability that the loop iterates at any given iteration. I am not sure how good estimate this is for loop preconditioning to be true (without profile histograms it is really hard to tell). > > I then thought I would need to scale the counts by the probability of the edge > being taken. The problem here is that the probabilities don't end up to 100%
So you are summing exit_edge->count ()? I am not sure how useful would be summit probabilities since they are conditional (relative to probability of entering BB you go to). How complicated CFG we now handle with vectorization? Honza > > so the scaled counts also looked kinda wonkey. Any suggestions? > > If you want some small examples to look at, testcases > ./gcc/testsuite/gcc.dg/vect/vect-early-break_90.c to > ./gcc/testsuite/gcc.dg/vect/vect-early-break_93.c > should be relevant here. > > Thanks, > Tamar
