[PATCH] D139692: [IndVars] Use symbolic max block exit count to handle the last iter

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added inline comments.


================
Comment at: llvm/lib/Transforms/Scalar/IndVarSimplify.cpp:1705
     if (SE->isLoopEntryGuardedByCond(L, CmpInst::ICMP_ULT, MaxBECount,
                                      ExactExitCount)) {
       foldExit(L, ExitingBB, false, DeadInsts);
----------------
mkazantsev wrote:
> nikic wrote:
> > As a followup, we should be able to use MaxExitCount here as well.
> Good catch, need to find a test on it.
Thinking more of it, I don't think it's right. Imagine case:
exit 1: exact (unknown) is `2`, symbolic max is `x`
exit 2: exact (unknown) is 0, symbolic max is `x + 1<nuw>`

Loop's symbolic max is `x`.

In this case we cannot eliminate the 2nd exit basing on fact that its maximum is more than some other maximum.

Unilke this, in my patch exit (in given block) earlier than proven max doesn't matter; logic with last iteration is only in the game when loop actually exits on max possible proven iteration (and therefore all subsequent exits don't execute).


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  https://reviews.llvm.org/D139692/new/

https://reviews.llvm.org/D139692