[PATCH] D92745: [LoopIdiomRecognize] Teach detectShiftUntilZeroIdiom to recognize loops where the counter is decrementing.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

Seems to look ok in principle, but needs some negative tests too.



================
Comment at: llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp:1484-1489
   for (BasicBlock::iterator Iter = LoopEntry->getFirstNonPHI()->getIterator(),
                             IterE = LoopEntry->end();
        Iter != IterE; Iter++) {
     Instruction *Inst = &*Iter;
     if (Inst->getOpcode() != Instruction::Add)
       continue;
----------------
Preexisting problem, but this is //kinda// broken.
If we have a second `add` in the loop,
and it happens to be before the one we are looking for,
we'll fail to detect the idiom.
The detection should not involve linear scan over instructions.


================
Comment at: llvm/test/Transforms/LoopIdiom/X86/ctlz.ll:531
+; Recognize CTLZ builtin pattern.
+; Here it will replace the loop -
+; assume builtin is always profitable.
----------------
Precommit this + use update script?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D92745/new/

https://reviews.llvm.org/D92745