[PATCH] D80120: [InstCombine] Sink pure instructions down to return and unreachable blocks

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev marked an inline comment as done.
mkazantsev added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp:3309
+    if (DestBlock->getUniquePredecessor() != I->getParent())
+      return false;
     for (BasicBlock::iterator Scan = I->getIterator(),
----------------
jdoerfert wrote:
> I first thought:
> This is more restrictive than you want it to be.
> `DestBlock != I->getParent()->getUniqueSuccessor` is what u want.
> (There is even a less strict restriction but that is more complicated.)
> That allows `DestBlock` to have multiple predecessors (maybe add at test).
> 
> Then I realized this is also the test outside. Though, I still think the above is what we want here.
This is not true. We don't want to sink from A to C in case:
  A    B
   \  /
    C
because BC may be a hot path and A is "nearly never executed" block.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D80120/new/

https://reviews.llvm.org/D80120