[LICM] Adjust how moving the re-hoist point works

In some cases the order that we hoist instructions in means that when rehoisting
(which uses the same order as hoisting) we can rehoist to a block A, then a
block B, then block A again. This currently causes an assertion failure as it
expects that when changing the hoist point it only ever moves to a block that
dominates the hoist point being moved from.

Fix this by moving the re-hoist point when it doesn't dominate the dominator of
hoisted instruction, or in other words when it wouldn't dominate the uses of
the instruction being rehoisted.

Differential Revision: https://reviews.llvm.org/D55266

llvm-svn: 350408

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 695eaf6cf4a..5ebf035bf51 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -809,14 +809,15 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
                         [&](Use &U) { return DT->dominates(I, U); })) {
         BasicBlock *Dominator =
             DT->getNode(I->getParent())->getIDom()->getBlock();
-        LLVM_DEBUG(dbgs() << "LICM rehoisting to " << Dominator->getName()
-                          << ": " << *I << "\n");
-        if (!HoistPoint || HoistPoint->getParent() != Dominator) {
+        if (!HoistPoint || !DT->dominates(HoistPoint->getParent(), Dominator)) {
           if (HoistPoint)
             assert(DT->dominates(Dominator, HoistPoint->getParent()) &&
                    "New hoist point expected to dominate old hoist point");
           HoistPoint = Dominator->getTerminator();
         }
+        LLVM_DEBUG(dbgs() << "LICM rehoisting to "
+                          << HoistPoint->getParent()->getName()
+                          << ": " << *I << "\n");
         moveInstructionBefore(*I, *HoistPoint, *SafetyInfo);
         HoistPoint = I;
         Changed = true;