[LICM] Don't try to sink values out of loops without any exits

There is no suitable basic block to sink instructions in loops without
exits.  The only way an instruction in a loop without exits can be used
is as an incoming value to a PHI.  In such cases, the incoming block for
the corresponding value is unreachable.

This fixes PR24013.

Differential Revision: http://reviews.llvm.org/D10903

llvm-svn: 241987

1 files changed, 12 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index f0e6d641b18..43fc50e588f 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -602,7 +602,8 @@ static bool sink(Instruction &I, const LoopInfo *LI, const DominatorTree *DT,
   // PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
   // the instruction.
   while (!I.use_empty()) {
-    Instruction *User = I.user_back();
+    Value::user_iterator UI = I.user_begin();
+    auto *User = cast<Instruction>(*UI);
     if (!DT->isReachableFromEntry(User->getParent())) {
       User->replaceUsesOfWith(&I, UndefValue::get(I.getType()));
       continue;
@@ -610,6 +611,16 @@ static bool sink(Instruction &I, const LoopInfo *LI, const DominatorTree *DT,
     // The user must be a PHI node.
     PHINode *PN = cast<PHINode>(User);
 
+    // Surprisingly, instructions can be used outside of loops without any
+    // exits.  This can only happen in PHI nodes if the incoming block is
+    // unreachable.
+    Use &U = UI.getUse();
+    BasicBlock *BB = PN->getIncomingBlock(U);
+    if (!DT->isReachableFromEntry(BB)) {
+      U = UndefValue::get(I.getType());
+      continue;
+    }
+
     BasicBlock *ExitBlock = PN->getParent();
     assert(ExitBlockSet.count(ExitBlock) &&
            "The LCSSA PHI is not in an exit block!");