Fix for PR17073 ( http://llvm.org/pr17073 ), simplifycfg illegally hoists an operation in a phi node that can trap.

This patch adds to an existing loop over phi nodes in SimplifyCondBranchToCondBranch() to check for trapping ops and bails out of the optimization if we find one of those.

The test cases verify that trapping ops are not hoisted and non-trapping ops are still optimized as expected.

llvm-svn: 212490

1 files changed, 20 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 1fea04a..4fd0b18 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2378,16 +2378,33 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
   // Do not perform this transformation if it would require
   // insertion of a large number of select instructions. For targets
   // without predication/cmovs, this is a big pessimization.
-  BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
 
+  // Also do not perform this transformation if any phi node in the common
+  // destination block can trap when reached by BB or PBB (PR17073). In that
+  // case, it would be unsafe to hoist the operation into a select instruction.
+
+  BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
   unsigned NumPhis = 0;
   for (BasicBlock::iterator II = CommonDest->begin();
-       isa<PHINode>(II); ++II, ++NumPhis)
+       isa<PHINode>(II); ++II, ++NumPhis) {
     if (NumPhis > 2) // Disable this xform.
       return false;
 
+    PHINode *PN = cast<PHINode>(II);
+    Value *BIV = PN->getIncomingValueForBlock(BB);
+    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BIV))
+      if (CE->canTrap())
+        return false;
+
+    unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
+    Value *PBIV = PN->getIncomingValue(PBBIdx);
+    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(PBIV))
+      if (CE->canTrap())
+        return false;
+  }
+
   // Finally, if everything is ok, fold the branches to logical ops.
-  BasicBlock *OtherDest  = BI->getSuccessor(BIOp ^ 1);
+  BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
 
   DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
                << "AND: " << *BI->getParent());