[BDCE] Skip metadata while replacing uses.

The fix committed in r288851 doesn't cover all the cases.
In particular, if we have an instruction with side effects
which has a no non-dbg use not depending on the bits, we still
perform RAUW destroying the dbg.value's first argument.
Prevent metadata from being replaced here to avoid the issue.

Differential Revision:  https://reviews.llvm.org/D27534

llvm-svn: 288987

2 files changed, 13 insertions, 4 deletions

diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index 451cb09ea9f..91a999b5800 100644
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -367,7 +367,7 @@ static bool contains(Value *Expr, Value *V) {
 }
 #endif // NDEBUG
 
-void Value::replaceAllUsesWith(Value *New) {
+void Value::doRAUW(Value *New, bool NoMetadata) {
   assert(New && "Value::replaceAllUsesWith(<null>) is invalid!");
   assert(!contains(New, this) &&
          "this->replaceAllUsesWith(expr(this)) is NOT valid!");
@@ -377,7 +377,7 @@ void Value::replaceAllUsesWith(Value *New) {
   // Notify all ValueHandles (if present) that this value is going away.
   if (HasValueHandle)
     ValueHandleBase::ValueIsRAUWd(this, New);
-  if (isUsedByMetadata())
+  if (!NoMetadata && isUsedByMetadata())
     ValueAsMetadata::handleRAUW(this, New);
 
   while (!use_empty()) {
@@ -398,6 +398,14 @@ void Value::replaceAllUsesWith(Value *New) {
     BB->replaceSuccessorsPhiUsesWith(cast<BasicBlock>(New));
 }
 
+void Value::replaceAllUsesWith(Value *New) {
+  doRAUW(New, false /* NoMetadata */);
+}
+
+void Value::replaceNonMetadataUsesWith(Value *New) {
+  doRAUW(New, true /* NoMetadata */);
+}
+
 // Like replaceAllUsesWith except it does not handle constants or basic blocks.
 // This routine leaves uses within BB.
 void Value::replaceUsesOutsideBlock(Value *New, BasicBlock *BB) {
diff --git a/llvm/lib/Transforms/Scalar/BDCE.cpp b/llvm/lib/Transforms/Scalar/BDCE.cpp
index 1baade43e85..251b3870776 100644
--- a/llvm/lib/Transforms/Scalar/BDCE.cpp
+++ b/llvm/lib/Transforms/Scalar/BDCE.cpp
@@ -40,7 +40,8 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
   bool Changed = false;
   for (Instruction &I : instructions(F)) {
     // If the instruction has side effects and no non-dbg uses,
-    // BDCE should skip it.
+    // skip it. This way we avoid computing known bits on an instruction
+    // that will not help us.
     if (I.mayHaveSideEffects() && I.use_empty())
       continue;
 
@@ -55,7 +56,7 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
       // undef, poison, etc.
       Value *Zero = ConstantInt::get(I.getType(), 0);
       ++NumSimplified;
-      I.replaceAllUsesWith(Zero);
+      I.replaceNonMetadataUsesWith(Zero);
       Changed = true;
     }
     if (!DB.isInstructionDead(&I))