Fix a layering violation: hasConstantValue, which is part of the PHINode

class, uses DominatorTree which is an analysis.  This change moves all of
the tricky hasConstantValue logic to SimplifyInstruction, and replaces it
with a very simple literal implementation.  I already taught users of
hasConstantValue that need tricky stuff to use SimplifyInstruction instead.
I didn't update InlineFunction because the IR looks like it might be in a
funky state at the point it calls hasConstantValue, which makes calling
SimplifyInstruction dangerous since it can in theory do a lot of tricky
reasoning.  This may be a pessimization, for example in the case where
all phi node operands are either undef or a fixed constant.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119459 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 56 deletions

diff --git a/lib/VMCore/Instructions.cpp b/lib/VMCore/Instructions.cpp
index 7306d86..9fc9ef0 100644
--- a/lib/VMCore/Instructions.cpp
+++ b/lib/VMCore/Instructions.cpp
@@ -19,7 +19,6 @@
 #include "llvm/Instructions.h"
 #include "llvm/Module.h"
 #include "llvm/Operator.h"
-#include "llvm/Analysis/Dominators.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/CallSite.h"
 #include "llvm/Support/ConstantRange.h"
@@ -164,61 +163,13 @@ void PHINode::resizeOperands(unsigned NumOps) {
 
 /// hasConstantValue - If the specified PHI node always merges together the same
 /// value, return the value, otherwise return null.
-///
-/// If the PHI has undef operands, but all the rest of the operands are
-/// some unique value, return that value if it can be proved that the
-/// value dominates the PHI. If DT is null, use a conservative check,
-/// otherwise use DT to test for dominance.
-///
-Value *PHINode::hasConstantValue(const DominatorTree *DT) const {
-  // If the PHI node only has one incoming value, eliminate the PHI node.
-  if (getNumIncomingValues() == 1) {
-    if (getIncomingValue(0) != this)   // not  X = phi X
-      return getIncomingValue(0);
-    return UndefValue::get(getType());  // Self cycle is dead.
-  }
-      
-  // Otherwise if all of the incoming values are the same for the PHI, replace
-  // the PHI node with the incoming value.
-  //
-  Value *InVal = 0;
-  bool HasUndefInput = false;
-  for (unsigned i = 0, e = getNumIncomingValues(); i != e; ++i)
-    if (isa<UndefValue>(getIncomingValue(i))) {
-      HasUndefInput = true;
-    } else if (getIncomingValue(i) != this) { // Not the PHI node itself...
-      if (InVal && getIncomingValue(i) != InVal)
-        return 0;  // Not the same, bail out.
-      InVal = getIncomingValue(i);
-    }
-  
-  // The only case that could cause InVal to be null is if we have a PHI node
-  // that only has entries for itself.  In this case, there is no entry into the
-  // loop, so kill the PHI.
-  //
-  if (InVal == 0) InVal = UndefValue::get(getType());
-  
-  // If we have a PHI node like phi(X, undef, X), where X is defined by some
-  // instruction, we cannot always return X as the result of the PHI node.  Only
-  // do this if X is not an instruction (thus it must dominate the PHI block),
-  // or if the client is prepared to deal with this possibility.
-  if (!HasUndefInput || !isa<Instruction>(InVal))
-    return InVal;
-  
-  Instruction *IV = cast<Instruction>(InVal);
-  if (DT) {
-    // We have a DominatorTree. Do a precise test.
-    if (!DT->dominates(IV, this))
-      return 0;
-  } else {
-    // If it is in the entry block, it obviously dominates everything.
-    if (IV->getParent() != &IV->getParent()->getParent()->getEntryBlock() ||
-        isa<InvokeInst>(IV))
-      return 0;   // Cannot guarantee that InVal dominates this PHINode.
-  }
-
-  // All of the incoming values are the same, return the value now.
-  return InVal;
+Value *PHINode::hasConstantValue() const {
+  // Exploit the fact that phi nodes always have at least one entry.
+  Value *ConstantValue = getIncomingValue(0);
+  for (unsigned i = 1, e = getNumIncomingValues(); i != e; ++i)
+    if (getIncomingValue(i) != ConstantValue)
+      return 0; // Incoming values not all the same.
+  return ConstantValue;
 }