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.
llvm-svn: 119459
diff --git a/llvm/lib/VMCore/Instructions.cpp b/llvm/lib/VMCore/Instructions.cpp
index 7306d86..9fc9ef0 100644
--- a/llvm/lib/VMCore/Instructions.cpp
+++ b/llvm/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 @@
/// 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;
}