When performing a conditional branch depending on the value of a comparison
%cmp (eg: A==B) we already replace %cmp with "true" under the true edge, and
with "false" under the false edge. This change enhances this to replace the
negated compare (A!=B) with "false" under the true edge and "true" under the
false edge. Reported to improve perlbench results by 1%.
llvm-svn: 151517
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 162ee2b..68281e6 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -96,12 +96,17 @@
uint32_t nextValueNumber;
Expression create_expression(Instruction* I);
+ Expression create_cmp_expression(unsigned Opcode,
+ CmpInst::Predicate Predicate,
+ Value *LHS, Value *RHS);
Expression create_extractvalue_expression(ExtractValueInst* EI);
uint32_t lookup_or_add_call(CallInst* C);
public:
ValueTable() : nextValueNumber(1) { }
uint32_t lookup_or_add(Value *V);
uint32_t lookup(Value *V) const;
+ uint32_t lookup_or_add_cmp(unsigned Opcode, CmpInst::Predicate Pred,
+ Value *LHS, Value *RHS);
void add(Value *V, uint32_t num);
void clear();
void erase(Value *v);
@@ -181,6 +186,25 @@
return e;
}
+Expression ValueTable::create_cmp_expression(unsigned Opcode,
+ CmpInst::Predicate Predicate,
+ Value *LHS, Value *RHS) {
+ assert((Opcode == Instruction::ICmp || Opcode == Instruction::FCmp) &&
+ "Not a comparison!");
+ Expression e;
+ e.type = CmpInst::makeCmpResultType(LHS->getType());
+ e.varargs.push_back(lookup_or_add(LHS));
+ e.varargs.push_back(lookup_or_add(RHS));
+
+ // Sort the operand value numbers so x<y and y>x get the same value number.
+ if (e.varargs[0] > e.varargs[1]) {
+ std::swap(e.varargs[0], e.varargs[1]);
+ Predicate = CmpInst::getSwappedPredicate(Predicate);
+ }
+ e.opcode = (Opcode << 8) | Predicate;
+ return e;
+}
+
Expression ValueTable::create_extractvalue_expression(ExtractValueInst *EI) {
assert(EI != 0 && "Not an ExtractValueInst?");
Expression e;
@@ -430,6 +454,19 @@
return VI->second;
}
+/// lookup_or_add_cmp - Returns the value number of the given comparison,
+/// assigning it a new number if it did not have one before. Useful when
+/// we deduced the result of a comparison, but don't immediately have an
+/// instruction realizing that comparison to hand.
+uint32_t ValueTable::lookup_or_add_cmp(unsigned Opcode,
+ CmpInst::Predicate Predicate,
+ Value *LHS, Value *RHS) {
+ Expression exp = create_cmp_expression(Opcode, Predicate, LHS, RHS);
+ uint32_t& e = expressionNumbering[exp];
+ if (!e) e = nextValueNumber++;
+ return e;
+}
+
/// clear - Remove all entries from the ValueTable.
void ValueTable::clear() {
valueNumbering.clear();
@@ -1987,14 +2024,35 @@
}
// If we are propagating an equality like "(A == B)" == "true" then also
- // propagate the equality A == B.
+ // propagate the equality A == B. When propagating a comparison such as
+ // "(A >= B)" == "true", replace all instances of "A < B" with "false".
if (ICmpInst *Cmp = dyn_cast<ICmpInst>(LHS)) {
- // Only equality comparisons are supported.
+ Value *Op0 = Cmp->getOperand(0), *Op1 = Cmp->getOperand(1);
+
+ // If "A == B" is known true, or "A != B" is known false, then replace
+ // A with B everywhere in the scope.
if ((isKnownTrue && Cmp->getPredicate() == CmpInst::ICMP_EQ) ||
- (isKnownFalse && Cmp->getPredicate() == CmpInst::ICMP_NE)) {
- Value *Op0 = Cmp->getOperand(0), *Op1 = Cmp->getOperand(1);
+ (isKnownFalse && Cmp->getPredicate() == CmpInst::ICMP_NE))
Changed |= propagateEquality(Op0, Op1, Root);
+
+ // If "A >= B" is known true, replace "A < B" with false everywhere.
+ CmpInst::Predicate NotPred = Cmp->getInversePredicate();
+ Constant *NotVal = ConstantInt::get(Cmp->getType(), isKnownFalse);
+ // Since we don't have the instruction "A < B" immediately to hand, work out
+ // the value number that it would have and use that to find an appropriate
+ // instruction (if any).
+ unsigned Num = VN.lookup_or_add_cmp(Cmp->getOpcode(), NotPred, Op0, Op1);
+ Value *NotCmp = findLeader(Root, Num);
+ if (NotCmp && isa<Instruction>(NotCmp)) {
+ unsigned NumReplacements =
+ replaceAllDominatedUsesWith(NotCmp, NotVal, Root);
+ Changed |= NumReplacements > 0;
+ NumGVNEqProp += NumReplacements;
}
+ // Ensure that any instruction in scope that gets the "A < B" value number
+ // is replaced with false.
+ addToLeaderTable(Num, NotVal, Root);
+
return Changed;
}