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[JumpThreading] NFC: Simplify ComputeValueKnownInPredecessorsImpl 

Summary:
ComputeValueKnownInPredecessorsImpl is the main folding mechanism in
JumpThreading.cpp.  To avoid potential infinite recursion while
chasing use-def chains, it uses:

  DenseSet<std::pair<Value *, BasicBlock *>> &RecursionSet

to keep track of Value-BB pairs that we've processed.

Now, when ComputeValueKnownInPredecessorsImpl recursively calls
itself, it always passes BB as is, so the second element is always BB.

This patch simplifes the function by dropping "BasicBlock *" from
RecursionSet.

Reviewers: wmi, efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77699
This commit is contained in:
Kazu Hirata 2020-04-07 17:41:50 -07:00
parent 09f345080e
commit 91eb442fde
2 changed files with 4 additions and 6 deletions
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5
llvm/include/llvm/Transforms/Scalar/JumpThreading.h
View File

@ -128,14 +128,13 @@ public:
bool ComputeValueKnownInPredecessorsImpl(
Value *V, BasicBlock *BB, jumpthreading::PredValueInfo &Result,
jumpthreading::ConstantPreference Preference,
DenseSet<std::pair<Value *, BasicBlock *>> &RecursionSet,
Instruction *CxtI = nullptr);
DenseSet<Value *> &RecursionSet, Instruction *CxtI = nullptr);
bool
ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
jumpthreading::PredValueInfo &Result,
jumpthreading::ConstantPreference Preference,
Instruction *CxtI = nullptr) {
DenseSet<std::pair<Value *, BasicBlock *>> RecursionSet;
DenseSet<Value *> RecursionSet;
return ComputeValueKnownInPredecessorsImpl(V, BB, Result, Preference,
RecursionSet, CxtI);
}

5
llvm/lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -611,14 +611,13 @@ static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
/// This returns true if there were any known values.
bool JumpThreadingPass::ComputeValueKnownInPredecessorsImpl(
Value *V, BasicBlock *BB, PredValueInfo &Result,
ConstantPreference Preference,
DenseSet<std::pair<Value *, BasicBlock *>> &RecursionSet,
ConstantPreference Preference, DenseSet<Value *> &RecursionSet,
Instruction *CxtI) {
// This method walks up use-def chains recursively. Because of this, we could
// get into an infinite loop going around loops in the use-def chain. To
// prevent this, keep track of what (value, block) pairs we've already visited
// and terminate the search if we loop back to them
if (!RecursionSet.insert(std::make_pair(V, BB)).second)
if (!RecursionSet.insert(V).second)
return false;
// If V is a constant, then it is known in all predecessors.