Add InstCombine/InstructionSimplify support for Freeze Instruction

Summary:
- Add llvm::SimplifyFreezeInst
- Add InstCombiner::visitFreeze
- Add llvm tests

Reviewers: majnemer, sanjoy, reames, lebedev.ri, spatel

Reviewed By: reames, lebedev.ri

Subscribers: reames, lebedev.ri, filcab, regehr, trentxintong, llvm-commits

Differential Revision: https://reviews.llvm.org/D29013

2 files changed, 30 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 83b767a8fd1..3ef8f9b0050 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -5241,6 +5241,19 @@ Value *llvm::SimplifyCall(CallBase *Call, const SimplifyQuery &Q) {
   return ConstantFoldCall(Call, F, ConstantArgs, Q.TLI);
 }
 
+/// Given operands for a Freeze, see if we can fold the result.
+static Value *SimplifyFreezeInst(Value *Op0) {
+  // Use a utility function defined in ValueTracking.
+  if (llvm::isGuaranteedNotToBeUndefOrPoison(Op0))
+    return Op0;
+  // We have room for improvement.
+  return nullptr;
+}
+
+Value *llvm::SimplifyFreezeInst(Value *Op0, const SimplifyQuery &Q) {
+  return ::SimplifyFreezeInst(Op0);
+}
+
 /// See if we can compute a simplified version of this instruction.
 /// If not, this returns null.
 
@@ -5383,6 +5396,9 @@ Value *llvm::SimplifyInstruction(Instruction *I, const SimplifyQuery &SQ,
     Result = SimplifyCall(cast<CallInst>(I), Q);
     break;
   }
+  case Instruction::Freeze:
+    Result = SimplifyFreezeInst(I->getOperand(0), Q);
+    break;
 #define HANDLE_CAST_INST(num, opc, clas) case Instruction::opc:
 #include "llvm/IR/Instruction.def"
 #undef HANDLE_CAST_INST
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index cc5acf5d010..b26c6294dce 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4221,6 +4221,20 @@ bool llvm::isOverflowIntrinsicNoWrap(const WithOverflowInst *WO,
   return llvm::any_of(GuardingBranches, AllUsesGuardedByBranch);
 }
 
+bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V) {
+  // If the value is a freeze instruction, then it can never
+  // be undef or poison.
+  if (isa<FreezeInst>(V))
+    return true;
+  // TODO: Some instructions are guaranteed to return neither undef
+  // nor poison if their arguments are not poison/undef.
+
+  // TODO: Deal with other Constant subclasses.
+  if (isa<ConstantInt>(V) || isa<GlobalVariable>(V))
+    return true;
+
+  return false;
+}
 
 OverflowResult llvm::computeOverflowForSignedAdd(const AddOperator *Add,
                                                  const DataLayout &DL,