Let EarlyCSE fold equivalent freeze instructions

[aqjune]

Summary

This patch makes EarlyCSE fold equivalent freeze instructions.

Another optimization that I think will be useful is to remove freeze if its operand is used as a branch condition or at llvm.assume:

  %c = ...
  br i1 %c, label %A, ..
A:
  %d = freeze %c ; %d can be optimized to %c because %c cannot be poison or undef (or 'br %c' would be UB otherwise)

If it make sense for EarlyCSE to support this as well, I will make a patch for this.

[lebedev.ri]

This LG.

[spatel]

Another optimization that I think will be useful is to remove freeze if its operand is used as a branch condition or at llvm.assume:

That doesn't sound like a CSE optimization. Maybe InstSimplify?

llvm/lib/Transforms/Scalar/EarlyCSE.cpp

static bool canHandle(Instruction *Inst) {
     // This can only handle non-void readnone functions.
     if (CallInst *CI = dyn_cast<CallInst>(Inst))
       return CI->doesNotAccessMemory() && !CI->getType()->isVoidTy();
     return isa<CastInst>(Inst) || isa<UnaryOperator>(Inst) ||
            isa<BinaryOperator>(Inst) || isa<GetElementPtrInst>(Inst) ||
            isa<CmpInst>(Inst) || isa<SelectInst>(Inst) ||
            isa<ExtractElementInst>(Inst) || isa<InsertElementInst>(Inst) ||
            isa<ShuffleVectorInst>(Inst) || isa<ExtractValueInst>(Inst) ||
-           isa<InsertValueInst>(Inst);
+           isa<InsertValueInst>(Inst) || isa<FreezeInst>(Inst);
   }
 };
 
 } // end anonymous namespace
 
 namespace llvm {
 
 template <> struct DenseMapInfo<SimpleValue> {
if (CmpInst::getInversePredicate(Pred) < Pred) {
       std::swap(A, B);
     }
     return hash_combine(Inst->getOpcode(), Pred, X, Y, A, B);
   }
 
   if (CastInst *CI = dyn_cast<CastInst>(Inst))
     return hash_combine(CI->getOpcode(), CI->getType(), CI->getOperand(0));
 
+  if (FreezeInst *FI = dyn_cast<FreezeInst>(Inst))
+    return hash_combine(FI->getOpcode(), FI->getOperand(0));
+
   if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst))
     return hash_combine(EVI->getOpcode(), EVI->getOperand(0),
                         hash_combine_range(EVI->idx_begin(), EVI->idx_end()));
 
   if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst))
     return hash_combine(IVI->getOpcode(), IVI->getOperand(0),
                         IVI->getOperand(1),
                         hash_combine_range(IVI->idx_begin(), IVI->idx_end()));
 
   assert((isa<CallInst>(Inst) || isa<GetElementPtrInst>(Inst) ||
           isa<ExtractElementInst>(Inst) || isa<InsertElementInst>(Inst) ||
-          isa<ShuffleVectorInst>(Inst) || isa<UnaryOperator>(Inst)) &&
+          isa<ShuffleVectorInst>(Inst) || isa<UnaryOperator>(Inst) ||
+          isa<FreezeInst>(Inst)) &&
          "Invalid/unknown instruction");
 
   // Mix in the opcode.
   return hash_combine(
       Inst->getOpcode(),
       hash_combine_range(Inst->value_op_begin(), Inst->value_op_end()));
 }
 

llvm/test/Transforms/EarlyCSE/basic.ll

 ; CHECK: call i32 @reads_c(i32 0)
 ; CHECK: store i32 2, i32* @c, align 4
   %load = load i32, i32* @c, align 4
   store i32 0, i32* @c, align 4
   %call = call i32 @reads_c(i32 0)
   store i32 2, i32* @c, align 4
   ret void
 }
+
+define i1 @cse_freeze(i1 %a) {
+entry:
+; CHECK-LABEL: @cse_freeze(
+; CHECK: %b = freeze i1 %a
+; CHECK: ret i1 %b
+  %b = freeze i1 %a
+  %c = freeze i1 %a
+  %and = and i1 %b, %c
+  ret i1 %and
+}