Let EarlyCSE fold equivalent freeze instructions

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.

Reviewers: spatel, reames, lebedev.ri

Reviewed By: lebedev.ri

Subscribers: lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75334

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index 588939d..39c86c7 100644 (file)
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -114,7 +114,7 @@ struct SimpleValue {
            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);
   }
 };
 
@@ -268,6 +268,9 @@ static unsigned getHashValueImpl(SimpleValue Val) {
   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()));
@@ -279,7 +282,8 @@ static unsigned getHashValueImpl(SimpleValue Val) {
 
   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.

diff --git a/llvm/test/Transforms/EarlyCSE/basic.ll b/llvm/test/Transforms/EarlyCSE/basic.ll
index 5797475..25d2dc3 100644 (file)
--- a/llvm/test/Transforms/EarlyCSE/basic.ll
+++ b/llvm/test/Transforms/EarlyCSE/basic.ll
@@ -291,3 +291,14 @@ entry:
   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
+}