[PATCH] D75334: Let EarlyCSE fold equivalent freeze instructions

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune created this revision.
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aqjune edited the summary of this revision.
aqjune added reviewers: spatel, reames.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D75334

Files:
  llvm/lib/Transforms/Scalar/EarlyCSE.cpp
  llvm/test/Transforms/EarlyCSE/basic.ll


Index: llvm/test/Transforms/EarlyCSE/basic.ll
===================================================================
--- llvm/test/Transforms/EarlyCSE/basic.ll
+++ llvm/test/Transforms/EarlyCSE/basic.ll
@@ -291,3 +291,14 @@
   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
+}
Index: llvm/lib/Transforms/Scalar/EarlyCSE.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -114,7 +114,7 @@
            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 @@
   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 @@
 
   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.


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