[PATCH] D75401: [InstSimplify] Remove freeze if its operand is used as a branch condition

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune created this revision.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.
aqjune added reviewers: xbolva00, spatel, lebedev.ri, reames.

    fr = freeze(c)
    br i1 c, BB1, BB2:
  BB1:
    use1(fr)
  BB2:
    use2(fr)

In BB1 and BB2, fr can be replaced with c because if c was undef or poison the branch would have triggered UB.

Checked with Alive2


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D75401

Files:
  llvm/lib/Analysis/InstructionSimplify.cpp
  llvm/test/Transforms/InstSimplify/freeze.ll


Index: llvm/test/Transforms/InstSimplify/freeze.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/freeze.ll
+++ llvm/test/Transforms/InstSimplify/freeze.ll
@@ -18,3 +18,19 @@
   %x = freeze i32 10
   ret i32 %x
 }
+
+define i1 @brcond(i1 %c) {
+; CHECK-LABEL: @brcond(
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[A:%.*]], label [[B:%.*]]
+; CHECK:       A:
+; CHECK-NEXT:    ret i1 [[C]]
+; CHECK:       B:
+; CHECK-NEXT:    ret i1 [[C]]
+;
+  %f = freeze i1 %c
+  br i1 %c, label %A, label %B
+A:
+  ret i1 %f
+B:
+  ret i1 %f
+}
Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -5361,16 +5361,29 @@
 }
 
 /// Given operands for a Freeze, see if we can fold the result.
-static Value *SimplifyFreezeInst(Value *Op0) {
+static Value *SimplifyFreezeInst(Value *Op0, const SimplifyQuery &Q) {
   // Use a utility function defined in ValueTracking.
   if (llvm::isGuaranteedNotToBeUndefOrPoison(Op0))
     return Op0;
+
+  if (Q.CxtI) {
+    // Given freeze(Op0), if Op0 is ever used as a branch condition, Op0 cannot
+    // be undef or poison.
+    //   v = freeze(cond)
+    //   br cond, BB1, BB2
+    // BB1:
+    //   use(v) -> use(cond)
+    auto BI = dyn_cast<BranchInst>(Q.CxtI->getParent()->getTerminator());
+    if (BI && BI->isConditional() && BI->getCondition() == Op0)
+      return Op0;
+  }
+
   // We have room for improvement.
   return nullptr;
 }
 
 Value *llvm::SimplifyFreezeInst(Value *Op0, const SimplifyQuery &Q) {
-  return ::SimplifyFreezeInst(Op0);
+  return ::SimplifyFreezeInst(Op0, Q);
 }
 
 /// See if we can compute a simplified version of this instruction.


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