[PATCH] D85533: [InstCombine] Optimize select(freeze(icmp eq/ne x, y), x, y)

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune created this revision.
aqjune added reviewers: nikic, lebedev.ri, spatel, efriedma.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.
aqjune requested review of this revision.

This patch adds an optimization that folds select(freeze(icmp eq/ne x, y), x, y)
to x or y.
This was needed to resolve slowdown after D84940 <https://reviews.llvm.org/D84940> is applied.

I tried to bake this logic into foldSelectInstWithICmp, but it wasn't clear.
This patch conservatively writes the pattern in a separate function,
foldSelectWithFrozenICmp.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D85533

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/select.ll


Index: llvm/test/Transforms/InstCombine/select.ll
===================================================================
--- llvm/test/Transforms/InstCombine/select.ll
+++ llvm/test/Transforms/InstCombine/select.ll
@@ -2540,9 +2540,7 @@
 
 define i32 @select_freeze_icmp_eq(i32 %x, i32 %y) {
 ; CHECK-LABEL: @select_freeze_icmp_eq(
-; CHECK-NEXT:    [[C:%.*]] = icmp eq i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[C_FR:%.*]] = freeze i1 [[C]]
-; CHECK-NEXT:    [[V:%.*]] = select i1 [[C_FR]], i32 [[X]], i32 [[Y]]
+; CHECK-NEXT:    [[V:%.*]] = freeze i32 [[Y:%.*]]
 ; CHECK-NEXT:    ret i32 [[V]]
 ;
   %c = icmp eq i32 %x, %y
@@ -2553,9 +2551,7 @@
 
 define i32 @select_freeze_icmp_ne(i32 %x, i32 %y) {
 ; CHECK-LABEL: @select_freeze_icmp_ne(
-; CHECK-NEXT:    [[C:%.*]] = icmp ne i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[C_FR:%.*]] = freeze i1 [[C]]
-; CHECK-NEXT:    [[V:%.*]] = select i1 [[C_FR]], i32 [[X]], i32 [[Y]]
+; CHECK-NEXT:    [[V:%.*]] = freeze i32 [[X:%.*]]
 ; CHECK-NEXT:    ret i32 [[V]]
 ;
   %c = icmp ne i32 %x, %y
Index: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2526,6 +2526,28 @@
   return nullptr;
 }
 
+static Value *foldSelectWithFrozenICmp(SelectInst &Sel, InstCombiner::BuilderTy &Builder) {
+  FreezeInst *FI = dyn_cast<FreezeInst>(Sel.getCondition());
+  if (!FI)
+    return nullptr;
+
+  Value *Cond = FI->getOperand(0);
+  Value *TrueVal = Sel.getTrueValue(), *FalseVal = Sel.getFalseValue();
+
+  // select (freeze(x == y)), x, y --> freeze(y)
+  // select (freeze(x != y)), x, y --> freeze(x)
+  CmpInst::Predicate Pred;
+  if (match(Cond, m_c_ICmp(Pred, m_Specific(TrueVal), m_Specific(FalseVal))) &&
+      (Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE)) {
+    auto *NewFr = Builder.CreateFreeze(Pred == ICmpInst::ICMP_EQ ?
+                                       FalseVal : TrueVal);
+    NewFr->takeName(&Sel);
+    return NewFr;
+  }
+
+  return nullptr;
+}
+
 Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   Value *CondVal = SI.getCondition();
   Value *TrueVal = SI.getTrueValue();
@@ -2977,5 +2999,8 @@
   if (Instruction *PN = foldSelectToPhi(SI, DT, Builder))
     return replaceInstUsesWith(SI, PN);
 
+  if (Value *Fr = foldSelectWithFrozenICmp(SI, Builder))
+    return replaceInstUsesWith(SI, Fr);
+
   return nullptr;
 }


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