[InstCombine] Simplify switch with selects

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -3334,6 +3334,37 @@ Instruction *InstCombinerImpl::visitBranchInst(BranchInst &BI) {
   return nullptr;
 }
 
+// Replaces (switch (select cond, X, C)/(select cond, C, X)) with (switch X) if
+// we can prove that both (switch C) and (switch X) go to the default when cond
+// is false/true.
+static Value *simplifySwitchOnSelectUsingRanges(SwitchInst &SI,
+                                                SelectInst *Select,
+                                                unsigned CstOpIdx) {
+  auto *C = dyn_cast<ConstantInt>(Select->getOperand(CstOpIdx));
+  if (!C)
+    return nullptr;
+
+  BasicBlock *CstBB = SI.findCaseValue(C)->getCaseSuccessor();
+  if (CstBB != SI.getDefaultDest())
+    return nullptr;
+  Value *X = Select->getOperand(3 - CstOpIdx);
+  ICmpInst::Predicate Pred;
+  const APInt *RHSC;
+  if (!match(Select->getCondition(),
+             m_ICmp(Pred, m_Specific(X), m_APInt(RHSC))))
+    return nullptr;
+  if (CstOpIdx == 1)
+    Pred = ICmpInst::getInversePredicate(Pred);
+
+  // See whether we can replace the select with X
+  ConstantRange CR = ConstantRange::makeExactICmpRegion(Pred, *RHSC);
+  for (auto Case : SI.cases())
+    if (!CR.contains(Case.getCaseValue()->getValue()))
+      return nullptr;
+
+  return X;
+}
+
 Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
   Value *Cond = SI.getCondition();
   Value *Op0;
@@ -3407,6 +3438,16 @@ Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
     }
   }
 
+  // Fold switch(select cond, X, Y) into switch(X/Y) if possible
+  if (auto *Select = dyn_cast<SelectInst>(Cond)) {
+    if (Value *V =
+            simplifySwitchOnSelectUsingRanges(SI, Select, /*CstOpIdx=*/1))
+      return replaceOperand(SI, 0, V);
+    if (Value *V =
+            simplifySwitchOnSelectUsingRanges(SI, Select, /*CstOpIdx=*/2))
+      return replaceOperand(SI, 0, V);
+  }
+
   KnownBits Known = computeKnownBits(Cond, 0, &SI);
   unsigned LeadingKnownZeros = Known.countMinLeadingZeros();
   unsigned LeadingKnownOnes = Known.countMinLeadingOnes();

llvm/test/Transforms/InstCombine/switch-select.ll

@@ -4,12 +4,9 @@
 define void @test_ult_rhsc(i8 %x) {
 ; CHECK-LABEL: define void @test_ult_rhsc(
 ; CHECK-SAME: i8 [[X:%.*]]) {
-; CHECK-NEXT:    [[VAL:%.*]] = add nsw i8 [[X]], -2
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[VAL]], 11
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[CMP]], i8 [[VAL]], i8 6
-; CHECK-NEXT:    switch i8 [[COND]], label [[BB1:%.*]] [
-; CHECK-NEXT:      i8 0, label [[BB2:%.*]]
-; CHECK-NEXT:      i8 10, label [[BB3:%.*]]
+; CHECK-NEXT:    switch i8 [[X]], label [[BB1:%.*]] [
+; CHECK-NEXT:      i8 2, label [[BB2:%.*]]
+; CHECK-NEXT:      i8 12, label [[BB3:%.*]]
 ; CHECK-NEXT:    ]
 ; CHECK:       bb1:
 ; CHECK-NEXT:    call void @func1()
@@ -43,9 +40,7 @@ bb3:
 define void @test_eq_lhsc(i8 %x) {
 ; CHECK-LABEL: define void @test_eq_lhsc(
 ; CHECK-SAME: i8 [[X:%.*]]) {
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X]], 4
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[CMP]], i8 6, i8 [[X]]
-; CHECK-NEXT:    switch i8 [[COND]], label [[BB1:%.*]] [
+; CHECK-NEXT:    switch i8 [[X]], label [[BB1:%.*]] [
 ; CHECK-NEXT:      i8 0, label [[BB2:%.*]]
 ; CHECK-NEXT:      i8 10, label [[BB3:%.*]]
 ; CHECK-NEXT:    ]