[CVP] processSwitch: Remove default case when switch cover all possible values.

Differential Revision: https://reviews.llvm.org/D106056

llvm/include/llvm/Transforms/Utils/Local.h

@@ -55,6 +55,7 @@ class MDNode;
 class MemorySSAUpdater;
 class PHINode;
 class StoreInst;
+class SwitchInst;
 class TargetLibraryInfo;
 class TargetTransformInfo;
 
@@ -236,6 +237,10 @@ CallInst *createCallMatchingInvoke(InvokeInst *II);
 /// This function converts the specified invoek into a normall call.
 void changeToCall(InvokeInst *II, DomTreeUpdater *DTU = nullptr);
 
+/// This function removes the default destination from the specified switch.
+void createUnreachableSwitchDefault(SwitchInst *Switch,
+                                    DomTreeUpdater *DTU = nullptr);
+
 ///===---------------------------------------------------------------------===//
 ///  Dbg Intrinsic utilities
 ///

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -341,7 +341,13 @@ static bool processSwitch(SwitchInst *I, LazyValueInfo *LVI,
     // ConstantFoldTerminator() as the underlying SwitchInst can be changed.
     SwitchInstProfUpdateWrapper SI(*I);
 
-    for (auto CI = SI->case_begin(), CE = SI->case_end(); CI != CE;) {
+    APInt Low =
+        APInt::getSignedMaxValue(Cond->getType()->getScalarSizeInBits());
+    APInt High =
+        APInt::getSignedMinValue(Cond->getType()->getScalarSizeInBits());
+
+    SwitchInst::CaseIt CI = SI->case_begin();
+    for (auto CE = SI->case_end(); CI != CE;) {
       ConstantInt *Case = CI->getCaseValue();
       LazyValueInfo::Tristate State =
           LVI->getPredicateAt(CmpInst::ICMP_EQ, Cond, Case, I,
@@ -374,9 +380,28 @@ static bool processSwitch(SwitchInst *I, LazyValueInfo *LVI,
         break;
       }
 
+      // Get Lower/Upper bound from switch cases.
+      Low = APIntOps::smin(Case->getValue(), Low);
+      High = APIntOps::smax(Case->getValue(), High);
+
       // Increment the case iterator since we didn't delete it.
       ++CI;
     }
+
+    // Try to simplify default case as unreachable
+    if (CI == SI->case_end() && SI->getNumCases() != 0 &&
+        !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg())) {
+      const ConstantRange SIRange =
+          LVI->getConstantRange(SI->getCondition(), SI);
+
+      // If the numbered switch cases cover the entire range of the condition,
+      // then the default case is not reachable.
+      if (SIRange.getSignedMin() == Low && SIRange.getSignedMax() == High &&
+          SI->getNumCases() == High - Low + 1) {
+        createUnreachableSwitchDefault(SI, &DTU);
+        Changed = true;
+      }
+    }
   }
 
   if (Changed)

llvm/lib/Transforms/Utils/Local.cpp

@@ -2182,6 +2182,30 @@ void llvm::changeToCall(InvokeInst *II, DomTreeUpdater *DTU) {
     DTU->applyUpdates({{DominatorTree::Delete, BB, UnwindDestBB}});
 }
 
+void llvm::createUnreachableSwitchDefault(SwitchInst *Switch,
+                                          DomTreeUpdater *DTU) {
+  LLVM_DEBUG(dbgs() << "Switch default is dead.\n");
+  auto *BB = Switch->getParent();
+  BasicBlock *NewDefaultBlock = SplitBlockPredecessors(
+      Switch->getDefaultDest(), Switch->getParent(), "", DTU);
+  auto *OrigDefaultBlock = Switch->getDefaultDest();
+  Switch->setDefaultDest(&*NewDefaultBlock);
+  if (DTU)
+    DTU->applyUpdates({{DominatorTree::Insert, BB, &*NewDefaultBlock},
+                       {DominatorTree::Delete, BB, OrigDefaultBlock}});
+
+  SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(), DTU);
+  SmallVector<DominatorTree::UpdateType, 2> Updates;
+  if (DTU)
+    for (auto *Successor : successors(NewDefaultBlock))
+      Updates.push_back({DominatorTree::Delete, NewDefaultBlock, Successor});
+  auto *NewTerminator = NewDefaultBlock->getTerminator();
+  new UnreachableInst(Switch->getContext(), NewTerminator);
+  NewTerminator->eraseFromParent();
+  if (DTU)
+    DTU->applyUpdates(Updates);
+}
+
 BasicBlock *llvm::changeToInvokeAndSplitBasicBlock(CallInst *CI,
                                                    BasicBlock *UnwindEdge,
                                                    DomTreeUpdater *DTU) {

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -4743,29 +4743,6 @@ static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
   return true;
 }
 
-static void createUnreachableSwitchDefault(SwitchInst *Switch,
-                                           DomTreeUpdater *DTU) {
-  LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
-  auto *BB = Switch->getParent();
-  BasicBlock *NewDefaultBlock = SplitBlockPredecessors(
-      Switch->getDefaultDest(), Switch->getParent(), "", DTU);
-  auto *OrigDefaultBlock = Switch->getDefaultDest();
-  Switch->setDefaultDest(&*NewDefaultBlock);
-  if (DTU)
-    DTU->applyUpdates({{DominatorTree::Insert, BB, &*NewDefaultBlock},
-                       {DominatorTree::Delete, BB, OrigDefaultBlock}});
-  SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(), DTU);
-  SmallVector<DominatorTree::UpdateType, 2> Updates;
-  if (DTU)
-    for (auto *Successor : successors(NewDefaultBlock))
-      Updates.push_back({DominatorTree::Delete, NewDefaultBlock, Successor});
-  auto *NewTerminator = NewDefaultBlock->getTerminator();
-  new UnreachableInst(Switch->getContext(), NewTerminator);
-  EraseTerminatorAndDCECond(NewTerminator);
-  if (DTU)
-    DTU->applyUpdates(Updates);
-}
-
 /// Turn a switch with two reachable destinations into an integer range
 /// comparison and branch.
 bool SimplifyCFGOpt::TurnSwitchRangeIntoICmp(SwitchInst *SI,

llvm/test/Transforms/CorrelatedValuePropagation/basic.ll

@@ -382,7 +382,7 @@ define i32 @switch_range(i32 %cond) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[S:%.*]] = urem i32 [[COND:%.*]], 3
 ; CHECK-NEXT:    [[S1:%.*]] = add nuw nsw i32 [[S]], 1
-; CHECK-NEXT:    switch i32 [[S1]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    switch i32 [[S1]], label [[UNREACHABLE1:%.*]] [
 ; CHECK-NEXT:    i32 1, label [[EXIT1:%.*]]
 ; CHECK-NEXT:    i32 2, label [[EXIT2:%.*]]
 ; CHECK-NEXT:    i32 3, label [[EXIT1]]
@@ -391,6 +391,43 @@ define i32 @switch_range(i32 %cond) {
 ; CHECK-NEXT:    ret i32 1
 ; CHECK:       exit2:
 ; CHECK-NEXT:    ret i32 2
+; CHECK:       unreachable1:
+; CHECK-NEXT:    unreachable
+; CHECK:       unreachable1.split:
+; CHECK-NEXT:    br label [[UNREACHABLE:%.*]]
+; CHECK:       unreachable:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %s = urem i32 %cond, 3
+  %s1 = add i32 %s, 1
+  switch i32 %s1, label %unreachable [
+  i32 1, label %exit1
+  i32 2, label %exit2
+  i32 3, label %exit1
+  ]
+
+exit1:
+  ret i32 1
+exit2:
+  ret i32 2
+unreachable:
+  ret i32 0
+}
+
+define i32 @switch_range_not_full(i32 %cond) {
+; CHECK-LABEL: @switch_range_not_full(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[S:%.*]] = urem i32 [[COND:%.*]], 3
+; CHECK-NEXT:    [[S1:%.*]] = add nuw nsw i32 [[S]], 1
+; CHECK-NEXT:    switch i32 [[S1]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i32 1, label [[EXIT1:%.*]]
+; CHECK-NEXT:    i32 3, label [[EXIT2:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       exit1:
+; CHECK-NEXT:    ret i32 1
+; CHECK:       exit2:
+; CHECK-NEXT:    ret i32 2
 ; CHECK:       unreachable:
 ; CHECK-NEXT:    ret i32 0
 ;
@@ -399,8 +436,7 @@ entry:
   %s1 = add i32 %s, 1
   switch i32 %s1, label %unreachable [
   i32 1, label %exit1
-  i32 2, label %exit2
-  i32 3, label %exit1
+  i32 3, label %exit2
   ]
 
 exit1: