[InstCombine] Factor out unreachable inst idiom creation [NFC]

In InstCombine, we use an idiom of "store i1 true, i1 undef" to indicate we've found a path which we've proven unreachable.  We can't actually insert the unreachable instruction since that would require changing the CFG.  We leave that to simplifycfg later.

This just factors out that idiom creation so we don't duplicate the same mostly undocument idiom creation in multiple places.

llvm-svn: 358600

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -4324,9 +4324,7 @@ Instruction *InstCombiner::visitCallBase(CallBase &Call) {
         // variety of reasons (e.g. it may be written in assembly).
         !CalleeF->isDeclaration()) {
       Instruction *OldCall = &Call;
-      new StoreInst(ConstantInt::getTrue(Callee->getContext()),
-                UndefValue::get(Type::getInt1PtrTy(Callee->getContext())),
-                                  OldCall);
+      CreateNonTerminatorUnreachable(OldCall);
       // If OldCall does not return void then replaceAllUsesWith undef.
       // This allows ValueHandlers and custom metadata to adjust itself.
       if (!OldCall->getType()->isVoidTy())
@@ -4356,13 +4354,8 @@ Instruction *InstCombiner::visitCallBase(CallBase &Call) {
       return nullptr;
     }
 
-    // This instruction is not reachable, just remove it.  We insert a store to
-    // undef so that we know that this code is not reachable, despite the fact
-    // that we can't modify the CFG here.
-    new StoreInst(ConstantInt::getTrue(Callee->getContext()),
-                  UndefValue::get(Type::getInt1PtrTy(Callee->getContext())),
-                  &Call);
-
+    // This instruction is not reachable, just remove it.
+    CreateNonTerminatorUnreachable(&Call);
     return eraseInstFromFunction(Call);
   }
 

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

@@ -656,6 +656,16 @@ public:
     return InsertValueInst::Create(Struct, Result, 0);
   }
 
+  /// Create and insert the idiom we use to indicate a block is unreachable
+  /// without having to rewrite the CFG from within InstCombine.
+  void CreateNonTerminatorUnreachable(Instruction *InsertAt) {
+    auto &Ctx = InsertAt->getContext();
+    new StoreInst(ConstantInt::getTrue(Ctx),
+                  UndefValue::get(Type::getInt1PtrTy(Ctx)),
+                  InsertAt);
+  }
+
+
   /// Combiner aware instruction erasure.
   ///
   /// When dealing with an instruction that has side effects or produces a void

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2475,9 +2475,8 @@ Instruction *InstCombiner::visitFree(CallInst &FI) {
 
   // free undef -> unreachable.
   if (isa<UndefValue>(Op)) {
-    // Insert a new store to null because we cannot modify the CFG here.
-    Builder.CreateStore(ConstantInt::getTrue(FI.getContext()),
-                        UndefValue::get(Type::getInt1PtrTy(FI.getContext())));
+    // Leave a marker since we can't modify the CFG here.
+    CreateNonTerminatorUnreachable(&FI);
     return eraseInstFromFunction(FI);
   }