[InstCombine] Insert instructions before adding them to worklist

Summary:
This patch adds instructions to the InstCombine worklist after they are properly inserted. This way we don't get `<badref>`s printed when logging added instructions.
It also adds a check in `Worklist::Add` that ensures that all added instructions have parents.

Simple test case that illustrates the difference when run with `--debug-only=instcombine`:

```
define i32 @test35(i32 %a, i32 %b) {
  %1 = or i32 %a, 1135
  %2 = or i32 %1, %b
  ret i32 %2
}
```

Before this patch:
```
INSTCOMBINE ITERATION #1 on test35
IC: ADDING: 3 instrs to worklist
IC: Visiting:   %1 = or i32 %a, 1135
IC: Visiting:   %2 = or i32 %1, %b
IC: ADD:   %2 = or i32 %a, %b
IC: Old =   %3 = or i32 %1, %b
    New =   <badref> = or i32 %2, 1135
IC: ADD:   <badref> = or i32 %2, 1135
...
```

With this patch:
```
INSTCOMBINE ITERATION #1 on test35
IC: ADDING: 3 instrs to worklist
IC: Visiting:   %1 = or i32 %a, 1135
IC: Visiting:   %2 = or i32 %1, %b
IC: ADD:   %2 = or i32 %a, %b
IC: Old =   %3 = or i32 %1, %b
    New =   <badref> = or i32 %2, 1135
IC: ADD:   %3 = or i32 %2, 1135
...
```

Reviewers: fhahn, davide, spatel, foad, grosser, nikic

Reviewed By: nikic

Subscribers: nikic, lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71093

llvm/include/llvm/Transforms/InstCombine/InstCombineWorklist.h

@@ -24,8 +24,8 @@ namespace llvm {
 /// InstCombineWorklist - This is the worklist management logic for
 /// InstCombine.
 class InstCombineWorklist {
-  SmallVector<Instruction*, 256> Worklist;
-  DenseMap<Instruction*, unsigned> WorklistMap;
+  SmallVector<Instruction *, 256> Worklist;
+  DenseMap<Instruction *, unsigned> WorklistMap;
 
 public:
   InstCombineWorklist() = default;
@@ -38,6 +38,9 @@ class InstCombineWorklist {
   /// Add - Add the specified instruction to the worklist if it isn't already
   /// in it.
   void Add(Instruction *I) {
+    assert(I);
+    assert(I->getParent() && "Instruction not inserted yet?");
+
     if (WorklistMap.insert(std::make_pair(I, Worklist.size())).second) {
       LLVM_DEBUG(dbgs() << "IC: ADD: " << *I << '\n');
       Worklist.push_back(I);

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -1189,15 +1189,17 @@ Instruction *InstCombiner::visitZExt(ZExtInst &CI) {
       // zext (or icmp, icmp) -> or (zext icmp), (zext icmp)
       Value *LCast = Builder.CreateZExt(LHS, CI.getType(), LHS->getName());
       Value *RCast = Builder.CreateZExt(RHS, CI.getType(), RHS->getName());
-      BinaryOperator *Or = BinaryOperator::Create(Instruction::Or, LCast, RCast);
+      Value *Or = Builder.CreateOr(LCast, RCast, CI.getName());
+      if (auto *OrInst = dyn_cast<Instruction>(Or))
+        Builder.SetInsertPoint(OrInst);
 
       // Perform the elimination.
       if (auto *LZExt = dyn_cast<ZExtInst>(LCast))
         transformZExtICmp(LHS, *LZExt);
       if (auto *RZExt = dyn_cast<ZExtInst>(RCast))
         transformZExtICmp(RHS, *RZExt);
 
-      return Or;
+      return replaceInstUsesWith(CI, Or);
     }
   }
 

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -3361,10 +3361,6 @@ bool InstCombiner::run() {
         // Move the name to the new instruction first.
         Result->takeName(I);
 
-        // Push the new instruction and any users onto the worklist.
-        Worklist.AddUsersToWorkList(*Result);
-        Worklist.Add(Result);
-
         // Insert the new instruction into the basic block...
         BasicBlock *InstParent = I->getParent();
         BasicBlock::iterator InsertPos = I->getIterator();
@@ -3376,6 +3372,10 @@ bool InstCombiner::run() {
 
         InstParent->getInstList().insert(InsertPos, Result);
 
+        // Push the new instruction and any users onto the worklist.
+        Worklist.AddUsersToWorkList(*Result);
+        Worklist.Add(Result);
+
         eraseInstFromFunction(*I);
       } else {
         LLVM_DEBUG(dbgs() << "IC: Mod = " << OrigI << '\n'

llvm/test/Transforms/InstCombine/zext-or-icmp.ll

@@ -15,7 +15,7 @@ define i8 @zext_or_icmp_icmp(i8 %a, i8 %b) {
 ; CHECK-NEXT:    %toBool2 = icmp eq i8 %b, 0
 ; CHECK-NEXT:    %toBool22 = zext i1 %toBool2 to i8
 ; CHECK-NEXT:    %1 = xor i8 %mask, 1
-; CHECK-NEXT:    %zext = or i8 %1, %toBool22
+; CHECK-NEXT:    %zext3 = or i8 %1, %toBool22
 ; CHECK-NEXT:    ret i8 %zext
 }