[OpaquePtr] Support changing load type in InstCombine

[nikic]

Summary

When the load type is changed to ptr, we need the load pointer type to also be ptr, because it's not allowed to create a pointer to an opaque pointer. This is achieved by adjusting the getPointerTo() API to return an opaque pointer for an opaque pointer base type.

[aeubanks]

lgtm with nit

llvm/lib/IR/Type.cpp

728–734

probably worth putting a comment in the header that this method should be removed after the opaque pointer transition

llvm/include/llvm/IR/Type.h

public:
   static PointerType *getInt1PtrTy(LLVMContext &C, unsigned AS = 0);
   static PointerType *getInt8PtrTy(LLVMContext &C, unsigned AS = 0);
   static PointerType *getInt16PtrTy(LLVMContext &C, unsigned AS = 0);
   static PointerType *getInt32PtrTy(LLVMContext &C, unsigned AS = 0);
   static PointerType *getInt64PtrTy(LLVMContext &C, unsigned AS = 0);
 
   /// Return a pointer to the current type. This is equivalent to
   /// PointerType::get(Foo, AddrSpace).
+  /// TODO: Remove this after opaque pointer transition is complete.
   PointerType *getPointerTo(unsigned AddrSpace = 0) const;
 
 private:
   /// Derived types like structures and arrays are sized iff all of the members
   /// of the type are sized as well. Since asking for their size is relatively
   /// uncommon, move this operation out-of-line.
   bool isSizedDerivedType(SmallPtrSetImpl<Type*> *Visited = nullptr) const;
 };

llvm/lib/IR/Type.cpp

PointerType::PointerType(Type *E, unsigned AddrSpace)
   setSubclassData(AddrSpace);
 }
 
 PointerType::PointerType(LLVMContext &C, unsigned AddrSpace)
     : Type(C, PointerTyID), PointeeTy(nullptr) {
   setSubclassData(AddrSpace);
 }
 
-PointerType *Type::getPointerTo(unsigned addrs) const {
-  return PointerType::get(const_cast<Type*>(this), addrs);
+PointerType *Type::getPointerTo(unsigned AddrSpace) const {
+  // Pointer to opaque pointer is opaque pointer.
+  if (auto *PTy = dyn_cast<PointerType>(this))
+    if (PTy->isOpaque())
+      return PointerType::get(getContext(), AddrSpace);
+
+  return PointerType::get(const_cast<Type*>(this), AddrSpace);
 }
 
 bool PointerType::isValidElementType(Type *ElemTy) {
   return !ElemTy->isVoidTy() && !ElemTy->isLabelTy() &&
          !ElemTy->isMetadataTy() && !ElemTy->isTokenTy() &&
          !ElemTy->isX86_AMXTy() &&
          !(ElemTy->isPointerTy() && cast<PointerType>(ElemTy)->isOpaque());
 }
 
 bool PointerType::isLoadableOrStorableType(Type *ElemTy) {
   return isValidElementType(ElemTy) && !ElemTy->isFunctionTy();
 }

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

 /// point the \c InstCombinerImpl currently is using.
 LoadInst *InstCombinerImpl::combineLoadToNewType(LoadInst &LI, Type *NewTy,
                                                  const Twine &Suffix) {
   assert((!LI.isAtomic() || isSupportedAtomicType(NewTy)) &&
          "can't fold an atomic load to requested type");
 
   Value *Ptr = LI.getPointerOperand();
   unsigned AS = LI.getPointerAddressSpace();
+  Type *NewPtrTy = NewTy->getPointerTo(AS);
   Value *NewPtr = nullptr;
   if (!(match(Ptr, m_BitCast(m_Value(NewPtr))) &&
-        NewPtr->getType()->getPointerElementType() == NewTy &&
-        NewPtr->getType()->getPointerAddressSpace() == AS))
-    NewPtr = Builder.CreateBitCast(Ptr, NewTy->getPointerTo(AS));
+        NewPtr->getType() == NewPtrTy))
+    NewPtr = Builder.CreateBitCast(Ptr, NewPtrTy);
 
   LoadInst *NewLoad = Builder.CreateAlignedLoad(
       NewTy, NewPtr, LI.getAlign(), LI.isVolatile(), LI.getName() + Suffix);
   NewLoad->setAtomic(LI.getOrdering(), LI.getSyncScopeID());
   copyMetadataForLoad(*NewLoad, LI);
   return NewLoad;
 }
 

llvm/test/Transforms/InstCombine/opaque-ptr.ll

 }
 
 define ptr @gep_constexpr_2(ptr %a) {
 ; CHECK-LABEL: @gep_constexpr_2(
 ; CHECK-NEXT:    ret ptr bitcast (i8* getelementptr (i8, i8* @g, i64 3) to ptr)
 ;
   ret ptr getelementptr (i8, ptr bitcast (i8* @g to ptr), i32 3)
 }
+
+define ptr @load_bitcast_1(ptr %a) {
+; CHECK-LABEL: @load_bitcast_1(
+; CHECK-NEXT:    [[B1:%.*]] = load ptr, ptr [[A:%.*]], align 8
+; CHECK-NEXT:    ret ptr [[B1]]
+;
+  %b = load i8*, ptr %a
+  %c = bitcast i8* %b to ptr
+  ret ptr %c
+}
+
+define ptr @load_bitcast_2(ptr %a) {
+; CHECK-LABEL: @load_bitcast_2(
+; CHECK-NEXT:    [[C1:%.*]] = load ptr, ptr [[A:%.*]], align 8
+; CHECK-NEXT:    ret ptr [[C1]]
+;
+  %b = bitcast ptr %a to i8**
+  %c = load i8*, i8** %b
+  %d = bitcast i8* %c to ptr
+  ret ptr %d
+}