Add support for non-zero null pointer for C and OpenCL

In amdgcn target, null pointers in global, constant, and generic address space take value 0 but null pointers in private and local address space take value -1. Currently LLVM assumes all null pointers take value 0, which results in incorrectly translated IR. To workaround this issue, instead of emit null pointers in local and private address space, a null pointer in generic address space is emitted and casted to local and private address space.

Tentative definition of global variables with non-zero initializer will have weak linkage instead of common linkage since common linkage requires zero initializer and does not have explicit section to hold the non-zero value.

Virtual member functions getNullPointer and performAddrSpaceCast are added to TargetCodeGenInfo which by default returns ConstantPointerNull and emitting addrspacecast instruction. A virtual member function getNullPointerValue is added to TargetInfo which by default returns 0. Each target can override these virtual functions to get target specific null pointer and the null pointer value for specific address space, and perform specific translations for addrspacecast.

Wrapper functions getNullPointer is added to CodegenModule and getTargetNullPointerValue is added to ASTContext to facilitate getting the target specific null pointers and their values.

This change has no effect on other targets except amdgcn target. Other targets can provide support of non-zero null pointer in a similar way.

This change only provides support for non-zero null pointer for C and OpenCL. Supporting for other languages will be added later incrementally.

Differential Revision: https://reviews.llvm.org/D26196

llvm-svn: 289252

10 files changed, 140 insertions, 42 deletions

diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 695e92ea1d0..46573477580 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -708,7 +708,7 @@ void CodeGenFunction::EmitScalarInit(const Expr *init, const ValueDecl *D,
     }
 
     auto ty = cast<llvm::PointerType>(tempLV.getAddress().getElementType());
-    llvm::Value *zero = llvm::ConstantPointerNull::get(ty);
+    llvm::Value *zero = CGM.getNullPointer(ty, tempLV.getType());
 
     // If __weak, we want to use a barrier under certain conditions.
     if (lifetime == Qualifiers::OCL_Weak)
diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index 98f476efe93..c0989883e17 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -1050,7 +1050,8 @@ static bool isSimpleZero(const Expr *E, CodeGenFunction &CGF) {
     return true;
   // (int*)0 - Null pointer expressions.
   if (const CastExpr *ICE = dyn_cast<CastExpr>(E))
-    return ICE->getCastKind() == CK_NullToPointer;
+    return ICE->getCastKind() == CK_NullToPointer &&
+        CGF.getTypes().isPointerZeroInitializable(E->getType());
   // '\0'
   if (const CharacterLiteral *CL = dyn_cast<CharacterLiteral>(E))
     return CL->getValue() == 0;
diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 27c8386ee46..f6573091776 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -16,6 +16,7 @@
 #include "CGObjCRuntime.h"
 #include "CGRecordLayout.h"
 #include "CodeGenModule.h"
+#include "TargetInfo.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/RecordLayout.h"
@@ -1262,6 +1263,10 @@ llvm::Constant *CodeGenModule::EmitConstantExpr(const Expr *E,
   return C;
 }
 
+llvm::Constant *CodeGenModule::getNullPointer(llvm::PointerType *T, QualType QT) {
+  return getTargetCodeGenInfo().getNullPointer(*this, T, QT);
+}
+
 llvm::Constant *CodeGenModule::EmitConstantValue(const APValue &Value,
                                                  QualType DestType,
                                                  CodeGenFunction *CGF) {
@@ -1293,6 +1298,7 @@ llvm::Constant *CodeGenModule::EmitConstantValue(const APValue &Value,
       llvm::ConstantInt::get(Int64Ty, Value.getLValueOffset().getQuantity());
 
     llvm::Constant *C = nullptr;
+
     if (APValue::LValueBase LVBase = Value.getLValueBase()) {
       // An array can be represented as an lvalue referring to the base.
       if (isa<llvm::ArrayType>(DestTy)) {
@@ -1323,7 +1329,9 @@ llvm::Constant *CodeGenModule::EmitConstantValue(const APValue &Value,
 
       // Convert to the appropriate type; this could be an lvalue for
       // an integer.
-      if (isa<llvm::PointerType>(DestTy)) {
+      if (auto PT = dyn_cast<llvm::PointerType>(DestTy)) {
+        if (Value.isNullPointer())
+          return getNullPointer(PT, DestType);
         // Convert the integer to a pointer-sized integer before converting it
         // to a pointer.
         C = llvm::ConstantExpr::getIntegerCast(
@@ -1510,7 +1518,7 @@ static llvm::Constant *EmitNullConstantForBase(CodeGenModule &CGM,
                                                const CXXRecordDecl *base);
 
 static llvm::Constant *EmitNullConstant(CodeGenModule &CGM,
-                                        const CXXRecordDecl *record,
+                                        const RecordDecl *record,
                                         bool asCompleteObject) {
   const CGRecordLayout &layout = CGM.getTypes().getCGRecordLayout(record);
   llvm::StructType *structure =
@@ -1520,25 +1528,29 @@ static llvm::Constant *EmitNullConstant(CodeGenModule &CGM,
   unsigned numElements = structure->getNumElements();
   std::vector<llvm::Constant *> elements(numElements);
 
+  auto CXXR = dyn_cast<CXXRecordDecl>(record);
   // Fill in all the bases.
-  for (const auto &I : record->bases()) {
-    if (I.isVirtual()) {
-      // Ignore virtual bases; if we're laying out for a complete
-      // object, we'll lay these out later.
-      continue;
-    }
+  if (CXXR) {
+    for (const auto &I : CXXR->bases()) {
+      if (I.isVirtual()) {
+        // Ignore virtual bases; if we're laying out for a complete
+        // object, we'll lay these out later.
+        continue;
+      }
 
-    const CXXRecordDecl *base = 
-      cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
+      const CXXRecordDecl *base =
+        cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
 
-    // Ignore empty bases.
-    if (base->isEmpty() ||
-        CGM.getContext().getASTRecordLayout(base).getNonVirtualSize().isZero())
-      continue;
-    
-    unsigned fieldIndex = layout.getNonVirtualBaseLLVMFieldNo(base);
-    llvm::Type *baseType = structure->getElementType(fieldIndex);
-    elements[fieldIndex] = EmitNullConstantForBase(CGM, baseType, base);
+      // Ignore empty bases.
+      if (base->isEmpty() ||
+          CGM.getContext().getASTRecordLayout(base).getNonVirtualSize()
+              .isZero())
+        continue;
+
+      unsigned fieldIndex = layout.getNonVirtualBaseLLVMFieldNo(base);
+      llvm::Type *baseType = structure->getElementType(fieldIndex);
+      elements[fieldIndex] = EmitNullConstantForBase(CGM, baseType, base);
+    }
   }
 
   // Fill in all the fields.
@@ -1562,8 +1574,8 @@ static llvm::Constant *EmitNullConstant(CodeGenModule &CGM,
   }
 
   // Fill in the virtual bases, if we're working with the complete object.
-  if (asCompleteObject) {
-    for (const auto &I : record->vbases()) {
+  if (CXXR && asCompleteObject) {
+    for (const auto &I : CXXR->vbases()) {
       const CXXRecordDecl *base = 
         cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
 
@@ -1605,6 +1617,10 @@ static llvm::Constant *EmitNullConstantForBase(CodeGenModule &CGM,
 }
 
 llvm::Constant *CodeGenModule::EmitNullConstant(QualType T) {
+  if (auto PT = T->getAs<PointerType>())
+    return getNullPointer(cast<llvm::PointerType>(getTypes().ConvertTypeForMem(T)),
+                      T);
+
   if (getTypes().isZeroInitializable(T))
     return llvm::Constant::getNullValue(getTypes().ConvertTypeForMem(T));
     
@@ -1620,10 +1636,8 @@ llvm::Constant *CodeGenModule::EmitNullConstant(QualType T) {
     return llvm::ConstantArray::get(ATy, Array);
   }
 
-  if (const RecordType *RT = T->getAs<RecordType>()) {
-    const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
-    return ::EmitNullConstant(*this, RD, /*complete object*/ true);
-  }
+  if (const RecordType *RT = T->getAs<RecordType>())
+    return ::EmitNullConstant(*this, RT->getDecl(), /*complete object*/ true);
 
   assert(T->isMemberDataPointerType() &&
          "Should only see pointers to data members here!");
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index dc8186563ce..2b54370c296 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -19,6 +19,7 @@
 #include "TargetInfo.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclObjC.h"
+#include "clang/AST/Expr.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/AST/StmtVisitor.h"
 #include "clang/Basic/TargetInfo.h"
@@ -171,9 +172,9 @@ public:
   }
 
   /// EmitPointerToBoolConversion - Perform a pointer to boolean conversion.
-  Value *EmitPointerToBoolConversion(Value *V) {
-    Value *Zero = llvm::ConstantPointerNull::get(
-                                      cast<llvm::PointerType>(V->getType()));
+  Value *EmitPointerToBoolConversion(Value *V, QualType QT) {
+    Value *Zero = CGF.CGM.getNullPointer(cast<llvm::PointerType>(V->getType()), QT);
+
     return Builder.CreateICmpNE(V, Zero, "tobool");
   }
 
@@ -591,7 +592,7 @@ Value *ScalarExprEmitter::EmitConversionToBool(Value *Src, QualType SrcType) {
     return EmitIntToBoolConversion(Src);
 
   assert(isa<llvm::PointerType>(Src->getType()));
-  return EmitPointerToBoolConversion(Src);
+  return EmitPointerToBoolConversion(Src, SrcType);
 }
 
 void ScalarExprEmitter::EmitFloatConversionCheck(
@@ -1394,11 +1395,23 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     return Builder.CreateBitCast(Src, DstTy);
   }
   case CK_AddressSpaceConversion: {
-    Value *Src = Visit(const_cast<Expr*>(E));
+    Expr::EvalResult Result;
+    if (E->EvaluateAsRValue(Result, CGF.getContext()) &&
+        Result.Val.isNullPointer()) {
+      // If E has side effect, it is emitted even if its final result is a
+      // null pointer. In that case, a DCE pass should be able to
+      // eliminate the useless instructions emitted during translating E.
+      if (Result.HasSideEffects)
+        Visit(E);
+      return CGF.CGM.getNullPointer(cast<llvm::PointerType>(
+          ConvertType(DestTy)), DestTy);
+    }
     // Since target may map different address spaces in AST to the same address
     // space, an address space conversion may end up as a bitcast.
-    return Builder.CreatePointerBitCastOrAddrSpaceCast(Src,
-                                                       ConvertType(DestTy));
+    auto *Src = Visit(E);
+    return CGF.CGM.getTargetCodeGenInfo().performAddrSpaceCast(CGF, Src,
+                                                               E->getType(),
+                                                               DestTy);
   }
   case CK_AtomicToNonAtomic:
   case CK_NonAtomicToAtomic:
@@ -1453,8 +1466,8 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     if (MustVisitNullValue(E))
       (void) Visit(E);
 
-    return llvm::ConstantPointerNull::get(
-                               cast<llvm::PointerType>(ConvertType(DestTy)));
+    return CGF.CGM.getNullPointer(cast<llvm::PointerType>(ConvertType(DestTy)),
+                              DestTy);
 
   case CK_NullToMemberPointer: {
     if (MustVisitNullValue(E))
@@ -1547,7 +1560,7 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
   case CK_IntegralToBoolean:
     return EmitIntToBoolConversion(Visit(E));
   case CK_PointerToBoolean:
-    return EmitPointerToBoolConversion(Visit(E));
+    return EmitPointerToBoolConversion(Visit(E), E->getType());
   case CK_FloatingToBoolean:
     return EmitFloatToBoolConversion(Visit(E));
   case CK_MemberPointerToBoolean: {
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 78f43bdef99..98b6215d0b4 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2611,9 +2611,16 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D,
   else
     GV->setDLLStorageClass(llvm::GlobalVariable::DefaultStorageClass);
 
-  if (Linkage == llvm::GlobalVariable::CommonLinkage)
+  if (Linkage == llvm::GlobalVariable::CommonLinkage) {
     // common vars aren't constant even if declared const.
     GV->setConstant(false);
+    // Tentative definition of global variables may be initialized with
+    // non-zero null pointers. In this case they should have weak linkage
+    // since common linkage must have zero initializer and must not have
+    // explicit section therefore cannot have non-zero initial value.
+    if (!GV->getInitializer()->isNullValue())
+      GV->setLinkage(llvm::GlobalVariable::WeakAnyLinkage);
+  }
 
   setNonAliasAttributes(D, GV);
 
diff --git a/clang/lib/CodeGen/CodeGenModule.h b/clang/lib/CodeGen/CodeGenModule.h
index be45bb8ea44..f014be95570 100644
--- a/clang/lib/CodeGen/CodeGenModule.h
+++ b/clang/lib/CodeGen/CodeGenModule.h
@@ -1155,6 +1155,11 @@ public:
   llvm::Value *
   createOpenCLIntToSamplerConversion(const Expr *E, CodeGenFunction &CGF);
 
+  /// Get target specific null pointer.
+  /// \param T is the LLVM type of the null pointer.
+  /// \param QT is the clang QualType of the null pointer.
+  llvm::Constant *getNullPointer(llvm::PointerType *T, QualType QT);
+
 private:
   llvm::Constant *
   GetOrCreateLLVMFunction(StringRef MangledName, llvm::Type *Ty, GlobalDecl D,
diff --git a/clang/lib/CodeGen/CodeGenTypes.cpp b/clang/lib/CodeGen/CodeGenTypes.cpp
index ebe55c70d81..1ec6607ccc2 100644
--- a/clang/lib/CodeGen/CodeGenTypes.cpp
+++ b/clang/lib/CodeGen/CodeGenTypes.cpp
@@ -736,10 +736,14 @@ CodeGenTypes::getCGRecordLayout(const RecordDecl *RD) {
   return *Layout;
 }
 
+bool CodeGenTypes::isPointerZeroInitializable(QualType T) {
+  assert (T->getAs<PointerType>() && "Invalid type");
+  return isZeroInitializable(T);
+}
+
 bool CodeGenTypes::isZeroInitializable(QualType T) {
-  // No need to check for member pointers when not compiling C++.
-  if (!Context.getLangOpts().CPlusPlus)
-    return true;
+  if (auto PT = T->getAs<PointerType>())
+    return Context.getTargetNullPointerValue(T) == 0;
 
   if (const auto *AT = Context.getAsArrayType(T)) {
     if (isa<IncompleteArrayType>(AT))
@@ -753,7 +757,7 @@ bool CodeGenTypes::isZeroInitializable(QualType T) {
   // Records are non-zero-initializable if they contain any
   // non-zero-initializable subobjects.
   if (const RecordType *RT = T->getAs<RecordType>()) {
-    const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
+    auto RD = cast<RecordDecl>(RT->getDecl());
     return isZeroInitializable(RD);
   }
 
diff --git a/clang/lib/CodeGen/CodeGenTypes.h b/clang/lib/CodeGen/CodeGenTypes.h
index 00df10dd138..2ce6591e4eb 100644
--- a/clang/lib/CodeGen/CodeGenTypes.h
+++ b/clang/lib/CodeGen/CodeGenTypes.h
@@ -352,6 +352,10 @@ public:  // These are internal details of CGT that shouldn't be used externally.
   /// zero-initialized (in the C++ sense) with an LLVM zeroinitializer.
   bool isZeroInitializable(QualType T);
 
+  /// Check if the pointer type can be zero-initialized (in the C++ sense)
+  /// with an LLVM zeroinitializer.
+  bool isPointerZeroInitializable(QualType T);
+
   /// IsZeroInitializable - Return whether a record type can be
   /// zero-initialized (in the C++ sense) with an LLVM zeroinitializer.
   bool isZeroInitializable(const RecordDecl *RD);
diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 79d481580ec..f70c6cb5de5 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -401,6 +401,20 @@ unsigned TargetCodeGenInfo::getOpenCLKernelCallingConv() const {
   return llvm::CallingConv::C;
 }
 
+llvm::Constant *TargetCodeGenInfo::getNullPointer(const CodeGen::CodeGenModule &CGM,
+    llvm::PointerType *T, QualType QT) const {
+  return llvm::ConstantPointerNull::get(T);
+}
+
+llvm::Value *TargetCodeGenInfo::performAddrSpaceCast(
+    CodeGen::CodeGenFunction &CGF, llvm::Value *Src, QualType SrcTy,
+    QualType DestTy) const {
+  // Since target may map different address spaces in AST to the same address
+  // space, an address space conversion may end up as a bitcast.
+  return CGF.Builder.CreatePointerBitCastOrAddrSpaceCast(Src,
+             CGF.ConvertType(DestTy));
+}
+
 static bool isEmptyRecord(ASTContext &Context, QualType T, bool AllowArrays);
 
 /// isEmptyField - Return true iff a the field is "empty", that is it
@@ -7075,8 +7089,10 @@ public:
   void setTargetAttributes(const Decl *D, llvm::GlobalValue *GV,
                            CodeGen::CodeGenModule &M) const override;
   unsigned getOpenCLKernelCallingConv() const override;
-};
 
+  llvm::Constant *getNullPointer(const CodeGen::CodeGenModule &CGM,
+      llvm::PointerType *T, QualType QT) const override;
+};
 }
 
 static void appendOpenCLVersionMD (CodeGen::CodeGenModule &CGM);
@@ -7140,6 +7156,24 @@ unsigned AMDGPUTargetCodeGenInfo::getOpenCLKernelCallingConv() const {
   return llvm::CallingConv::AMDGPU_KERNEL;
 }
 
+// Currently LLVM assumes null pointers always have value 0,
+// which results in incorrectly transformed IR. Therefore, instead of
+// emitting null pointers in private and local address spaces, a null
+// pointer in generic address space is emitted which is casted to a
+// pointer in local or private address space.
+llvm::Constant *AMDGPUTargetCodeGenInfo::getNullPointer(
+    const CodeGen::CodeGenModule &CGM, llvm::PointerType *PT,
+    QualType QT) const {
+  if (CGM.getContext().getTargetNullPointerValue(QT) == 0)
+    return llvm::ConstantPointerNull::get(PT);
+
+  auto &Ctx = CGM.getContext();
+  auto NPT = llvm::PointerType::get(PT->getElementType(),
+      Ctx.getTargetAddressSpace(LangAS::opencl_generic));
+  return llvm::ConstantExpr::getAddrSpaceCast(
+      llvm::ConstantPointerNull::get(NPT), PT);
+}
+
 //===----------------------------------------------------------------------===//
 // SPARC v8 ABI Implementation.
 // Based on the SPARC Compliance Definition version 2.4.1.
diff --git a/clang/lib/CodeGen/TargetInfo.h b/clang/lib/CodeGen/TargetInfo.h
index e46382596af..223d6d047af 100644
--- a/clang/lib/CodeGen/TargetInfo.h
+++ b/clang/lib/CodeGen/TargetInfo.h
@@ -220,6 +220,22 @@ public:
 
   /// Get LLVM calling convention for OpenCL kernel.
   virtual unsigned getOpenCLKernelCallingConv() const;
+
+  /// Get target specific null pointer.
+  /// \param T is the LLVM type of the null pointer.
+  /// \param QT is the clang QualType of the null pointer.
+  /// \return ConstantPointerNull with the given type \p T.
+  /// Each target can override it to return its own desired constant value.
+  virtual llvm::Constant *getNullPointer(const CodeGen::CodeGenModule &CGM,
+      llvm::PointerType *T, QualType QT) const;
+
+  /// Perform address space cast of an expression of pointer type.
+  /// \param V is the LLVM value to be casted to another address space.
+  /// \param SrcTy is the QualType of \p V.
+  /// \param DestTy is the destination QualType.
+  virtual llvm::Value *performAddrSpaceCast(CodeGen::CodeGenFunction &CGF,
+      llvm::Value *V, QualType SrcTy, QualType DestTy) const;
+
 };
 
 } // namespace CodeGen