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

1 files changed, 35 insertions, 1 deletions

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.