[clang][SPIR][SPIRV] Materialize non-generic null pointers via addrspacecast #161773

wenju-he · 2025-10-03T04:21:59Z

LLVM models ConstantPointerNull as all-zero, but some GPUs (e.g. AMDGPU and our downstream GPU target) use a non-zero sentinel for null in private / local address spaces.
SPIR-V is a supported input for our GPU target. This PR preserves a canonical zero form in the generic AS while allowing later lowering to substitute the target’s real sentinel.

…acecast LLVM models ConstantPointerNull as all-zero, but some GPUs (e.g. AMDGPU and our downstream GPU target) use a non-zero sentinel for null in private / local address spaces. SPIR-V is a supported input for our GPU target. This PR preserves a canonical zero form in the generic AS while allowing later lowering to substitute the target’s real sentinel.

llvmbot · 2025-10-03T04:22:28Z

@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Wenju He (wenju-he)

Changes

LLVM models ConstantPointerNull as all-zero, but some GPUs (e.g. AMDGPU and our downstream GPU target) use a non-zero sentinel for null in private / local address spaces.
SPIR-V is a supported input for our GPU target. This PR preserves a canonical zero form in the generic AS while allowing later lowering to substitute the target’s real sentinel.

Full diff: https://github.com/llvm/llvm-project/pull/161773.diff

3 Files Affected:

(modified) clang/lib/CodeGen/Targets/SPIR.cpp (+24)
(modified) clang/test/CodeGenOpenCL/builtins.cl (+3-3)
(modified) clang/test/CodeGenSYCL/address-space-conversions.cpp (+2-2)

diff --git a/clang/lib/CodeGen/Targets/SPIR.cpp b/clang/lib/CodeGen/Targets/SPIR.cpp
index 4aa63143a66cd..b1c1d65b4f5e1 100644
--- a/clang/lib/CodeGen/Targets/SPIR.cpp
+++ b/clang/lib/CodeGen/Targets/SPIR.cpp
@@ -61,6 +61,9 @@ class CommonSPIRTargetCodeGenInfo : public TargetCodeGenInfo {
       QualType SampledType, CodeGenModule &CGM) const;
   void
   setOCLKernelStubCallingConvention(const FunctionType *&FT) const override;
+  llvm::Constant *getNullPointer(const CodeGen::CodeGenModule &CGM,
+                                 llvm::PointerType *T,
+                                 QualType QT) const override;
 };
 class SPIRVTargetCodeGenInfo : public CommonSPIRTargetCodeGenInfo {
 public:
@@ -240,6 +243,27 @@ void CommonSPIRTargetCodeGenInfo::setOCLKernelStubCallingConvention(
       FT, FT->getExtInfo().withCallingConv(CC_SpirFunction));
 }
 
+// LLVM currently assumes a null pointer has the bit pattern 0, but some GPU
+// targets use a non-zero encoding for null in certain address spaces.
+// Because SPIR(-V) is a virtual target and the bit pattern of a non-generic
+// null is unspecified, materialize non-generic null via an addrspacecast from
+// the generic null.
+// This allows later lowering to substitute the target’s real sentinel value.
+llvm::Constant *
+CommonSPIRTargetCodeGenInfo::getNullPointer(const CodeGen::CodeGenModule &CGM,
+                                            llvm::PointerType *PT,
+                                            QualType QT) const {
+  auto &Ctx = CGM.getContext();
+  if (PT->getAddressSpace() ==
+      Ctx.getTargetAddressSpace(LangAS::opencl_generic))
+    return llvm::ConstantPointerNull::get(PT);
+
+  auto NPT = llvm::PointerType::get(
+      PT->getContext(), Ctx.getTargetAddressSpace(LangAS::opencl_generic));
+  return llvm::ConstantExpr::getAddrSpaceCast(
+      llvm::ConstantPointerNull::get(NPT), PT);
+}
+
 LangAS
 SPIRVTargetCodeGenInfo::getGlobalVarAddressSpace(CodeGenModule &CGM,
                                                  const VarDecl *D) const {
diff --git a/clang/test/CodeGenOpenCL/builtins.cl b/clang/test/CodeGenOpenCL/builtins.cl
index aa666c7b671b9..708d1b84f4838 100644
--- a/clang/test/CodeGenOpenCL/builtins.cl
+++ b/clang/test/CodeGenOpenCL/builtins.cl
@@ -62,19 +62,19 @@ void testBranchingOnAddressSpaceCast(generic long* ptr) {
     if (to_global(ptr))
         (void)0;
     // CHECK:       [[P:%[0-9]+]] = call spir_func [[GLOBAL_VOID:ptr addrspace\(1\)]] @__to_global([[GENERIC_VOID:ptr addrspace\(4\)]] {{%[0-9]+}})
-    // CHECK-NEXT:  [[BOOL:%[a-z0-9]+]] = icmp ne ptr addrspace(1) [[P]], null
+    // CHECK-NEXT:  [[BOOL:%[a-z0-9]+]] = icmp ne ptr addrspace(1) [[P]], addrspacecast (ptr addrspace(4) null to ptr addrspace(1))
     // CHECK-NEXT:  br i1 [[BOOL]]
 
     if (to_local(ptr))
         (void)0;
     // CHECK:       [[P:%[0-9]+]] = call spir_func [[LOCAL_VOID:ptr addrspace\(3\)]] @__to_local([[GENERIC_VOID]] {{%[0-9]+}})
-    // CHECK-NEXT:  [[BOOL:%[a-z0-9]+]] = icmp ne ptr addrspace(3) [[P]], null
+    // CHECK-NEXT:  [[BOOL:%[a-z0-9]+]] = icmp ne ptr addrspace(3) [[P]], addrspacecast (ptr addrspace(4) null to ptr addrspace(3))
     // CHECK-NEXT:  br i1 [[BOOL]]
 
     if (to_private(ptr))
         (void)0;
     // CHECK:       [[P:%[0-9]+]] = call spir_func [[PRIVATE_VOID:ptr]] @__to_private([[GENERIC_VOID]] {{%[0-9]+}})
-    // CHECK-NEXT:  [[BOOL:%[a-z0-9]+]] = icmp ne ptr [[P]], null
+    // CHECK-NEXT:  [[BOOL:%[a-z0-9]+]] = icmp ne ptr [[P]], addrspacecast (ptr addrspace(4) null to ptr)
     // CHECK-NEXT:  br i1 [[BOOL]]
 }
 
diff --git a/clang/test/CodeGenSYCL/address-space-conversions.cpp b/clang/test/CodeGenSYCL/address-space-conversions.cpp
index fa7acb0d99433..ea52be53ea906 100644
--- a/clang/test/CodeGenSYCL/address-space-conversions.cpp
+++ b/clang/test/CodeGenSYCL/address-space-conversions.cpp
@@ -35,9 +35,9 @@ void tmpl(T t) {}
   // CHECK-DAG: [[LOC]].ascast = addrspacecast ptr [[LOC]] to ptr addrspace(4)
   // CHECK-DAG: [[PRIV]].ascast = addrspacecast ptr [[PRIV]] to ptr addrspace(4)
   LOC = nullptr;
-  // CHECK-DAG: store ptr addrspace(3) null, ptr addrspace(4) [[LOC]].ascast, align 8
+  // CHECK-DAG: store ptr addrspace(3) addrspacecast (ptr addrspace(4) null to ptr addrspace(3)), ptr addrspace(4) [[LOC]].ascast, align 8
   GLOB = nullptr;
-  // CHECK-DAG: store ptr addrspace(1) null, ptr addrspace(4) [[GLOB]].ascast, align 8
+  // CHECK-DAG: store ptr addrspace(1) addrspacecast (ptr addrspace(4) null to ptr addrspace(1)), ptr addrspace(4) [[GLOB]].ascast, align 8
 
   // Explicit conversions
   // From named address spaces to default address space

Copilot

Pull Request Overview

This PR modifies the SPIR and SPIRV code generation to properly handle null pointers in non-generic address spaces. Instead of using direct null values for private/local address spaces, it materializes these nulls via addrspacecast from generic null pointers to accommodate GPU targets that use non-zero sentinel values for null in certain address spaces.

Implements a new getNullPointer method in SPIR target code generation that creates addrspacecast expressions for non-generic address spaces
Updates test expectations to verify the new addrspacecast pattern instead of direct null comparisons
Preserves canonical zero form in generic address space while allowing target-specific null representations

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
clang/lib/CodeGen/Targets/SPIR.cpp	Adds `getNullPointer` override that materializes non-generic nulls via addrspacecast from generic null
clang/test/CodeGenSYCL/address-space-conversions.cpp	Updates test expectations to verify addrspacecast pattern for null pointer stores
clang/test/CodeGenOpenCL/builtins.cl	Updates test expectations to verify addrspacecast pattern for null pointer comparisons

Copilot

Pull Request Overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

arsenm · 2025-10-03T06:58:06Z

clang/lib/CodeGen/Targets/SPIR.cpp

+// the generic null.
+// This allows later lowering to substitute the target’s real sentinel value.
+llvm::Constant *
+CommonSPIRTargetCodeGenInfo::getNullPointer(const CodeGen::CodeGenModule &CGM,


Can you directly copy the AMDGPU implementation, which checks against getTargetNullPointerValue

Can you directly copy the AMDGPU implementation, which checks against getTargetNullPointerValue

getTargetNullPointerValue requires defining getNullPointerValue in BaseSPIRTargetInfo, like

llvm-project/clang/lib/Basic/Targets/AMDGPU.h

Lines 424 to 430 in 441f0c7

uint64_t getNullPointerValue(LangAS AS) const override {

// FIXME: Also should handle region.

return (AS == LangAS::opencl_local || AS == LangAS::opencl_private ||

AS == LangAS::sycl_local || AS == LangAS::sycl_private)

? ~0

: 0;

}

However, we don't know the return value of getNullPointerValue in BaseSPIRTargetInfo except for generic address space, right?

Hm, seems like getNullPointerValue should be able to fail. My main worry here is special casing generic address space, as opposed to default address space

Hm, seems like getNullPointerValue should be able to fail. My main worry here is special casing generic address space, as opposed to default address space

thanks @arsenm, you're right that the default AS is not checked.

I copied the logic from ASTContext::getTargetNullPointerValue into CommonSPIRTargetCodeGenInfo::getNullPointer in 5c5e13e to avoid changing return type of both getNullPointerValue and getTargetNullPointerValue to std::optional<uint64_t>. Since getTargetNullPointerValue is used in many places, changing its return type looks like unnecessary churn.

…deGenInfo::getNullPointer

llvmbot added clang Clang issues not falling into any other category clang:codegen IR generation bugs: mangling, exceptions, etc. labels Oct 3, 2025

wenju-he requested review from Copilot, schittir and Keenuts October 3, 2025 04:22

Copilot AI reviewed Oct 3, 2025

View reviewed changes

wenju-he requested review from arsenm, bader and Copilot October 3, 2025 04:22

Copilot AI reviewed Oct 3, 2025

View reviewed changes

arsenm requested a review from shiltian October 3, 2025 06:56

arsenm reviewed Oct 3, 2025

View reviewed changes

move getTargetNullPointerValue implementation into CommonSPIRTargetCo…

5c5e13e

…deGenInfo::getNullPointer

bader requested a review from arsenm October 3, 2025 17:34

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[clang][SPIR][SPIRV] Materialize non-generic null pointers via addrspacecast #161773

[clang][SPIR][SPIRV] Materialize non-generic null pointers via addrspacecast #161773

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	uint64_t getNullPointerValue(LangAS AS) const override {
	// FIXME: Also should handle region.
	return (AS == LangAS::opencl_local \|\| AS == LangAS::opencl_private \|\|
	AS == LangAS::sycl_local \|\| AS == LangAS::sycl_private)
	? ~0
	: 0;
	}

[clang][SPIR][SPIRV] Materialize non-generic null pointers via addrspacecast #161773

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