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

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Merged
merged 3 commits into from
Oct 9, 2025
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[clang][SPIR][SPIRV] Materialize non-generic null pointers via addrspacecast #161773

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4612597
[clang][SPIR][SPIRV] Materialize non-generic null pointers via addrsp…
wenju-he Oct 3, 2025
5c5e13e
move getTargetNullPointerValue implementation into CommonSPIRTargetCo…
wenju-he Oct 3, 2025
b95e77a
ternary operator, update comment
wenju-he Oct 8, 2025
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28 changes: 28 additions & 0 deletions clang/lib/CodeGen/Targets/SPIR.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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:
Expand Down Expand Up @@ -240,6 +243,31 @@ 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.
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Suggested change
// This allows later lowering to substitute the target’s real sentinel value.
// This allows later lowering to substitute the target's real sentinel value.

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done in b95e77a

llvm::Constant *
CommonSPIRTargetCodeGenInfo::getNullPointer(const CodeGen::CodeGenModule &CGM,
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Can you directly copy the AMDGPU implementation, which checks against getTargetNullPointerValue

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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?

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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

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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.

llvm::PointerType *PT,
QualType QT) const {
LangAS AS;
if (QT->getUnqualifiedDesugaredType()->isNullPtrType())
AS = LangAS::Default;
else
AS = QT->getPointeeType().getAddressSpace();
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Initialize with ?: instead of conditional assign

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Initialize with ?: instead of conditional assign

done in b95e77a

if (AS == LangAS::Default || AS == LangAS::opencl_generic)
return llvm::ConstantPointerNull::get(PT);

auto &Ctx = CGM.getContext();
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 {
Expand Down
6 changes: 3 additions & 3 deletions clang/test/CodeGenOpenCL/builtins.cl
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Expand Up @@ -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]]
}

4 changes: 2 additions & 2 deletions clang/test/CodeGenSYCL/address-space-conversions.cpp
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Expand Up @@ -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
Expand Down