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

[Wenju He via cfe-commits]

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@@ -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,
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wenju-he wrote:

> 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 5c5e13ee5e95791c9795714fec01efbe92dbecd4 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. 

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