[clang] [llvm] [AMDGPU] Implement variadic functions by IR lowering (PR #93362)

[Jon Chesterfield via llvm-commits]

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@@ -197,12 +206,20 @@ ABIArgInfo AMDGPUABIInfo::classifyKernelArgumentType(QualType Ty) const {
   return ABIArgInfo::getDirect(LTy, 0, nullptr, false);
 }
 
-ABIArgInfo AMDGPUABIInfo::classifyArgumentType(QualType Ty,
+ABIArgInfo AMDGPUABIInfo::classifyArgumentType(QualType Ty, bool Variadic,
                                                unsigned &NumRegsLeft) const {
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JonChesterfield wrote:

This was subtle. Structs that aren't packed into integers and passed in registers fall through to default handling which sets CanBeFlattened, saying that it's OK to spread the struct across multiple arguments. This is then very difficult to reassemble robustly using the va_arg(x, type) interface - one needs to compute how type is likely to have been spread out across part of the call frame.

Noting that these values aren't being usefully passed in registers anyway, the `if (Variadic) {}` sets up call instructions that pass values by value (not byval) and declares that every value shall be exactly four byte aligned (including doubles, as that's something Matt suggested for amdgpu some time ago). This means the frame setup implementation and the case analysis for testing are very straightforward.

https://github.com/llvm/llvm-project/pull/93362