[clang] [Sparc][clang] make `_Complex` ABI GCC-compatible (PR #212340)

[Folkert de Vries via cfe-commits]

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@@ -277,6 +313,22 @@ ABIArgInfo SparcV9ABIInfo::classifyType(QualType Ty, unsigned SizeLimit,
       return ABIArgInfo::getExtend(Ty);
     }
 
+  // When being GCC-compatible, cast a complex char, short and int to an integer
+  // type of the right size to get the correct scalar-like behavior. Other
+  // complex types fall through and are treated like a struct containing the
+  // real and imaginary parts, e.g. `{ i64, i64 }` or `{ double, double }`.
+  if (IsComplexGnuABI) {
+    const auto *CT = Ty->getAs<ComplexType>();
+    if (CT && CT->getElementType()->isIntegerType()) {
+      uint64_t ElementTypeSize = Context.getTypeSize(CT->getElementType());
+      if (ElementTypeSize < 32) {
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folkertdev wrote:

I've changed that to `<= 32`, it already worked in practice before (so, fallthrough had the same effect) but based on the GCC source this is probably more correct. I tried `<= 64` too but that failed.

https://github.com/gcc-mirror/gcc/blob/28f2250ad1d37f49edb0efd1d88e3877257ab4a1/gcc/config/sparc/sparc.cc#L6646-L6655

I'm validating the implementation with a cross-compiling version of https://github.com/Gankra/abi-cafe, which just fuzzes the cartesian product of clang (on this branch) versus GCC with a wide variety of signatures. 

https://github.com/llvm/llvm-project/pull/212340