[clang] [Clang][CodeGen][X86] don't coerce int128 into `{i64,i64}` for SysV-like ABIs (PR #135230)

[Eli Friedman via cfe-commits]

================
@@ -2595,6 +2595,14 @@ GetX86_64ByValArgumentPair(llvm::Type *Lo, llvm::Type *Hi,
 
 ABIArgInfo X86_64ABIInfo::
 classifyReturnType(QualType RetTy) const {
+  // return int128 as i128
+  if (const BuiltinType *BT = RetTy->getAs<BuiltinType>()) {
+    BuiltinType::Kind k = BT->getKind();
+    if (k == BuiltinType::Int128 || k == BuiltinType::UInt128) {
+      return ABIArgInfo::getDirect();
+    }
+  }
+
   // AMD64-ABI 3.2.3p4: Rule 1. Classify the return type with the
   // classification algorithm.
   X86_64ABIInfo::Class Lo, Hi;
----------------
efriedma-quic wrote:

Sorry about the delay here.

I think this does the right thing, but in a pretty unintuitive way; I had to read through the code multiple times to convince myself this actually works the way it's supposed to.

I'd rather not have GetINTEGERTypeAtOffset return nullptr; instead, the callers of GetINTEGERTypeAtOffset should explicitly check for an i128 return type, and return early.  And assert `Hi == Integer`.

https://github.com/llvm/llvm-project/pull/135230