[clang][RISCV] support BITINT with mixed-type (llvm#156592)

Implement riscv-non-isa/riscv-elf-psabi-doc#419.

This patch makes the type extension based on the variable type for
BIGINT, rather than using sign extension for all cases.

Author: BeMg

clang/lib/CodeGen/Targets/RISCV.cpp

@@ -680,22 +680,22 @@ ABIArgInfo RISCVABIInfo::classifyArgumentType(QualType Ty, bool IsFixed,
     if (const auto *ED = Ty->getAsEnumDecl())
       Ty = ED->getIntegerType();
 
-    // All integral types are promoted to XLen width
-    if (Size < XLen && Ty->isIntegralOrEnumerationType()) {
-      return extendType(Ty, CGT.ConvertType(Ty));
-    }
-
     if (const auto *EIT = Ty->getAs<BitIntType>()) {
-      if (EIT->getNumBits() < XLen)
+
+      if (XLen == 64 && EIT->getNumBits() == 32)
         return extendType(Ty, CGT.ConvertType(Ty));
-      if (EIT->getNumBits() > 128 ||
-          (!getContext().getTargetInfo().hasInt128Type() &&
-           EIT->getNumBits() > 64))
-        return getNaturalAlignIndirect(
-            Ty, /*AddrSpace=*/getDataLayout().getAllocaAddrSpace(),
-            /*ByVal=*/false);
+
+      if (EIT->getNumBits() <= 2 * XLen)
+        return ABIArgInfo::getExtend(Ty, CGT.ConvertType(Ty));
+      return getNaturalAlignIndirect(
+          Ty, /*AddrSpace=*/getDataLayout().getAllocaAddrSpace(),
+          /*ByVal=*/false);
     }
 
+    // All integral types are promoted to XLen width
+    if (Size < XLen && Ty->isIntegralOrEnumerationType())
+      return extendType(Ty, CGT.ConvertType(Ty));
+
     return ABIArgInfo::getDirect();
   }