[clang] [llvm] [SystemZ] Do not extend integer arguments in z/OS XPLINK64 ABI (PR #206833)

[Zibi Sarbinowski via cfe-commits]

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@@ -822,9 +822,16 @@ ABIArgInfo ZOSXPLinkABIInfo::classifyArgumentType(QualType Ty, bool IsNamedArg,
     return getNaturalAlignIndirect(Ty, getDataLayout().getAllocaAddrSpace(),
                                    RAA == CGCXXABI::RAA_DirectInMemory);
 
-  // Integers and enums are extended to full register width.
+  // Promotable integer arguments are passed without extension attributes.
+  // The XPLINK64 ABI specification mandates that scalar values are widened to
+  // 64 bits only for *return* values, not for arguments; other compilers (e.g.
+  // xlc) do not guarantee the upper bits are sign- or zero-extended on the
+  // caller side.  Omitting signext/zeroext here ensures the backend does not
+  // emit a redundant extension instruction on the callee side and avoids
+  // incorrect code when interoperating with xlc.  Return values still use
+  // getExtend() because the spec does mandate extension there.
   if (isPromotableIntegerTypeForABI(Ty))
-    return ABIArgInfo::getExtend(Ty, CGT.ConvertType(Ty));
+    return ABIArgInfo::getDirect(CGT.ConvertType(Ty));
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zibi2 wrote:

right, it should be for sub 64-bit only.

https://github.com/llvm/llvm-project/pull/206833