Enable NativeVarargs for CoreCLR

This will allow coreclr run programs which have native varargs.
It also re-enables the ArgIterator type for managed to managed native
vararg calls. This will allow the use of the __arglist keyword.

Limitations:

NYI statements have been added for all Unix Targets.

With this change __arglist (native varargs) will be supported, but not
yet tested on:

Amd64 Windows
x86 Windows
Arm32 Windows
Arm64 Windows

This change does not re-enable native vararg tests. This will be done
in a seperate change.

Author: jashook

src/System.Private.CoreLib/src/System/ArgIterator.cs

@@ -27,7 +27,6 @@ public ref struct ArgIterator
         private IntPtr ArgPtr;                  // Pointer to remaining args.
         private int RemainingArgs;           // # of remaining args.
 
-#if VARARGS_ENABLED //The JIT doesn't support Varargs calling convention.
         [MethodImplAttribute(MethodImplOptions.InternalCall)]
         private extern ArgIterator(IntPtr arglist);
 
@@ -180,6 +179,5 @@ public int GetRemainingCount()
         {
             throw new PlatformNotSupportedException(SR.PlatformNotSupported_ArgIterator); // https://github.com/dotnet/coreclr/issues/9204
         }
-#endif //VARARGS_ENABLED
     }
 }

src/jit/lclvars.cpp

@@ -598,6 +598,18 @@ void Compiler::lvaInitUserArgs(InitVarDscInfo* varDscInfo)
                 isHfaArg = varTypeIsFloating(hfaType);
             }
         }
+        else if (info.compIsVarArgs)
+        {
+#ifdef _TARGET_UNIX_
+            // Currently native varargs is not implemented on non windows targets.
+            // 
+            // Note that some targets like Arm64 Unix should not need much work as
+            // the ABI is the same. While other targets may only need small changes 
+            // such as amd64 Unix, which just expects RAX to pass numFPArguments.
+            NYI("InitUserArgs for Vararg callee is not yet implemented on non Windows targets.");
+#endif
+        }
+
         if (isHfaArg)
         {
             // We have an HFA argument, so from here on out treat the type as a float or double.

src/jit/morph.cpp

@@ -2742,6 +2742,18 @@ GenTreeCall* Compiler::fgMorphArgs(GenTreeCall* call)
     bool callIsVararg = call->IsVarargs();
 #endif
 
+#ifdef _TARGET_UNIX_
+    if (callIsVararg)
+    {
+        // Currently native varargs is not implemented on non windows targets.
+        // 
+        // Note that some targets like Arm64 Unix should not need much work as
+        // the ABI is the same. While other targets may only need small changes 
+        // such as amd64 Unix, which just expects RAX to pass numFPArguments.
+        NYI("Morhing Vararg call not yet implemented on non Windows targets.");
+    }
+#endif
+
 #ifdef UNIX_AMD64_ABI
     // If fgMakeOutgoingStructArgCopy is called and copies are generated, hasStackArgCopy is set
     // to make sure to call EvalArgsToTemp. fgMakeOutgoingStructArgCopy just marks the argument

src/vm/callingconvention.h

@@ -417,11 +417,10 @@ class ArgIteratorTemplate : public ARGITERATOR_BASE
 
 #ifdef _TARGET_AMD64_
 #ifdef UNIX_AMD64_ABI
-        PORTABILITY_ASSERT("ArgIteratorTemplate::IsVarArgPassedByRef");                
         return FALSE;
-#else // UNIX_AMD64_ABI
+#else // !UNIX_AMD64_ABI
         return IsArgPassedByRef(size);
-#endif // UNIX_AMD64_ABI
+#endif // !UNIX_AMD64_ABI
 
 #else
         return (size > ENREGISTERED_PARAMTYPE_MAXSIZE);

src/vm/jitinterface.cpp

@@ -489,13 +489,6 @@ CEEInfo::ConvToJitSig(
         IfFailThrow(sig.GetCallingConvInfo(&data));
         sigRet->callConv = (CorInfoCallConv) data;
 
-        if ((isCallConv(sigRet->callConv, IMAGE_CEE_CS_CALLCONV_VARARG)) ||
-            (isCallConv(sigRet->callConv, IMAGE_CEE_CS_CALLCONV_NATIVEVARARG)))
-        {
-            // This signature corresponds to a method that uses varargs, which are not supported.
-             COMPlusThrow(kInvalidProgramException, IDS_EE_VARARG_NOT_SUPPORTED);
-        }
-
         // Skip number of type arguments
         if (sigRet->callConv & IMAGE_CEE_CS_CALLCONV_GENERIC)
           IfFailThrow(sig.GetData(NULL));