[AVR] Support zero-sized arguments in defined methods

It is sufficient to skip emission of these arguments as we have nothing
to actually pass through the function call.

The AVR-GCC reference has nothing to say about zero-sized arguments,
presumably because C/C++ doesn't support them. This means we don't have
to worry about ABI differences.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294119 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/AVR/AVRISelLowering.cpp

@@ -932,6 +932,12 @@ static void analyzeStandardArguments(TargetLowering::CallLoweringInfo *CLI,
   bool UsesStack = false;
   for (unsigned i = 0, pos = 0, e = Args.size(); i != e; ++i) {
     unsigned Size = Args[i];
+
+    // If we have a zero-sized argument, don't attempt to lower it.
+    // AVR-GCC does not support zero-sized arguments and so we need not
+    // worry about ABI compatibility.
+    if (Size == 0) continue;
+
     MVT LocVT = (IsCall) ? (*Outs)[pos].VT : (*Ins)[pos].VT;
 
     // If we have plenty of regs to pass the whole argument do it.