public void AddABIAttributesForByValueStructure(Function function, int paramIndex)
{
// ByVal pointers indicate by value semantics. The actual semantics are along the lines of
// "pass the arg as copy on the arguments stack and set parameter implicitly to that copy's address"
// (src: https://github.com/ldc-developers/ldc/issues/937 ) [e.g. caller copies byval args]
//
// LLVM recognizes this pattern and has a pass to map to an efficient register usage whenever plausible.
// Though it seems Clang doesn't apply the attribute in all cases, for x86 it doesn't appear to ever use
// it, for Cortex-Mx it seems to use it only for larger structs, otherwise it uses an [ n x i32]. (Max
// value of n is not known) and performs casts. Thus, on cortex-m the function parameters are handled
// quite differently by clang, which seems odd to put such target dependent differences into the front-end.
var argType = function.Parameters[paramIndex].NativeType as IPointerType;
if (argType == null || !argType.ElementType.IsStruct)
{
throw new ArgumentException("Signature for specified parameter must be a pointer to a structure");
}
var layout = function.ParentModule.Layout;
function.AddAttributes(FunctionAttributeIndex.Parameter0 + paramIndex
, function.Context.CreateAttribute(AttributeKind.ByVal)
, function.Context.CreateAttribute(AttributeKind.Alignment, layout.AbiAlignmentOf(argType.ElementType))
);
}
