On Nov 12, 2010, at 7:52 PM, Michael Spencer wrote:
> On Fri, Nov 12, 2010 at 10:14 PM, John McCall <[email protected]> wrote:
>> On Oct 18, 2010, at 11:54 PM, Michael Spencer wrote:
>>> The Microsoft ABI requires that structs passed by value as "expanded"
>>> arguments maintain padding. This patch implements this and fixes
>>> <http://llvm.org/bugs/show_bug.cgi?id=8398>.
>>
>> Sorry this has taken so long. Can you explain how this approach is
>> different from just not passing the struct as expanded at all, i.e. passing
>> it byval?
>>
>> John.
>
> I thought byval was a pointer thing, would it work in this case?
As I understand it, byval arguments are passed in memory by copying the
pointed-to object into the appropriate position in the arguments, which is
pretty much exactly what you want. I think the pointer-is-secretly-a-struct
thing is just a throwback to before we had first-class aggregates.
I'm also pretty sure that expansion won't work because it screws up
register-passing CCs, of which there are several we need to support on MS
platforms. For example:
struct A { int x; double d; };
void __fastcall foo(struct A, void *, void *); // the pointers should be
passed in registers, not the int and the padding pseudo-argument.
John.
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