RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct

[Richard Smith via cfe-commits]

On Mon, Feb 8, 2016 at 2:49 PM, H.J. Lu <hjl.tools at gmail.com> wrote:
> On Mon, Feb 8, 2016 at 2:42 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>> Do we really need an 'empty type' special case?
>>
>> The x86_64 psABI already seems clear that empty types with size <= 16
>> are not passed at all. Following the algorithm in section 3.2.3, each
>> eightbyte is classified as NO_CLASS, and thus is not passed. So the
>> proposed change would only affect the behavior of types larger than 16
>> bytes that contain no data. It doesn't seem worth breaking ABI to more
>> efficiently pass those.
>>
>
> Clang isn't consistent between ia32 and x86-64.  GCC isn't compatible
> with clang.  My proposal will make GCC and clang compatible.  It also
> makes GCC and clang behave the same on both ia32 and x86-64.

OK, but if the ABI is already saying what we want, and the only
problem is that GCC and Clang don't conform to the ABI in some cases,
then we just need to fix the compilers and not the ABI. Which cases do
we think the existing ABI rule misclassifies?