[cfe-dev] [RFC] Adding AIX power alignment rule in clang front end

[Xiangling Liao via cfe-dev]

Currently we are trying to implement AIX special alignment rule on the top
of Itanium ABI in Clang. On AIX,  in aggregates, the first member of
double/long double is aligned according to its natural alignment value;
subsequent double/long double of the aggregate are aligned on 4-byte
boundaries.



This rule is recursive. It applies all the way to the inner most nested
members. An example is as the following:



struct D {
        double d;
        int i;
};



struct IntFirst {
        int i;
        struct D d;
};



The size of struct D is 16, class alignment is 8;

The size of struct IntFirst is 20, class alignment is 4;



So our current idea is to have an alignment field to record nested struct
info and pass the special alignment value all the way back to the ourter
most aggregate.



And based on this idea, there are two possible ways.



1.

The first one is that we introduced a new field in class *ASTRecordLayout*

Called `*AIXOffsetAlignment*`  which represents the special AIX alignment
for the object that

contains floating-point member or sub-member. This is for AIX-ABI only.



The proposed change has been put on Phabricator under review:
https://reviews.llvm.org/D78563



*Pros:*

- The design is cleaner. It is clear that such field is for AIX and it does
not affect any other target

   in terms of functionality.



*Cons:*

- All other targets using ItaniumRecordLayoutBuilder will also have to pay
the price to keep

   updating this `*AIXOffsetAlignment` *along the common code path by using
*`UpdateAlignment*`.



2.

The second possible way we have in mind is to overload the usage of current
Microsoft-ABI only alignment field *`RequiredAlignment*` and rename it to `
*TargetSpecialAlignment*` instead of the above first way to create a new
alignment field.

This alignment field will function the same in Itanium ABI part for AIX.
Meanwhile, because the current `RequiredAlignment` is only used in MS code,
renaming will not affect MS code path functionally.



*Pro:*

- Class ASTRecordLayout does not need to construct one more field
`AIXOffsetAlignment` compared to the method one.

- Instead of having each target add new fields for their special alignment
rules, we could have one field that handles all the potential alignment
differences for every target.



*Cons:*

-  Still, all other targets using ItaniumRecordLayoutBuilder will also have
to pay the price to

   keep updating this `TargetSpecialAlignment` along the common code path
by using

   `UpdateAlignment`.

-  By overloading the usage, it may create confusion when more targets try
to utilize it in the

   future.

-  The degree of willingness of sharing this current MS-only alignment
field is unknown.





I would like to see how people think about the above two ways. Which one is
better? Any concerns about either one? Or any other ideas to suggest?



Please let me know if there are any questions about my current proposal and
design. Your feedback is appreciated.



Regards,



Xiangling Liao
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