[Cxx-abi-dev] Overlong bit fields and 128-bit integers

[Dennis Handly]

>From: David Vandevoorde <daveed at edg.com>
>As you can tell from the subject line keywords we're well in "odd corner
>case" territory here, but we received an ABI compatibility problem
>report in this area.

Yes, we found this using Code Sourcery's ABI test suite.

Consider:
	struct S {
	  int i;
	  long long x: 130;
	  int j;
	};

What's the offset of j?

>Here we're dealing with the second bullet because sizeof(long long)*8 <
>130 (long long is 8 bytes).  We emulate GCC modes that permit 128-bit
integral types, and in those modes, T' is __int128, a 16-byte aligned
>type.  That means that the addition of __int128 has changed the layout
>of a structure that doesn't involve __int128 at all!

Yes, that was most discouraging.  And not forward compatible, by just adding
wider types.

>GCC keeps the alignment of the bit field at an 8-byte boundary in this
>case, and that makes sense to me, but for:
	struct S2 {
	  int i;
	  __int128 x: 130;
	  int j;
	};

>it does the same, which does not make sense to me:  If I used a __int128
>type for the field, I presumably want the associated alignment used.

Right.

>I'd like to therefore propose changing this such that:
>- for standard C++11 integral types use the existing rule
> - for integral types not defined in C++11, limit T' to the standard
>C++11 types and the declared bit field container type

>What do you all think?
	Daveed

Sounds good.  Do we know who will be broken after this change?
gcc is the above broken case of __int128?