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Re: GCC libatomic ABI specification draft


On Fri, 20 Jan 2017, Richard Henderson wrote:

> > You can't have a 4-aligned type of size 3.  Sizes must be multiples of 
> > alignment (otherwise arrays don't work).  The type of a 3-sized field 
> > in a packed struct that syntactically might be a 4-aligned type (e.g. 
> > by using attributes on char-array types) is actually a different type 
> > having an alignment of 1.  It's easier to simply regard all types 
> > inside packed structs as 1-aligned (which is IMO what we try to do).
> > 
> > That is, the byte after a 4-aligned "3-sized" type is always padding.
> [ I read Bin Fan's original email some months ago, but I don't have it handy
> now.  Take faulty memory with a grain of salt. ]
> I thought this was about libatomic being presented with an unaligned 3-byte
> structure that happens to sit within an aligned 4-byte word, and choosing to
> atomically operate on the 4-byte word instead of taking a lock on the side.

Ah well, in that case I lost context as well ;)


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