Altivec + 16 byte alignment

[Geoff Keating]

kenner@vlsi1.ultra.nyu.edu (Richard Kenner) writes:

>     BIGGEST_ALIGNMENT is 128, but STACK_BOUNDARY is 64 unless you give 
>     -mabi=altivec (in 3.2+). 
> 
> I must say that I find this confusing.  Although the documentation is
> unclear, I've always thought of BIGGEST_ALIGNMENT as being the largest
> alignment we can guarantee and I think there's code that depends on that.

BIGGEST_ALIGNMENT is defined in the documentation as:

`BIGGEST_ALIGNMENT'
     Biggest alignment that any data type can require on this machine,
     in bits.

> But if STACK_BOUNDARY < BIGGEST_ALIGNMENT, that's clearly not true.  

I don't see that.  The prologue can always try to align the stack
itself, although at some runtime cost.  I think that's what
PREFERRED_STACK_BOUNDARY is all about.

> Has the definition changed?

It's been like this since at least 1997.

>   If so, how *do* you find the largest alignment you can guarantee?

For error checking?  MAX_OFILE_ALIGNMENT is probably the value you
want.

-- 
- Geoffrey Keating <geoffk@geoffk.org>