IA64 record alignment rules, and modes?

[James E Wilson]

Gary Funck wrote:
> Question: If we assume that a TImode would've been a more efficient mode
> to represent the record type above, would it not have been acceptable for
> the compiler to promote the alignment of this type to 128, given there
> are no apparent restrictions otherwise, or are there other C conventions
> at work that dictate otherwise?  Is there a configuration tweak that
> would've led to using TImode rather than BLKmode?

The ABI says that the type has alignment of 64-bits, because that is the 
largest alignment among the types used in the structure.

It is not OK to promote the alignment of the type to 128-bits, because 
we might be given a pointer to an object created by a different compiler 
which has only the 64-bit alignment required by the ABI.  If we assume a 
larger alignment, then we might generate code that fails (or rather in 
this case triggers an unwanted kernel unaligned access fixup).  Also, 
this might break the ABI, since structures with different alignments may 
be passed differently as arguments.  Also, as others mentioned, this 
breaks structure layout when this structure is nested inside other 
structures.  There may also be other problems.

It would be OK to promote the alignment of a variable with this type to 
128-bits.  We could then perhaps generate more efficient code to access 
this variable, and if we take a pointer and pass it to another compiler, 
there won't be any problems.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com