Re: function parms in regs, patch 3 of 3

[Jim Wilson <wilson at tuliptree dot org>]

This is the patch I really wanted to comment on, which is why I am 
willing to review this set of patches.

I believe that we already have so many macros that interact in confusing 
ways that we should avoid adding new ones.  I think a better approach is 
to return more info from FUNCTION_ARG, in a more structured form, so 
that we can get away from the ever expanding set of macros that we have.
The PARALLEL stuff is a step in this direction.  The structure of the 
info is ugly, but that can always be improved.  I'd like to see the 
entire interface rewritten, but it isn't clear if anyone will ever have 
the time for it, and we could only do this in a .0 release.

Your revised patch which only defines BLOCK_REG_PADDING for rs6000 is a 
major improvement.  I think there is too much risk in trying to define a 
default for all targets.

There is a known IA-64 ABI problem that your patch doesn't help with. 
The IA-64 ABI treats homogeneous floating-point aggregates (HFAs) 
specially.  A structure all of whose fields are the same FP type is an 
HFA for instance.  An HFA is passed in FP regs, one field per FP reg, 
until the FP regs are full, and then we pass the rest in integer regs.
The part passed in integer regs is placed in the same place it would 
have gone if the entire structure was passed in integer regs.  Suppose 
we have a structure of 5 floats, and there are 3 FP registers left.  The 
first 3 fields are passed in FP registers.  The fourth one is passed 
left-justified (little-endian) in the second integer register, because 
it is the second half of the second word.  The fifth one is passed 
right-justified (little-endian) in the third integer register, because 
it is the first half of the third word.  Now the problem here is that a 
float field in an integer register can require either left or right 
justification depending on the context.  I don't think your 
BLOCK_REG_PADDING macro can handle this.  It doesn't have enough info. 
FUNCTION_ARG does have enough info.  If the PARALLEL stuff had 3 items 
location/size/alignment instead of the current 2 location/size, then I 
believe we could fix this problem, your powerpc problem, and probably 
also some ABI problems in other ports that we may or may not know about.

Having said that, I really should take a closer look at the patch to see 
what it actually does.  And I need to look at the second patch of the set.

Jim