[Bug target/50883] [ARM] Suboptimal optimization for small structures

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50883

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #10)
> (In reply to Richard Earnshaw from comment #9)
> > Part of the problem here is that the gimple expansion does not see the
> > argument unpacking or understand how the parameters are passed; so this is
> > only exposed after lowering to RTL and we are then reliant on the less
> > powerful RTL optimations to get good code.
> 
> Right, i was thinking about how we expose so of it like having a late pass
> which does the splitting if it is possible. This is part of the reason why I
> created the meta bug to record all of these issues in one place to make it
> easier to start working on a pass like that (maybe for gcc 13 but I have so
> much stuff on my plate right now).

Btw, I thought about this issue quite extensively together with Michael Matz
and it's going to be a mess, in particular on the call side.  Representing
things on GIMPLE will also be fun.  The idea for incoming arguments was
to make the "default definitions" explicit.  For aggregates that would mean
having sth like

 struct S { double x; double y; };
 void foo (struct S s)
 {
   register double s$x __asm__("%xmm0");
   register double s$y __asm__("%xmm1");
   <bb2:>
     s.x = s$x;
     s.y = s$y; 
 }

or for registers (ick)

 void foo (_Complex double z)
 {
   register double z$r __asm__("%xmm0");
   register double z$i __asm__("%xmm1");
   <bb2:>
     z_1(D) = COMPLEX_EXPR <z$r, z$i>;
 }

but it's unclear how exactly this will help if it's just done right before
RTL expansion.  It will also be fun when an aggregate is passed partly
on the stack and partly in registers.

A similar approach could work for the return.

For calls we have representational issues, esp. for the return.  Some
cases could be improved but it might also result in for example too
large lifetime of return slots if the expose those.