Changing the frame layout of H8 port to reduce code size

[Kazu Hirata <kazu at cs dot umass dot edu>]

Hi,

I am thinking about changing the frame layout of H8 port to reduce
code size and would like to ask for comments.  Currently, the frame
layout used in h8300-{elf,hms}-gcc looks like

  argument
    :
  argument
  program counter
  frame pointer (can be omitted)
  local
    :
  local
  saved register
    :
  saved register <- sp

with the stack pointer pointing to the stack location containing the
last saved register.  I am proposing to flip the order of the locals
and the saved registers like so

  argument
    :
  argument
  program counter
  frame pointer (can be omitted)
  saved register
    :
  saved register
  local
    :
  local          <- sp

Assuming that we use -fomit-frame-pointer and save three registers,
the first long word of the local in the original layout is accessed
like

  mov.l @(12,er7),er0

whereas in my proposed layout, the first long word is pointed to by
the stack pointer (with no offset), so the following would suffice,
saving two bytes.

  mov.l @er7,er0

Also if you want to pass a stack location to a function, you might see

  sub.l er0,er0
  add.b #12,er0
  add.l er7,er0
  jsr   @_foo

But this would become

  mov.l er7,er0
  jsr   @_foo

As another example,

  sub.l er0,er0
  add.b #16,er0
  add.l er7,er0
  jsr   @_foo

would become

  mov.l er7,er0
  adds  #4,er0
  jsr   @_foo

A quick test with uClinux for H8 shows that all the object files
shrink or retain the same size.

Any comments?  You won't be affected unless your application digs up
the stack upward.  Otherwise, you're in trouble.  If that's the case,
I'll refrain from checking in the above change or add a switch to go
back to the old layout (or a switch to enable the new layout).  Note
that I am not changing the calling conventions, so linking code
compiled with different versions of GCC would be OK.

p.s.
i386 port of GCC uses this kind of frame layout.  That is, it saves
registers and then allocates locals.

Kazu Hirata