[sdcc-devel] Register allocation & PIC Port

[Scott D. <sc...@da...>]

I was out of town for three days and was overwelmed by the activity in
SDCC when I got back! Great work guys! Unfortunately, I deleted Sandeep's
message about register allocation...

In the PIC Port, I just implemented something similar to what you
(Sandeep) described, but it's at a much higher level. (Since I've deleted
the message, I may be mis-remembering the details). The PIC has no
accessible stack. Consequently, there's no push/pop available for passing
parameters to and from functions. To get around this, I created a special
block of "registers" in ralloc.c dedicated to passing parameters. (One of
the benefits of the PIC is that all RAM can be accessed like a register.)
I think what I've implemented could be incorporated at the iCode layer
since these special registers have a very short live range. OTOH, there's
a problem in some cases where the registers are getting duplicated. E.g. a
temporary is "pushed" into these special registers only to be "popped" in
the callee's context.

Avoiding this kind of register duplication is very difficult without
performing a call tree analysis. Guess what I intend to embark upon next
:).

BTW, The PIC port is starting to stabilize quite a bit now. There's still
no linker support, but for simple, single-file applications, SDCC is
almost usable for the PIC!

Scott