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Michael Zucchi

 B.E. (Comp. Sys. Eng.)

  also known as zed
  & handle of notzed

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Tuesday, 25 March 2014, 17:30

epiphany stack frame

I came across this months ago but forgot some of the finer details.

I thought that 8-byte empty area looked odd in generated code but I think it's there to simplify stacking saved registers due to the lack of pre-decrement addressing modes (otherwise it doesn't make much sense - leaf function use wouldn't matter if it had such addressing modes).

It's in the gcc source.

/* EPIPHANY stack frames look like:

             Before call                       After call
        +-----------------------+       +-----------------------+
        |                       |       |                       |
   high |  local variables,     |       |  local variables,     |
   mem  |  reg save area, etc.  |       |  reg save area, etc.  |
        |                       |       |                       |
        +-----------------------+       +-----------------------+
        |                       |       |                       |
        |  arguments on stack.  |       |  arguments on stack.  |
        |                       |       |                       |
  SP+8->+-----------------------+FP+8m->+-----------------------+
        | 2 word save area for  |       |  reg parm save area,  |
        | leaf funcs / flags    |       |  only created for     |
  SP+0->+-----------------------+       |  variable argument    |
                                        |  functions            |
                                 FP+8n->+-----------------------+
                                        |                       |
                                        |  register save area   |
                                        |                       |
                                        +-----------------------+
                                        |                       |
                                        |  local variables      |
                                        |                       |
                                  FP+0->+-----------------------+
                                        |                       |
                                        |  alloca allocations   |
                                        |                       |
                                        +-----------------------+
                                        |                       |
                                        |  arguments on stack   |
                                        |                       |
                                  SP+8->+-----------------------+
   low                                  | 2 word save area for  |
   memory                               | leaf funcs / flags    |
                                  SP+0->+-----------------------+

Tagged hacking, parallella.
inlining register reads | compiler strangeness
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