[power] Why is reserved_stack_space added twice when computing slot_offset?

[gretay-js]

reserved_stack_space is already taken into account by frame_size, and then added on explicitly again for Incoming in slot_offset.

ocaml/asmcomp/power/emit.mlp

Line 61 in 03c33f5

| Incoming n -> frame_size() + reserved_stack_space + n

[stedolan]

After some confused googling, this code turns out to be correct. In the power ABI, there's a reserved area at the bottom of the stack, so stack layouts look like this (growing downwards):

   |          |
   |  locals  |
   |          |
   +----------+
   |          |
   |   args   |
   |          |
   +----------+
   |          |
   | reserved |
   |          |
   +----------+ <-- caller SP
   |          |
   |  locals  |
   |          |
   +----------+
   |          |
   | reserved |
   |          |
   +----------+ <-- callee SP

For the caller to access its incoming arguments, it must add to SP its own frame size (its reserved space + its locals) and then add the callers reserved space. There are two reserved spaces between the callee's stack pointer and the callee's incoming stack arguments!

[lpw25]

Maybe a comment in the code is in order?