Re: [9fans] assembly syntax in plan 9

["Joel C. Salomon" <joelcsalomon@gmail.com>]

On 01/16/2012 08:08 AM, Charles Forsyth wrote:
> Plan 9 doesn't use a base pointer, because everything can be addressed
> relative to the stack pointer, and the loader keeps track of the SP
> level. thus FP is a virtual register, that the loader implements by
> replacing offsets relative to it by the current appropriate offset from
> the hardware stack pointer register (whatever that might be on a given
> platform). That's esp on the x86. the TEXT directive specifies the
> space a function requires for its stack frame, and the loader then adds
> appropriate code at start and end to provide it. 0(FP) is the first
> argument, 4(FP) is the second, and so on. 0(SP) is the bottom of the
> current frame, and 0(SP), 4(SP) etc are referenced to build the
> arguments for outgoing calls (but that space must be accounted for in
> the TEXT directive).

This would make it difficult to implement C99's variable-length
(actually, run-time-determined--length) arrays.  The best compiler-only
change I can think of would be to define a hidden variable `size_t
__size_of_all_vlas`, and add code to adjust SP by that amount before &
after each function call.

[Or we could just skip C99, and make the compiler C11-compliant by
pre-defining __STDC_NO_VLA__. ☺]

> (it's probably not very different in effect from -fno-frame-pointer or
> whatever it is for gcc, which also doesn't use ebp except that is
> implemented entirely by the compiler.)

Google turns up <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39337>,
indicating that GCC had issues combining VLAs and -fomit-frame-pointer;
I don't know how they managed the combination.

--Joel