Re: ANN: Cortex GNAT RTS 2018-07-15

[Simon Wright <simon@pushface.org>]

Paul Rubin <no.email@nospam.invalid> writes:

> Can you say about how hard it is to port GNAT RTS to a new board,
> given a reasonable C environment for the board?  How much code space
> and ram is required?

I don't know about the AdaCore RTS, but I imagine it'll be much the same
as this one (after you've got to grips with the deeply nested,
use-clause-happy code :-)

For this one, I started on 1 July and had a working RTS (bar an issue
with the board clock, discovered later) by 4 July.

The issues (once you have a base RTS for one ARM board) are

* startup, any RAM/Flash/FPU settings, clock
* interrupts, interrupt priority range, names

For the micro:bit, compiling with -O0 and assertions enabled, I get, for
the RTS + other support, in a program with delays, a task, and an
interrupt-handling PO,

text
        libgcc      1380
        gnat       10132
        freertos    7724
        libc         280

rodata
        gnat        1252

data
        gnat         220
        freertos       4

bss
        gnat          68
        freertos     340

The base RAM use (data + bss) for this program was 900 bytes. Out of the
remaining RAM (15484), it used (via heap allocations)

initial/interrupt stack   1024
environment task stack    1536
1 task stack              1024
others?                    828

and the free heap was 11072 (those numbers don't quite add up, I think
there's some overhead in the heap management maybe).

I'm not sure why the environment task stack needs to be so high (it does
elaboration + Ada main). The task stack could probably be lower, one
outstanding job is to work out a way of measuring task stack usage.