[chbot] Compiler optimisation

Charles Manning cdhmanning at gmail.com
Fri Jul 2 01:22:21 BST 2021

If you want optimisations that are still debuggable then try -Og. That will
optimise as far as it can without killing debuggability.

If your optimised code is running differently to non-optimised, then the
first thing to check is that you're using volatile where you need it.

A trick I've pulled in the past is to split my makefile to build two types
of object file:  *.o and, say, *.og with *.o built with -O2 or whatever and
-Og or -O0 for the *.og files.
They will still link fine.

On Fri, Jul 2, 2021 at 12:12 PM Robin Gilks <gb7ipd at gmail.com> wrote:

> Does anyone have any tips on how to debug an issue that only arises when
> GCC optimisation is increased from -O0 to -O1 (or above).
> I have an issue with the hardware jpeg encoder on an stm32f7 MCU when I
> crank up the optimise (the first time I've tried it as I prefer -O0 for
> debugging). I've tried putting:
>      #pragma GCC optimize("O0")
> at the start of the 3 files that directly implement the jpeg stuff to turn
> off optimise on them as it looks like the conversion functions that
> generate the YCbCr 4:2:0 data from the ARGB8888 ready for the hardware
> engine have bits optimised away!
> I would expect the lookup tables generated by the code for these
> conversions would be the same irrespective of optimisation level but that's
> the only place I can think to look (the '.lst' files output from the
> compiler are way too different to make any sensible comparisons between
> optimisation levels)
> Ideas? Cheers
> --
> Robin Gilks
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