[chbot] Compiler optimisation

[Robin Gilks]

I've managed to get the change in behaviour down to a single file now and
have started to sprinkle 'volatiles' about the place.

Question - if an element of a structure is defined as volatile, does the
instantiation of the structure need to be also or does the whole struct
effectively become volatile.

Cheers

On Fri, Jul 2, 2021 at 12:23 PM Charles Manning <cdhmanning at gmail.com>
wrote:

> 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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