TWI Optimization Issue?

RE: TWI Optimization Issue?

Jørgen Holmefjord — Wed, 10 Jun 2020 14:42:43 GMT

This is a compiler-specific question that you should direct to the developers of the compilers/IDE you are using. Modified source files should be rebuilt when changes are detected, I would expect this to be the case with modified dependency header files as well.

RE: TWI Optimization Issue?

JayDev — Mon, 08 Jun 2020 16:45:33 GMT

Ok, I confirmed that it now appears to work, even at optimization level 2, so all this could be an issue with the project needing to be cleaned between builds?

Can you explain what exactly is being built between projects when I click build? It appears that it's not actually building enough to properly represent the code, if I follow this correctly.

RE: TWI Optimization Issue?

JayDev — Mon, 08 Jun 2020 16:42:31 GMT

I might've figured out the issue. I ended up stepping through the project and it went to the true statement. I guess something wasn't cleaned up between builds? Sounds like I might need to clean between every build, if I'm interpreting this correctly.

RE: TWI Optimization Issue?

JayDev — Mon, 08 Jun 2020 16:26:18 GMT

In hindsight, it makes sense it would optimize it out if the only answer it ever returns is false, regardless of whether both sides match or not.

RE: TWI Optimization Issue?

JayDev — Mon, 08 Jun 2020 16:24:47 GMT

Hmm, it looks like that was indeed the issue. I'm kind of surprised the check would be optimized out.

I do have a related question though. It has to do with this section of the code:

    if(m_sample==WHO_AM_I_RESPONSE)
        return true;
    else
        return false;

So, my "WHO_AM_I_RESPONSE" is setup as:

#define WHO_AM_I_RESPONSE 0x1D

I can confirm that my m_sample is being returned as a uint8_t with a value of 0x1D. When I'm comparing those, I'm finding the value of the who am I response is something like 0x0000000000000000000001D or something along those lines. To resolve this issue, I cast my #define variable so that now my code looks like this:

    if(m_sample==(uint8_t)WHO_AM_I_RESPONSE)
        return true;
    else
        return false;

I can hover over m_sample and see the value is 0x1d and do the same with the cast variable of the who am I response (which also provides 0x1d) but find that it still returns false.

I decided to change the who am I response from a #define to:

uint8_t WHO_AM_I_RESPONSE = 0x1d;

in case the issue is related to comparing a #define (even if it's cast properly) to a declared variable but still got the same issue. Then I cast m_sample to a uint8_t (just in case). That ALSO failed.

So now, I'm changing my optimization setting from 0 to none and still no luck.

Is there something wrong with my if statement that I'm not seeing? If both sides are 0x1d, I don't see how it's possible to be returning false in any of these cases. Any help you can give me would be greatly appreciated! Thanks!

RE: TWI Optimization Issue?

Jørgen Holmefjord — Fri, 05 Jun 2020 12:41:32 GMT

Hi,

If the compiler has decided that the outcome of the error checking is known at compile-time, the instructions may be optimized away. If there are no operations at the line, there is no place for the debugger to break. You can reduce/disable the optimization level in the project options. If you want check the error code, you can print it using NRF_LOG or similar.

Best regards,
Jørgen