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arm-clang removes variable marked as used

Roman Khazanskii

I'm using Keil 5.23 and trying to use arm-clang compiler v6.6.

I want to create an array that will contain compilation date and time. I need this array to be placed in flash memory, so I made it const.
I don't need to access this array, so I made it static.

Now I have a problem. If I declare it like this:

static const char date_time[] =
    {
        "compilation time: " __DATE__ ", " __TIME__
    };

It gets removed by compiler, even though I set -O0. Adding __attribute__((used)). According to this page www.keil.com/.../armclang_mig_chr1398848377314.htm even objects marked as 'used' can be removed by linker nonetheless.
So I need to add a linker option --keep=symbol_name or --noremove.

If I add --noremove, linker keeps to much data, bloating binary size from 4KiB to 17KiB.
If I use --keep, linker tells me that it can't find that symbol:

.\obj\dummy.axf: Warning: L6320W: Ignoring --keep command. Cannot find argument 'date_time'.


Even if I don't declare it as static.

Adding volatile doesn't help either.

Please, tell me, what am I doing wrong? How can I tell compiler and linker not to remove this array?

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  • 0

    I don't know the answer for sure but I see a few clues.

    1) I think you are skimming and making assumptions that are probably not correct.

    Note:
In Arm Compiler 6, functions marked with __attribute__((used))
can still be removed by linker unused section removal. To prevent the linker from
removing these sections, you can use either the --keep=symbol or the
--no_remove armlink options. In Arm Compiler 5, functions marked with
__attribute__((used)) are not removed by the linker.

    Your static const char is not a function. If --keep is looking for a function with that name it is not going to find that name.

    2) There is no global symbol that the linker can see for date_time. You made it static. If you want a global symbol date_time that the linker can see you need to remove the static qualifier.

    3) I looks like what you may want to happen is for the section that contains the static char const to be kept. You could try --keep=YourFileName.o

    I would probably try removing the static and saying --keep=date_time and see if that works. That seems the closest to what you wanted (my assumption here)

    Next would be keep the section that contains it. I don't think you need to remove the static qualifier in this case. I am not sure how important that qualifier is to you (I would have to make another assumption)

    I am sure there are more ways to solve this and as I said, I have not actually solved this yet, I am just providing observations.

Reply
  • 0

    I don't know the answer for sure but I see a few clues.

    1) I think you are skimming and making assumptions that are probably not correct.

    Note:
In Arm Compiler 6, functions marked with __attribute__((used))
can still be removed by linker unused section removal. To prevent the linker from
removing these sections, you can use either the --keep=symbol or the
--no_remove armlink options. In Arm Compiler 5, functions marked with
__attribute__((used)) are not removed by the linker.

    Your static const char is not a function. If --keep is looking for a function with that name it is not going to find that name.

    2) There is no global symbol that the linker can see for date_time. You made it static. If you want a global symbol date_time that the linker can see you need to remove the static qualifier.

    3) I looks like what you may want to happen is for the section that contains the static char const to be kept. You could try --keep=YourFileName.o

    I would probably try removing the static and saying --keep=date_time and see if that works. That seems the closest to what you wanted (my assumption here)

    Next would be keep the section that contains it. I don't think you need to remove the static qualifier in this case. I am not sure how important that qualifier is to you (I would have to make another assumption)

    I am sure there are more ways to solve this and as I said, I have not actually solved this yet, I am just providing observations.

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