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    • generating position independent code problem on STM32F4

    Posted by SyUm75 on 2017-11-13 10:13

    Dear Bernard,

    thanks for your quick reply, I did not see the post (did not receive the email).

    I have in my project a part I'd like to compile with the fPIC option.
    I made a call to a test_function from a function in the code compiled with the fPIC option. The test_function also resides in a file compiled with the fPIC option.

    Here is an excerpt of the generated assembly (copied from the disassembly view in the eclipse plugin) when using -fPIC option:

    135 int ret = test_function(4);
    08050802: movs r0, #4
    08050804: bl 0x8050964

    It appears the generated fPIC code does NOT use relative call inside the fPIC compiled module.

    But maybe I have been tricked by the assembly display or do I miss something?

    Thanks for your help,
    Best regards,
    Sylvain

    Reads: 3165
    Posted by dautrevaux on 2017-11-13 18:38 France

    Hi Sylvain,

    As I said in my previous post, BL is ALWAYS using relative addressing; to use absolute adressing you would have to use code like this:
    LD R12,=test_function
    BLX R12
    where the first instruction will in turn be displayed in disassembly as two instruction: a MOVW #xxxx to load the 16 lsb (setting the msb to 0) and a MOVT #xxxx to load the 16 msb

    Thus the code you show is indeed using relative addressing, so is perfectly valid in PIC mode.

    Bernard (Ac6)