Re: ARM/THUMB long calls stubs

[Sami Kantoluoto <sami dot kantoluoto at embedtronics dot fi>]

On Fri, Aug 06, 2004 at 11:25:04AM +0100, Nick Clifton wrote:
> Hi Sami,
> 
> >So foo() gets called directly with 'bl' but bar() (in test2.c) gets called
> >as "wanted".
> 
> Right - this is because foo() is defined in the same file as the 
> function that calls it: _start().  GCC makes the (reasonable) assumption 
> that since both foo() and _start() are going to be compiled together and 
> placed into the same output section that it knows how far apart they are 
> and that it can avoid the overhead of the long-call.  In fact at high 
> optimisation levels it might even in-line the code from foo() into _start().

My example was missing __attribute__ ((section(".foo"))) from foo(). But
putting that there nothing changes (except output section of foo()).

I think gcc should use the long call (with -mlong-calls / #pragma) if called
and caller functions are in different sections even if the functions are in
the same file. 

> The way to avoid this "feature" (I hesitate to call it a bug) is to 
> compile your code with the "-ffunction-sections" switch added to the GCC 
> command line.  This tells GCC to place each function into its own 
> separate section, and GCC is smart enough to know that different 
> sections may not end up close to each other, so it cannot optimise away 
> the use of the long call.

Ah, I'll try that.

PS. # arm-elf-gcc -mthumb -mlong-calls -Wall -nostdlib -o test test.c test2.c


     -sk

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Attachment: test2.c
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