[arm-gnu] some questions about gcc

[yann.poupet@xxxxxxx]

Hi,

I used CS Q12009 to compile some files and I have some questions.

Consider the following C code:

int a(int b)
{
  int c = c;
  return c + b;
}

With the following command:
arm-none-eabi-gcc -Wall -c -o uninit -Os uninit.c
it compiles fine without any warning.

Whereas with the following:

int a(int b)
{
  int c;
  c = c;
  return b + c;
}

I have a warning : "'c' is used uninitialized in this function"

Is there a reason ?

Also, in both case, compiled code is :

00000000 <a>:
   0:   e12fff1e        bx      lr
                        0: R_ARM_V4BX   *ABS*

which is the same binary that would be produced by compiling the following:
int a(int b)
{
  return b;
}

I guess this is a "normal" behaviour since c is not initialized gcc considers it is zero (it's only a guess).



Consider this other c code:

int a(int b)
{
    register int c __asm__("r4");
    return (b + c);
}

This one compiles fine and does what I want, which is return r0 + r4.
However, objdump shows r4 is saved then restored from the stack :

   0:   e52d4004        push    {r4}            ; (str r4, [sp, #-4]!)
   4:   e0800004        add     r0, r0, r4
   8:   e8bd0010        pop     {r4}
   c:   e12fff1e        bx      lr

which is useless since r4 is not modified. Is there a reason ?
Note that if using r3 instead of r4, push/pop does not happen, probably because according to the ABI {r0-r3,ip} don't need to be saved/restored. But still I wonder why r4 is saved/restored.


Thank you.

Regards,

Yann Poupet.