Re: [arm-gnu] Why is R3 pushed/popped on function entry/exit (Cortex-M3)?

[Travis Geiselbrecht <Travis.Geiselbrecht@xxxxxxxx>]

That's probably to maintain 8 byte alignment on the stack, another requirement of EABI. Sometimes the compiler will push extra registers to keep an even amount on the stack.

On May 19, 2010, at 12:43 PM, Andrew Sterian wrote:

> Can someone help me understand why R3 takes part in the PUSH/POP
> instructions for this:
> 
> --------------<test.c>-----------------
> extern int func(int i);
> 
> int test(int i)
> {
>  return func(i)+5;
> }
> --------------<test.c>-----------------
> 
> Compiled with (2010q1 release G++ Lite):
> 
> arm-none-eabi-gcc -Wall -march=armv7-m -mcpu=cortex-m3 -mthumb
> -mfix-cortex-m3-ldrd -Os -S test.c
> 
> Result:
> --------------<test.s>-----------------
> 	push	  {r3, lr}
> 	bl	  func
> 	adds	  r0, r0, #5
> 	pop	  {r3, pc}
> --------------<test.s>-----------------
> 
> My understanding from the ARM Procedure Call Standard is that only R4
> upwards need to be saved in a function call. Why wouldn't it be enough
> to just PUSH/POP the LR?
> 
> Thanks,
> 
> -- 
> Andrew Sterian