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

[Andrew Sterian <steriana@xxxxxxxxx>]

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