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From: "joey.ye at arm dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/55757] Suboptimal interrupt prologue/epilogue for ARMv7-M (Cortex-M3)
Date: Fri, 21 Dec 2012 03:23:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55757

Joey Ye <joey.ye at arm dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |joey.ye at arm dot com

--- Comment #4 from Joey Ye <joey.ye at arm dot com> 2012-12-21 03:23:07 UTC ---
> An interrupt handler function (void something(void)), but without attribute,
> doing something inside (posts a FreeRTOS semaphore, calls vPortYieldFromISR()
> if it's needed) actually saves a lot of registers on entry:
>     23b4:    b507          push    {r0, r1, r2, lr}
Pushing of scratch registers can be used to 
1. align stack, which Richard has explained
2. allocate stack frame, as a code size optimization of sub sp, #x

Explain with following example:
extern void bar(int *, int *);
void foo()
{
    int a, b;
    bar(&a, &b);
}
Built with -Os -mcpu=cortex-m3:
push {r0, r1, r2, lr} 

Here, pushing of r0 and r1 allocates a 8-byte frame for local variables.
Pushing of r2 is to make sp aligned to 8 bytes together with pushing lr. Values
of r0-r2 pushed to stack don't really matter.

But built with -O2:
    push    {lr}
    sub sp, sp, #12

Former is better on code size, latter wins on performance. Hopefully this
explains everything.