[Bug rtl-optimization/55757] New: Suboptimal interrupt prologue/epilogue for ARMv7-M (Cortex-M3)

["freddie_chopin at op dot pl" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55757

             Bug #: 55757
           Summary: Suboptimal interrupt prologue/epilogue for ARMv7-M
                    (Cortex-M3)
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: freddie_chopin@op.pl


With a Cortex-M3 microcontroller (ARMv7-M) and an empty interrupt handler
function:

void DMA_IRQHandler(void) __attribute((interrupt));
void DMA_IRQHandler(void)
{

}

Results in suboptimal prologue/epilogue:

000023b4 <DMA_IRQHandler>:
void DMA_IRQHandler(void) __attribute((interrupt));
void DMA_IRQHandler(void)
{
    23b4:    4668          mov    r0, sp
    23b6:    f020 0107     bic.w    r1, r0, #7
    23ba:    468d          mov    sp, r1
    23bc:    b501          push    {r0, lr}
}
    23be:    e8bd 4001     ldmia.w    sp!, {r0, lr}
    23c2:    4685          mov    sp, r0
    23c4:    4770          bx    lr
    ...

Without the __attribute__ the function is fine, just a single "bx lr".

This behavior is incorrect not only because r0 and lr are unused, but also
because on ARMv7-M these registers (r0-r3, lr, ip, sp, pc, psr) are saved by
hardware, so there's no point in saving them again.