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From: "freddie_chopin at op dot pl" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/55757] Suboptimal interrupt prologue/epilogue for ARMv7-M (Cortex-M3)
Date: Thu, 20 Dec 2012 15:24:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55757

--- Comment #1 from Freddie Chopin <freddie_chopin at op dot pl> 2012-12-20 15:23:25 UTC ---
BTW - it seems that optimization settings don't make any difference here - the
code below was compiled with -Os, on all other levels (1,2,3) the assembly
looks like this:

00002e90 <DMA_IRQHandler>:
void DMA_IRQHandler(void) __attribute((interrupt));
void DMA_IRQHandler(void)
{
    2e90:    4668          mov    r0, sp
    2e92:    f020 0107     bic.w    r1, r0, #7
    2e96:    468d          mov    sp, r1
    2e98:    b401          push    {r0}
}
    2e9a:    bc01          pop    {r0}
    2e9c:    4685          mov    sp, r0
    2e9e:    4770          bx    lr


So it just saves r0 only, without saving lr. It's actually 2 bytes smaller than
the assembly done for size optimizations (;

Without optimization (-O0) I get:

0000473c <DMA_IRQHandler>:
void DMA_IRQHandler(void) __attribute((interrupt));
void DMA_IRQHandler(void)
{
    473c:    4668          mov    r0, sp
    473e:    f020 0107     bic.w    r1, r0, #7
    4742:    468d          mov    sp, r1
    4744:    b481          push    {r0, r7}
    4746:    af00          add    r7, sp, #0
}
    4748:    46bd          mov    sp, r7
    474a:    bc81          pop    {r0, r7}
    474c:    4685          mov    sp, r0
    474e:    4770          bx    lr

The commandline options used to compile:

arm-none-eabi-gcc -c -mcpu=cortex-m3 -mthumb -O0 -ffunction-sections
-fdata-sections -Wall -Wstrict-prototypes -Wextra -std=gnu99 -g -ggdb3
-fverbose-asm -Wa,-ahlms=out/uart.lst   -MD -MP -MF out/uart.d <some include
dirs> <input file> <output file>