void isr() __attribute__((interrupt));
void isr() { }

void non_isr() { }

// compiled with:
// arm-none-eabi-gcc -O3 -o test.o -mthumb -march=armv7-m -c test.c

// results in (arm-none-eabi-objdump -S test.o):
test.o:     file format elf32-littlearm

Disassembly of section .text:

00000000 <isr>:
   0:   4668        mov r0, sp
   2:   f020 0107   bic.w   r1, r0, #7
   6:   468d        mov sp, r1
   8:   b401        push    {r0}
   a:   bc01        pop {r0}
   c:   4685        mov sp, r0
   e:   4770        bx  lr

00000010 <non_isr>:
  10:   4770        bx  lr
  12:   bf00        nop