Teensy 3: Structure packing has an effect on the generated asm where it "shouldn't"
I added this line to the linker script in order to be able to make _SPI0 extern:
I then declared a class which would have all SPI registers as members (only one is needed here to show the effect):
The generated assembler is exactly the same as when using SPI0_MCR from the teensy3 header file:
When I uncomment the packed attribute, the output changes to:
There is an extra ldr instruction (at 468) which, to my understanding, serves no purpose. Why does it do that? Adding the attribute aligned(4) fixes this (i.e. when using __attribute__((packed,aligned(4))) ). The extra nop is needed for padding so that the address constant is aligned.
Regards
Christoph
I added this line to the linker script in order to be able to make _SPI0 extern:
Code:
_SPI0 = 0x4002C000I then declared a class which would have all SPI registers as members (only one is needed here to show the effect):
Code:
class SPI
{
public:
volatile uint32_t MCR;
};// __attribute__((packed));
extern SPI _SPI0;
int main(void)
{
_SPI0.MCR = 0;
}
Code:
00000464 <main>:
464: 4b01 ldr r3, [pc, #4] ; (46c <main+0x8>)
466: 2000 movs r0, #0
468: 6018 str r0, [r3, #0]
46a: 4770 bx lr
46c: 4002c000 .word 0x4002c000
Code:
00000464 <main>:
464: 4b02 ldr r3, [pc, #8] ; (470 <main+0xc>)
466: 2000 movs r0, #0
468: 681a ldr r2, [r3, #0]
46a: 6018 str r0, [r3, #0]
46c: 4770 bx lr
46e: bf00 nop
470: 4002c000 .word 0x4002c000Regards
Christoph