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Reordering instructions

Arlleex over 4 years ago

Greetings!

Keil uVision 5, ARM Compiler 6.12, -Os balanced, Cortex-M3. Just as an example

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GPIOC->OR = (GPIOC->OR & ~0xFF) | 0x12;
while(1)
{
  u32 i = 100000;
  ...
}
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

GPIOC->OR = (GPIOC->OR & ~0xFF) | 0x12;

while(1)
{
  u32 i = 100000;
  ...
}

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MOVS r1, #0x12
LDR  r0, [r12, #0x00]
BFI  r0, r1, #0, #8
MOVW r1, #0x86A0
STR  r0, [r12, #0x00]
MOVT r1, #0x01
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

MOVS r1, #0x12
LDR  r0, [r12, #0x00]
BFI  r0, r1, #0, #8
MOVW r1, #0x86A0
STR  r0, [r12, #0x00]
MOVT r1, #0x01

As you can see, there was a reordering of instructions. It is quite legal, I understand that perfectly.
But how do you make the compiler not do this here? I want nothing superfluous to be wedged into the GPIOC->OR change

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MOVS r1, #0x12
LDR  r0, [r12, #0x00]
BFI  r0, r1, #0, #8
STR  r0, [r12, #0x00]
MOVW r1, #0x86A0
MOVT r1, #0x01
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

MOVS r1, #0x12
LDR  r0, [r12, #0x00]
BFI  r0, r1, #0, #8
STR  r0, [r12, #0x00]

MOVW r1, #0x86A0
MOVT r1, #0x01

What optimizer barrier should I use and how?