Hi all,
I am using the Delay function as follows.
static void Delay(volatile int d) {
d = d*20;
while (d--) asm volatile("NOP");
}
However, sometimes when adding printf code, Delay function becomes slow.
So, when I analyzed the cause, I was able to see that the assembly code changes a lot if volatile is used as the argument of the Delay function.
If volatile is not added, the slowdown seems to disappear.
I used STM32H743XI and set the optimization option to O2.
- When volatile is not included in the argument assembly code
=> static void Delay(int d)
0806a27c <Delay>:
806a27c: eb00 0080 add.w r0, r0, r0, lsl #2
806a280: 0080 lsls r0, r0, #2
806a282: 1e43 subs r3, r0, #1
806a284: b118 cbz r0, 806a28e <Delay+0x12>
806a286: bf00 nop
806a288: f113 33ff adds.w r3, r3, #4294967295
806a28c: d2fb bcs.n 806a286 <Delay+0xa>
806a28e: 4770 bx lr
- Assembly code when volatile is added to the argument
=> static void Delay(volatile int d)
0806a27c <Delay>:
806a27c: b082 sub sp, #8
806a27e: 9001 str r0, [sp, #4]
806a280: 9b01 ldr r3, [sp, #4]
806a282: eb03 0383 add.w r3, r3, r3, lsl #2
806a286: 009b lsls r3, r3, #2
806a288: 9301 str r3, [sp, #4]
806a28a: 9b01 ldr r3, [sp, #4]
806a28c: 1e5a subs r2, r3, #1
806a28e: 9201 str r2, [sp, #4]
806a290: b12b cbz r3, 806a29e <Delay+0x22>
806a292: bf00 nop
806a294: 9b01 ldr r3, [sp, #4]
806a296: 1e5a subs r2, r3, #1
806a298: 9201 str r2, [sp, #4]
806a29a: 2b00 cmp r3, #0
806a29c: d1f9 bne.n 806a292 <Delay+0x16>
806a29e: b002 add sp, #8
806a2a0: 4770 bx lr
806a2a2: bf00 nop
I do not know why the code changes a lot by adding volatile, so I ask this question.
I'd like to know the definitive answer to whether this can slow down the code a lot.
Thank you.
Best Regards,
Gyosun.
