c - 优化级别打破了 C 代码顺序

Question

我有以下 5 行代码，并希望这行与 O2 或 O3 完全按照此顺序执行:

    PORT->Group[GPIO_PORTB].OUTCLR.reg = (volatile uint32_t) 1 << 9;
    TC3->COUNT16.COUNT.reg =  (volatile uint16_t) 0;
    TC3->COUNT16.CC[0].reg = (volatile uint16_t) vusb_driver->in_data->bitlength;
    SERCOM0->SPI.DATA.reg = (volatile uint32_t) 0x54;
    DMAC->Channel[USB_SEND_SD_DMA_CH].CHCTRLA.reg = (volatile uint8_t) DMAC_CHCTRLA_ENABLE;

如果我使用 O2 或 O3 进行优化，代码会在第 264 行中断，因为该行必须在第 265 行之前执行:

  261:      PORT->Group[GPIO_PORTB].OUTCLR.reg = (volatile uint32_t) 1 << 9;
200001EE   ldr  r1, [pc, #84]        
   263:         TC3->COUNT16.CC[0].reg = (volatile uint16_t) vusb_driver->in_data->bitlength;
200001F0   ldr  r5, [pc, #84]        
   264:         SERCOM0->SPI.DATA.reg = (volatile uint32_t) 0x54;
200001F2   ldr  r4, [pc, #88]        
   265:         DMAC->Channel[USB_SEND_SD_DMA_CH].CHCTRLA.reg = (volatile uint8_t) DMAC_CHCTRLA_ENABLE;
200001F4   ldr  r0, [pc, #88]        
   261:         PORT->Group[GPIO_PORTB].OUTCLR.reg = (volatile uint32_t) 1 << 9;
200001F6   mov.w    r6, #512         
200001FA   str.w    r6, [r1, #148]       
   262:         TC3->COUNT16.COUNT.reg =  (volatile uint16_t) 0;
200001FE   strh r2, [r3, #20]        
   263:         TC3->COUNT16.CC[0].reg = (volatile uint16_t) vusb_driver->in_data->bitlength;
20000200   ldr  r2, [r5]         
20000202   ldr  r2, [r2, #20]        
20000204   ldrh.w   r2, [r2, #72]        
20000208   strh r2, [r3, #28]        
   264:         SERCOM0->SPI.DATA.reg = (volatile uint32_t) 0x54;
2000020A   movs r5, #84      
   265:         DMAC->Channel[USB_SEND_SD_DMA_CH].CHCTRLA.reg = (volatile uint8_t) DMAC_CHCTRLA_ENABLE;
2000020C   movs r2, #2       
   264:         SERCOM0->SPI.DATA.reg = (volatile uint32_t) 0x54;
2000020E   str  r5, [r4, #40]   

 

Answer 1

您对 volatile 的使用不正确，您应该将目标对象定义为 volatile以确保它们完全按照程序的顺序写入。