如何控制C编译器优化的内容？

Question

我正在使用Silicon Labs IDE和SDCC编译器在C语言中为嵌入式设备编写固件.器件架构基于8051系列.有问题的功能如下所示.该功能用于设置MCU上的端口以驱动步进电机.它被一个中断处理程序调用.big switch语句只是将端口设置为下一个电机步骤的正确值.该功能的底部部分查看来自霍尔效应传感器的输入和移动的多个步骤,以便检测电机是否已停止.问题是,由于某种原因,看起来像这样的第二个IF语句if (StallDetector > (GapSize + 20)) { HandleStallEvent(); }似乎总是被优化出来.如果我尝试在HandleStallEvent()通话时设置断点,IDE会给我一条消息,说"此行号没有地址关联".我在阅读汇编时并不是很擅长讲述它正在做什么,但我已经从下面的asm输出中粘贴了一个片段.任何帮助将非常感激.

void OperateStepper(void)
{
    //static bit LastHomeMagState = HomeSensor;
    static bit LastPosMagState = PosSensor;
    if(PulseMotor)
    {
        if(MoveDirection == 1) // Go clockwise
        {
            switch(STEPPER_POSITION) 
            {
                case 'A': 
                     STEPPER_POSITION = 'B';
                     P1 = 0xFD;
                     break;
                case 'B':
                     STEPPER_POSITION = 'C';
                     P1 = 0xFF;
                     break;
                case 'C':
                     STEPPER_POSITION = 'D';
                     P1 = 0xFE;
                     break;
                case 'D':
                     STEPPER_POSITION = 'A';
                     P1 = 0xFC;
                     break; 
                default:
                     STEPPER_POSITION = 'A';
                     P1 = 0xFC;
            }   //end switch
        }
        else                // Go CounterClockwise
        {
            switch(STEPPER_POSITION) 
            {
                case 'A': 
                     STEPPER_POSITION = 'D';
                     P1 = 0xFE;
                     break;
                case 'B': 
                     STEPPER_POSITION = 'A';
                     P1 = 0xFC;
                     break;
                case 'C': 
                     STEPPER_POSITION = 'B';
                     P1 = 0xFD;
                     break;
                case 'D': 
                     STEPPER_POSITION = 'C';
                     P1 = 0xFF;
                     break; 
                default: 
                     STEPPER_POSITION = 'A';
                     P1 = 0xFE;
            }   //end switch
        }   //end else

        MotorSteps++;
        StallDetector++;

        if(PosSensor != LastPosMagState)
        {
            StallDetector = 0;

            LastPosMagState = PosSensor;
        }
        else
        {
            if (PosSensor == ON) 
            {
                if (StallDetector > (MagnetSize + 20))
                {
                    HandleStallEvent();
                }
            }
            else if (PosSensor == OFF) 
            {
                if (StallDetector > (GapSize + 20))
                {
                    HandleStallEvent();
                }
            }
        }

    }   //end if PulseMotor
}

...以及此函数底部的asm输出...

;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:653: if(PosSensor != LastPosMagState)
    mov c,_P1_4
    jb  _OperateStepper_LastPosMagState_1_1,00158$
    cpl c
00158$:
    jc  00126$
    C$MotionControl.c$655$3$7 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:655: StallDetector = 0;
    clr a
    mov _StallDetector,a
    mov (_StallDetector + 1),a
    C$MotionControl.c$657$3$7 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:657: LastPosMagState = PosSensor;
    mov c,_P1_4
    mov _OperateStepper_LastPosMagState_1_1,c
    ret
00126$:
    C$MotionControl.c$661$2$8 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:661: if (PosSensor == ON) 
    jb  _P1_4,00123$
    C$MotionControl.c$663$4$9 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:663: if (StallDetector > (MagnetSize + 20))
    mov a,_MagnetSize
    mov r2,a
    rlc a
    subb    a,acc
    mov r3,a
    mov a,#0x14
    add a,r2
    mov r2,a
    clr a
    addc    a,r3
    mov r3,a
    clr c
    mov a,r2
    subb    a,_StallDetector
    mov a,r3
    subb    a,(_StallDetector + 1)
    jnc 00130$
    C$MotionControl.c$665$5$10 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:665: HandleStallEvent();
    ljmp    _HandleStallEvent
00123$:
    C$MotionControl.c$668$2$8 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:668: else if (PosSensor == OFF) 
    jnb _P1_4,00130$
    C$MotionControl.c$670$4$11 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:670: if (StallDetector > (GapSize + 20))
    mov a,#0x14
    add a,_GapSize
    mov r2,a
    clr a
    addc    a,(_GapSize + 1)
    mov r3,a
    clr c
    mov a,r2
    subb    a,_StallDetector
    mov a,r3
    subb    a,(_StallDetector + 1)
    jnc 00130$
    C$MotionControl.c$672$5$12 ==.
;   C:\SiLabs\Optec Programs\HSFW_HID_SDCC_2\MotionControl.c:672: HandleStallEvent();
    C$MotionControl.c$678$2$1 ==.
    XG$OperateStepper$0$0 ==.
    ljmp    _HandleStallEvent
00130$:
    ret

在我看来,编译器并没有从asm的外观中优化掉第二个if语句但是如果是这样的话为什么IDE不允许我这样设置断点呢？也许这只是一个愚蠢的IDE!

Answer 1

它被称为"尾调用优化".

在调用HandleStallEvent()之后,OperateStepper()没有做任何事情,因此返回它没有意义.你只是对RET做一个RET,这是浪费指令和堆栈槽.

阅读"Lambda:The Ultimate ..."MIT AI Lab备忘录了解更多详情.

在HandleStallEvent()例程中设置断点,而不是调用.