Duff's device（C循环展开）

最新推荐文章于 2024-06-13 15:54:31 发布

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最新推荐文章于 2024-06-13 15:54:31 发布

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本文详细介绍了Duff'sdevice循环展开技术的原理与应用，通过实例展示了如何利用该技术提高程序效率，并提供了通用化的实现方式及实际调用场景，揭示了其在代码优化中的无穷魅力。

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Duff's device实际上就是用循环展开的方式来提高程序效率的办法，循环展开可以有效利用CPU资源提升效率，当然我们也可以不借助它，可以分两步走，第一步展开，第二步计算剩下部分，但是代码不简洁，通用性也没有这个强。

下面介绍下Duff's device，假定要copy 20 个字节

int count;                        // Set to 20
{
    int n = (count + 7) / 8;      // n is now 3.  (The "while" is going
                                  //              to be run three times.)

    switch (count % 8) {          // The remainder is 4 (20 modulo 8) so
                                  // jump to the case 4

    case 0:                       // [skipped]
             do {                 // [skipped]
                 *to = *from++;   // [skipped]
    case 7:      *to = *from++;   // [skipped]
    case 6:      *to = *from++;   // [skipped]
    case 5:      *to = *from++;   // [skipped]
    case 4:      *to = *from++;   // Start here.  Copy 1 byte  (total 1)
    case 3:      *to = *from++;   // Copy 1 byte (total 2)
    case 2:      *to = *from++;   // Copy 1 byte (total 3)
    case 1:      *to = *from++;   // Copy 1 byte (total 4)
           } while (--n > 0);     // N = 3 Reduce N by 1, then jump up
                                  //       to the "do" if it's still
    }                             //        greater than 0 (and it is)
}

下面给出一个通用的Duff's device，以及对它的调用

#define DUFF_DEVICE_8(aCount, aAction)	\
do {	\
int count_ = (aCount);	\
int times_ = (count_ + 7) >> 3;	\
switch (count_ & 7){	\
case 0: do { aAction;	\
case 7: aAction;	\
case 6: aAction;	\
case 5: aAction;	\
case 4: aAction;	\
case 3: aAction;	\
case 2: aAction;	\
case 1: aAction;	\
} while (--times_ > 0);	\
}	\
} while (0)

在下面的调用代码中看到了Duff's device的无穷魅力。

int CopyToEEPROM(const void* pSource, void* pDest, unsigned len) {

	// Anything to copy?
	if (len > 0) {
		const char* s = (const char*)pSource;
		char* d = (char*)pDest;

		// First, need to copy data to page buffer
		DUFF_DEVICE_8(len, *d++ = *s++);

		// Start EEPROM write cycle
		HAL_EEPROM_WRITE();

		// Check if data was transferred correctly
		s = (const char*)pSource;
		d = (char*)pDest;
		DUFF_DEVICE_8(len， if (*d++ != *s++) return 1; /* Fail */);
	}
	return 0; // Success
}

http://www.drdobbs.com/a-reusable-duff-device/184406208