php

尝试化简php-mm中关于small_bin的index计算 

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#include<stdio.h>

int zend_mm_small_size_to_bit(int size){
	return (__builtin_clz(size) ^ 0x1f) + 1;
}

int php(int size){
	unsigned int t1, t2;

	if (size <= 64) {
		/* we need to support size == 0 ... */
		return (size - !!size) >> 3;
	} else {
		t1 = size - 1;//311
		t2 = zend_mm_small_size_to_bit(t1) - 3;//
		t1 = t1 >> t2;
		//printf("t1:%d\n",t1);
		t2 = t2 - 3;
		t2 = t2 << 2;
		//printf("t2:%d\n",t2);
	return (int)(t1 + t2);
	}
}

int my0(int size){
	unsigned t1,high,offset;
	t1 = size-1;
	high = zend_mm_small_size_to_bit(t1)-1;
	offset = (t1 - (1<<high) + (1 << (3+high-5))) >> (3+high-5);
	//printf("offset:%d\n",offset);
	return offset + ((high-4)<<2)-1;
}

int my1(int size){
	unsigned t1,high,offset;
	t1 = size-1;
	high = zend_mm_small_size_to_bit(t1)-1;
	offset = (t1 - (1<<high) + (1 << (high-2))) >> (high-2);
	//printf("offset:%d\n",offset);
	return offset + ((high-4)<<2)-1;
}


int my2(int size){
	unsigned t1,high,offset;
	t1 = size-1;
	high = zend_mm_small_size_to_bit(t1)-3;
	offset = (t1 - (1<<(high+2)) + (1 << (high))) >> (high);
	//printf("offset:%d\n",offset);
	return offset + ((high-2)<<2)-1;
}

int my3(int size){
	unsigned t1,high,offset;
	t1 = size-1;
	high = zend_mm_small_size_to_bit(t1)-3;
	offset = ((t1 - (1<<(high+2))) >> (high)) + 1;
	//printf("offset:%d\n",offset);
	return offset + ((high-2)<<2)-1;
}


int my4(int size){
	unsigned t1,high,offset;
	t1 = size-1;
	high = zend_mm_small_size_to_bit(t1)-3;
	offset = ((t1 - (1<<(high+2))) >> (high));
	//printf("offset:%d\n",offset);
	return offset + ((high-2)<<2);
}


int my5(int size){
	unsigned t1,high,offset;
	t1 = size-1;
	high = zend_mm_small_size_to_bit(t1)-3;
	offset = (t1 >> high) -(1<<2) ;
	//printf("offset:%d\n",offset);
	return offset + ((high-2)<<2);
}


int my6(int size){
	unsigned t1,high,offset;
	t1 = size-1;
	high = zend_mm_small_size_to_bit(t1)-3;
	offset = (t1 >> high);
	//printf("offset:%d\n",offset);
	return offset + ((high-3)<<2);
}


int main(){
/*
8,16,24,32,40,48,56,64 (+8) 
80,96,112,128 (16)
160,192,224,256 (32)
320,384,448,512 (64)
640,768,886,1024 (128)
1280,1536,1792,2048 (256)
2560,3072 (512)
*/
	printf("%d\n",my6(2600));
	printf("%d\n",php(2600));
	return 1;
}

small_index在8-64之前按8递增，所以直接除8即可（考虑size==0的情况，!!0==1）

对于t1和t2的我理解是：t1表示区间偏移，t2区间。整体上区间可以划分位(1-64],(64,128],(128,256],(256,512],(512,1024],(1024,2048],(2048,3072]

第一个区间按8划分偏移，第二个区间按16划分偏移，，，，

可以看出来是有规律的，需要申请的内存大小可能落在其中一个区间里面，区间里面在细分。

如果要判断落在那个区间，直觉上要看size最高位的1落在哪个地方，举个例子如果落在2^6上，则一定在(64,128]上(假设已经排除了<=64)。

最高位用bsr算，则为第7bit位，我们需要再算它在这个区间上的偏移，需要先减去前一个区间最大的binsize，然后再除以现区间的划分size。