RISC-V的确是个好东西，可是，免费的东西往往需要付出代价才能得到了，最近遇到了一个算法中的问题，追了好久，最终追到了这个库函数中，没想到，这个库函数居然还隐藏着一些猫腻。值得记下来啊。
首先上一个在X86平台下的例子：
1 #include <stdio.h>
/* Position of the most significant bit of x */
#define gap8_fl1(x) (31 - __builtin_clz((x))) static int array[32];
int main()
{
int test = 0xFFFFF00; while(test > 0)
{
test = test>>1;
printf("test:%x gap8:%d \n",test,gap8_fl1(test));
} return 0;
}
~
这个是编译器的库函数，表示要获取你输入参数的位数。
测试结果：
test:7ffff80 gap8:26
test:3ffffc0 gap8:25
test:1ffffe0 gap8:24
test:fffff0 gap8:23
test:7ffff8 gap8:22
test:3ffffc gap8:21
test:1ffffe gap8:20
test:fffff gap8:19
test:7ffff gap8:18
test:3ffff gap8:17
test:1ffff gap8:16
test:ffff gap8:15
test:7fff gap8:14
test:3fff gap8:13
test:1fff gap8:12
test:fff gap8:11
test:7ff gap8:10
test:3ff gap8:9
test:1ff gap8:8
test:ff gap8:7
test:7f gap8:6
test:3f gap8:5
test:1f gap8:4
test:f gap8:3
test:7 gap8:2
test:3 gap8:1
test:1 gap8:0
test:0 gap8:0
让我们再看一下在RISC-V平台下的打印：
test:7ffff80 gap8:26
test:3ffffc0 gap8:25
test:1ffffe0 gap8:24
test:fffff0 gap8:23
test:7ffff8 gap8:22
test:3ffffc gap8:21
test:1ffffe gap8:20
test:fffff gap8:19
test:7ffff gap8:18
test:3ffff gap8:17
test:1ffff gap8:16
test:ffff gap8:15
test:7fff gap8:14
test:3fff gap8:13
test:1fff gap8:12
test:fff gap8:11
test:7ff gap8:10
test:3ff gap8:9
test:1ff gap8:8
test:ff gap8:7
test:7f gap8:6
test:3f gap8:5
test:1f gap8:4
test:f gap8:3
test:7 gap8:2
test:3 gap8:1
test:1 gap8:0
test:0 gap8:32
这里就有一个差别，就是当x等于０的时候，__builtin_clz((x)的值在risc-v中居然是-1,这个坑真是的不小啊。算是学习了。