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util: add clz() and use __builtin_clz() if available

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Add a macro for the clz (count leading zeros) operation.

Use the GCC __builtin_clz() for clz() if it is available and fall
back to a shift based implementation on systems that do not set
HAVE___BUILTIN_CLZ.
This commit is contained in:
Bryn M. Reeves 2016-12-12 12:03:50 +00:00
parent 83a9cd5258
commit 107bc13db3
1 changed files with 41 additions and 0 deletions
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41
lib/misc/util.h
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@ -35,6 +35,47 @@
#define uninitialized_var(x) x = x
#endif
/*
* GCC 3.4 adds a __builtin_clz, which uses the count leading zeros (clz)
* instruction on arches that have one. Provide a fallback using shifts
* and comparisons for older compilers.
*/
#ifdef HAVE___BUILTIN_CLZ
#define clz(x) __builtin_clz((x))
#else /* ifdef HAVE___BUILTIN_CLZ */
unsigned _dm_clz(unsigned x)
{
int n;
if ((int)x <= 0) return (~x >> 26) & 32;
n = 1;
if ((x >> 16) == 0) {
n = n + 16;
x = x << 16;
}
if ((x >> 24) == 0) {
n = n + 8;
x = x << 8;
}
if ((x >> 28) == 0) {
n = n + 4;
x = x << 4;
}
if ((x >> 30) == 0) {
n = n + 2;
x = x << 2;
}
n = n - (x >> 31);
return n;
}
#define clz(x) _dm_clz((x))
#endif /* ifdef HAVE___BUILTIN_CLZ */
#define KERNEL_VERSION(major, minor, release) (((major) << 16) + ((minor) << 8) + (release))
/* Define some portable printing types */