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libdrgn: improve and document bit operations

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fls() can be implemented with __bitop(), and we can get rid of clz() since
it's only used by fls().
This commit is contained in:
Omar Sandoval 2020-05-08 00:08:41 -07:00
parent f49d68d8f9
commit 340e00dfb5
1 changed files with 30 additions and 14 deletions
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44
libdrgn/util.h
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@ -101,26 +101,41 @@
#define __bitop(x, unique_x, op) ({ \
__auto_type unique_x = (x); \
\
static_assert(sizeof(unique_x) <= sizeof(unsigned long long), \
"type is too large"); \
(unsigned int)(sizeof(unique_x) <= sizeof(unsigned int) ? \
__builtin_##op(unique_x) : \
op(unique_x) : \
sizeof(unique_x) <= sizeof(unsigned long) ? \
__builtin_##op##l(unique_x) : \
__builtin_##op##ll(unique_x)); \
op##l(unique_x) : \
op##ll(unique_x)); \
})
#define clz(x) __bitop(x, __UNIQUE_ID(__x), clz)
#define ctz(x) __bitop(x, __UNIQUE_ID(__x), ctz)
/**
* Return the number of trailing least significant 0-bits in @p x. This is
* undefined if @p x is zero.
*/
#define ctz(x) __bitop(x, __UNIQUE_ID(__x), __builtin_ctz)
#define __fls(x, unique_x) ({ \
__auto_type unique_x = (x); \
\
unique_x ? 1U + ((8U * max(sizeof(unique_x), \
sizeof(unsigned int)) - 1U) ^ \
clz(unique_x)) : 0U; \
})
/*
* The straightfoward implementation is bits - clz. However, as noted by the
* folly implementation: "If X is a power of two, X - Y = 1 + ((X - 1) ^ Y).
* Doing this transformation allows GCC to remove its own xor that it adds to
* implement clz using bsr."
*
* This doesn't do the normal macro argument safety stuff because it should only
* be used via __bitop() which already does it.
*/
#define ____fls(x, type, suffix) \
(x ? 1 + ((8 * sizeof(type) - 1) ^ __builtin_clz##suffix(x)) : 0)
#define __fls(x) ____fls(x, unsigned int,)
#define __flsl(x) ____fls(x, unsigned long, l)
#define __flsll(x) ____fls(x, unsigned long long, ll)
#define fls(x) __fls(x, __UNIQUE_ID(__x))
/**
* Return one plus the index of the most significant 1-bit of @p x or 0 if @p x
* is 0.
*/
#define fls(x) __bitop(x, __UNIQUE_ID(__x), __fls)
#define __next_power_of_two(x, unique_x) ({ \
__auto_type unique_x = (x); \
@ -136,6 +151,7 @@
*/
#define next_power_of_two(x) __next_power_of_two(x, __UNIQUE_ID(__x))
/** Iterate over each 1-bit in @p mask. This modifies @c mask. */
#define for_each_bit(i, mask) \
for (i = -1; mask && (i = ctz(mask), mask &= mask - 1, 1);)