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basic/util: add macro variants of log2 functions

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The macro variants can be used in static initializers.

The same guard against calling __builtin_clz(0) is added as for
__builtin_clzll(0), since that's undefined behaviour too. Our code
wouldn't call it, but this avoids a potential pitfall with the macro.
All variants map 0→0. Otherwise we'd often have to handle 0 specially
in callers.

__builtin_clz takes unsigned as the argument, so there's no LOG2I macro.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek
2021-11-29 11:12:52 +01:00
parent 58c34be864
commit f4ada1b42f
2 changed files with 81 additions and 24 deletions
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src/basic/util.h
+23 -13
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@@ -22,11 +22,20 @@ void in_initrd_force(bool value);
int on_ac_power(void);
static inline unsigned log2u64(uint64_t n) {
/* Note: log2(0) == log2(1) == 0 here and below. */
#define CONST_LOG2ULL(x) ((x) > 1 ? (unsigned) __builtin_clzll(x) ^ 63U : 0)
#define NONCONST_LOG2ULL(x) ({ \
unsigned long long _x = (x); \
_x > 1 ? (unsigned) __builtin_clzll(_x) ^ 63U : 0; \
})
#define LOG2ULL(x) __builtin_choose_expr(__builtin_constant_p(x), CONST_LOG2ULL(x), NONCONST_LOG2ULL(x))
static inline unsigned log2u64(uint64_t x) {
#if __SIZEOF_LONG_LONG__ == 8
return (n > 1) ? (unsigned) __builtin_clzll(n) ^ 63U : 0;
return LOG2ULL(x);
#else
#error "Wut?"
# error "Wut?"
#endif
}
@@ -34,26 +43,27 @@ static inline unsigned u32ctz(uint32_t n) {
#if __SIZEOF_INT__ == 4
return n != 0 ? __builtin_ctz(n) : 32;
#else
#error "Wut?"
# error "Wut?"
#endif
}
static inline unsigned log2i(int x) {
assert(x > 0);
#define CONST_LOG2U(x) ((x) > 1 ? __SIZEOF_INT__ * 8 - __builtin_clz(x) - 1 : 0)
#define NONCONST_LOG2U(x) ({ \
unsigned _x = (x); \
_x > 1 ? __SIZEOF_INT__ * 8 - __builtin_clz(_x) - 1 : 0; \
})
#define LOG2U(x) __builtin_choose_expr(__builtin_constant_p(x), CONST_LOG2U(x), NONCONST_LOG2U(x))
return __SIZEOF_INT__ * 8 - __builtin_clz(x) - 1;
static inline unsigned log2i(int x) {
return LOG2U(x);
}
static inline unsigned log2u(unsigned x) {
assert(x > 0);
return sizeof(unsigned) * 8 - __builtin_clz(x) - 1;
return LOG2U(x);
}
static inline unsigned log2u_round_up(unsigned x) {
assert(x > 0);
if (x == 1)
if (x <= 1)
return 0;
return log2u(x - 1) + 1;