Count leading zeroes using builtins if available.
Signed-off-by: Jarno Rajahalme <jrajaha...@nicira.com>
---
lib/util.c | 25 +++++++++++++++++++++++++
lib/util.h | 26 ++++++++++++++++++++++++++
tests/library.at | 1 +
tests/test-util.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 105 insertions(+)
diff --git a/lib/util.c b/lib/util.c
index c2ffab2..118ffb1 100644
--- a/lib/util.c
+++ b/lib/util.c
@@ -892,6 +892,7 @@ log_2_ceil(uint32_t n)
#if __GNUC__ >= 4
/* Defined inline in util.h. */
#else
+/* Returns the number of trailing 0-bits in 'n'. Undefined if 'n' == 0. */
int
raw_ctz(uint64_t n)
{
@@ -914,6 +915,30 @@ raw_ctz(uint64_t n)
return count;
}
+
+/* Returns the number of leading 0-bits in 'n'. Undefined if 'n' == 0. */
+int
+raw_clz(uint64_t n)
+{
+ uint64_t k;
+ int count = 63;
+
+#define CLZ_STEP(X) \
+ k = n >> (X); \
+ if (k) { \
+ count -= X; \
+ n = k; \
+ }
+ CLZ_STEP(32);
+ CLZ_STEP(16);
+ CLZ_STEP(8);
+ CLZ_STEP(4);
+ CLZ_STEP(2);
+ CLZ_STEP(1);
+#undef CLZ_STEP
+
+ return count;
+}
#endif
/* Returns the number of 1-bits in 'x', between 0 and 32 inclusive. */
diff --git a/lib/util.h b/lib/util.h
index acb2d8f..1ee532a 100644
--- a/lib/util.h
+++ b/lib/util.h
@@ -301,9 +301,21 @@ raw_ctz(uint64_t n)
? __builtin_ctz(n)
: __builtin_ctzll(n));
}
+
+static inline int
+raw_clz(uint64_t n)
+{
+ /* With GCC 4.7 on 32-bit x86, if a 32-bit integer is passed as 'n', using
+ * a plain __builtin_ctzll() here always generates an out-of-line function
+ * call. The test below helps it to emit a single 'bsf' instruction. */
+ return (__builtin_constant_p(n <= UINT32_MAX) && n <= UINT32_MAX
+ ? __builtin_clz(n) + 32
+ : __builtin_clzll(n));
+}
#else
/* Defined in util.c. */
int raw_ctz(uint64_t n);
+int raw_clz(uint64_t n);
#endif
/* Returns the number of trailing 0-bits in 'n', or 32 if 'n' is 0. */
@@ -320,6 +332,20 @@ ctz64(uint64_t n)
return n ? raw_ctz(n) : 64;
}
+/* Returns the number of leading 0-bits in 'n', or 32 if 'n' is 0. */
+static inline int
+clz(uint32_t n)
+{
+ return n ? raw_clz(n) - 32 : 32;
+}
+
+/* Returns the number of leading 0-bits in 'n', or 64 if 'n' is 0. */
+static inline int
+clz64(uint64_t n)
+{
+ return n ? raw_clz(n) : 64;
+}
+
/* Returns the rightmost 1-bit in 'x' (e.g. 01011000 => 00001000), or 0 if 'x'
* is 0. */
static inline uintmax_t
diff --git a/tests/library.at b/tests/library.at
index 6e28573..57cdd6c 100644
--- a/tests/library.at
+++ b/tests/library.at
@@ -112,6 +112,7 @@ AT_CLEANUP
m4_foreach(
[testname],
[[ctz],
+ [clz],
[round_up_pow2],
[round_down_pow2],
[count_1bits],
diff --git a/tests/test-util.c b/tests/test-util.c
index 7183f46..3caf712 100644
--- a/tests/test-util.c
+++ b/tests/test-util.c
@@ -113,6 +113,58 @@ test_ctz(int argc OVS_UNUSED, char *argv[] OVS_UNUSED)
check_ctz64(0, 64);
}
+static void
+check_clz(uint32_t x, int n)
+{
+ if (clz(x) != n) {
+ fprintf(stderr, "clz(%"PRIu32") is %d but should be %d\n",
+ x, clz(x), n);
+ abort();
+ }
+}
+
+static void
+check_clz64(uint64_t x, int n)
+{
+ if (clz64(x) != n) {
+ fprintf(stderr, "clz64(%"PRIu64") is %d but should be %d\n",
+ x, clz64(x), n);
+ abort();
+ }
+}
+
+static void
+test_clz(int argc OVS_UNUSED, char *argv[] OVS_UNUSED)
+{
+ int n;
+
+ for (n = 0; n < 32; n++) {
+ /* Check minimum x such that f(x) == n. */
+ check_clz((1u << 31) >> n, n);
+
+ /* Check maximum x such that f(x) == n. */
+ check_clz(UINT32_MAX >> n, n);
+
+ /* Check a random value in the middle. */
+ check_clz((random_uint32() | 1u << 31) >> n, n);
+ }
+
+ for (n = 0; n < 64; n++) {
+ /* Check minimum x such that f(x) == n. */
+ check_clz64((UINT64_C(1) << 63) >> n, n);
+
+ /* Check maximum x such that f(x) == n. */
+ check_clz64(UINT64_MAX >> n, n);
+
+ /* Check a random value in the middle. */
+ check_clz64((random_uint64() | UINT64_C(1) << 63) >> n, n);
+ }
+
+ /* Check clz(0). */
+ check_clz(0, 32);
+ check_clz64(0, 64);
+}
+
/* Returns a random number in the range 'min'...'max' inclusive. */
static uint32_t
random_in_range(uint32_t min, uint32_t max)
@@ -964,6 +1016,7 @@ test_ovs_scan(int argc OVS_UNUSED, char *argv[] OVS_UNUSED)
static const struct command commands[] = {
{"ctz", 0, 0, test_ctz},
+ {"clz", 0, 0, test_clz},
{"round_up_pow2", 0, 0, test_round_up_pow2},
{"round_down_pow2", 0, 0, test_round_down_pow2},
{"count_1bits", 0, 0, test_count_1bits},
--
1.7.10.4
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