Add a new notion of "stamps" to be a fast 32-bit monotonic timestamp

The goal here is to replace our use of msec-based timestamps with
something less precise, but easier to calculate.  We're doing this
because calculating lots of msec-based timestamps requires lots of
64/32 division operations, which can be inefficient on 32-bit
platforms.

We make sure that these stamps can be calculated using only the
coarse monotonic timer and 32-bit bitwise operations.

1 files changed, 17 insertions, 0 deletions

diff --git a/src/common/compat_time.h b/src/common/compat_time.h
index 5ea4aae42b..462dceccea 100644
--- a/src/common/compat_time.h
+++ b/src/common/compat_time.h
@@ -117,6 +117,23 @@ uint64_t monotime_coarse_absolute_msec(void);
 #define monotime_coarse_absolute_msec monotime_absolute_msec
 #endif /* defined(MONOTIME_COARSE_FN_IS_DIFFERENT) */
 
+/**
+ * Return a "timestamp" approximation for a coarse monotonic timer.
+ * This timestamp is meant to be fast to calculate and easy to
+ * compare, and have a unit of something roughly around 1 msec.
+ *
+ * It will wrap over from time to time.
+ *
+ * It has no defined zero point.
+ */
+uint32_t monotime_coarse_to_stamp(const monotime_coarse_t *t);
+/**
+ * Convert a difference, expressed in the units of monotime_coarse_to_stamp,
+ * into an approximate number of milliseconds.
+ */
+uint64_t monotime_coarse_stamp_units_to_approx_msec(uint64_t units);
+uint32_t monotime_coarse_get_stamp(void);
+
 #if defined(MONOTIME_COARSE_TYPE_IS_DIFFERENT)
 int64_t monotime_coarse_diff_nsec(const monotime_coarse_t *start,
     const monotime_coarse_t *end);