From 49e14405cb513003433bc953eda8767113ac8b2b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Mon, 26 Aug 2024 19:13:11 +0300
Subject: [PATCH] clock: Fix calculation for number of bits needed to store a
 64 bit value

It was using log2(n) but what actually is needed is log2(n) + 1. Also add a
fast-path that uses __builtin_clzll() on gcc/clang.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7431>
---
 subprojects/gstreamer/gst/gstutils.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/subprojects/gstreamer/gst/gstutils.c b/subprojects/gstreamer/gst/gstutils.c
index 9cce0480a2..e691eea9a5 100644
--- a/subprojects/gstreamer/gst/gstutils.c
+++ b/subprojects/gstreamer/gst/gstutils.c
@@ -4454,10 +4454,15 @@ gst_util_group_id_next (void)
   return ret;
 }
 
-/* Compute log2 of the passed 64-bit number by finding the highest set bit */
+/* Compute the number of bits needed at least to store `in` */
 static guint
-gst_log2 (GstClockTime in)
+gst_bit_storage_uint64 (guint64 in)
 {
+#if defined(__GNUC__) && __GNUC__ >= 4
+  return in ? 64 - __builtin_clzll (in) : 1;
+#else
+  /* integer log2(v) from:
+     <http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog> */
   const guint64 b[] =
       { 0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000, 0xFFFFFFFF00000000LL };
   const guint64 S[] = { 1, 2, 4, 8, 16, 32 };
@@ -4471,7 +4476,8 @@ gst_log2 (GstClockTime in)
     }
   }
 
-  return count;
+  return count + 1;             // + 1 to get the number of storage bits needed
+#endif
 }
 
 /**
@@ -4649,7 +4655,7 @@ gst_calculate_linear_regression (const GstClockTime * xy,
    * That means that each number must require at most (63 - 1) / 2 bits = 31
    * bits of storage.
    */
-  max_bits = gst_log2 (MAX (1, MAX (xmax - xmin, ymax - ymin)));
+  max_bits = gst_bit_storage_uint64 (MAX (1, MAX (xmax - xmin, ymax - ymin)));
   if (max_bits > 31)
     pshift = max_bits - 31;