From b69f994a8f284b7931f5383a8de323a2986d86e5 Mon Sep 17 00:00:00 2001
From: "dgrowley@gmail.com"
Date: Tue, 7 Apr 2020 11:02:47 +1200
Subject: [PATCH v7 1/3] Add functions to calculate the next power of 2
There are many areas in the code where we need to determine the next
highest power of 2 of a given number. We tend to always do that in an
ad-hoc way each time, generally with some tight for loop which performs a
bitshift left once per loop and goes until it finds a number above the
given number.
Here we add two generic functions which make use of the existing
pg_leftmost_one_pos* functions which, when available will allow us to
calculate the next power of 2 without any looping.
Here we don't add any code which uses these new functions. That will be
done in followup commits.
---
src/include/port/pg_bitutils.h | 72 ++++++++++++++++++++++++++++++++++
1 file changed, 72 insertions(+)
diff --git a/src/include/port/pg_bitutils.h b/src/include/port/pg_bitutils.h
index 498e532308..4ca92f076d 100644
--- a/src/include/port/pg_bitutils.h
+++ b/src/include/port/pg_bitutils.h
@@ -129,6 +129,78 @@ pg_rightmost_one_pos64(uint64 word)
#endif /* HAVE__BUILTIN_CTZ */
}
+/*
+ * pg_nextpower2_32
+ * Returns the next highest power of 2 of 'num', or 'num', if it's
+ * already a power of 2.
+ *
+ * 'num' mustn't be 0 or be above PG_UINT32_MAX / 2 + 1.
+ */
+static inline uint32
+pg_nextpower2_32(uint32 num)
+{
+ Assert(num > 0 && num <= PG_UINT32_MAX / 2 + 1);
+
+ /*
+ * A power 2 number has only 1 bit set. Subtracting 1 from such a number
+ * will turn on all previous bits resulting in no common bits being set
+ * between num and num-1.
+ */
+ if ((num & (num - 1)) == 0)
+ return num; /* already power 2 */
+
+ return ((uint32) 1) << (pg_leftmost_one_pos32(num) + 1);
+}
+
+/*
+ * pg_nextpower2_64
+ * Returns the next highest power of 2 of 'num', or 'num', if it's
+ * already a power of 2.
+ *
+ * 'num' mustn't be 0 or be above PG_UINT64_MAX / 2 + 1.
+ */
+static inline uint64
+pg_nextpower2_64(uint64 num)
+{
+ Assert(num > 0 && num <= PG_UINT64_MAX / 2 + 1);
+
+ /*
+ * A power 2 number has only 1 bit set. Subtracting 1 from such a number
+ * will turn on all previous bits resulting in no common bits being set
+ * between num and num-1.
+ */
+ if ((num & (num - 1)) == 0)
+ return num; /* already power 2 */
+
+ return ((uint64) 1) << (pg_leftmost_one_pos64(num) + 1);
+}
+
+/*
+ * pg_ceil_log2_32
+ * Returns equivalent of ceil(log2(num))
+ */
+static inline uint32
+pg_ceil_log2_32(uint32 num)
+{
+ if (num < 2)
+ return 0;
+ else
+ return pg_leftmost_one_pos32(num - 1) + 1;
+}
+
+/*
+ * pg_ceil_log2_64
+ * Returns equivalent of ceil(log2(num))
+ */
+static inline uint64
+pg_ceil_log2_64(uint64 num)
+{
+ if (num < 2)
+ return 0;
+ else
+ return pg_leftmost_one_pos64(num - 1) + 1;
+}
+
/* Count the number of one-bits in a uint32 or uint64 */
extern int (*pg_popcount32) (uint32 word);
extern int (*pg_popcount64) (uint64 word);
--
2.25.1