changeset 0655a3d869ad in /z/repo/gem5
details: http://repo.gem5.org/gem5?cmd=changeset;node=0655a3d869ad
description:
misc: Add functions for doing popcount and power-of-two checking
Adds two public domain algorithms for determining number of set bits
and also
whether a value is a power of two, uses the builtin that is available
in GCC
and clang for popcount.
diffstat:
src/base/bitfield.hh | 33 +++++++++++++++++++++++++++++++++
1 files changed, 33 insertions(+), 0 deletions(-)
diffs (41 lines):
diff -r 0644819fc32f -r 0655a3d869ad src/base/bitfield.hh
--- a/src/base/bitfield.hh Sat Sep 20 17:18:26 2014 -0400
+++ b/src/base/bitfield.hh Thu Apr 24 17:41:26 2014 +0100
@@ -178,4 +178,37 @@
return lsb;
}
+/**
+ * Checks if a number is a power of two, or zero.
+ */
+template <class T>
+inline bool
+isPow2(T v) {
+ return (v & (v - 1)) == (T)0;
+}
+
+/**
+ * Returns the number of set ones in the provided value.
+ * PD algorithm from
+ * http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
+ */
+inline int
+popCount(uint64_t val) {
+#ifndef __has_builtin
+ #define __has_builtin(foo) 0
+#endif
+#if defined(__GNUC__) || (defined(__clang__) &&
__has_builtin(__builtin_popcountl))
+ return __builtin_popcountl(val);
+#else
+ const uint64_t m1 = 0x5555555555555555; // ..010101b
+ const uint64_t m2 = 0x3333333333333333; // ..110011b
+ const uint64_t m4 = 0x0f0f0f0f0f0f0f0f; // ..001111b
+ const uint64_t sum = 0x0101010101010101;
+
+ val -= (val >> 1) & m1; // 2 bits count -> 2 bits
+ val = (val & m2) + ((val >> 2) & m2); // 4 bits count -> 4 bits
+ val = (val + (val >> 4)) & m4; // 8 bits count -> 8 bits
+ return (val * sum) >> 56; // horizontal sum
+#endif // defined(__GNUC__) || (defined(__clang__) &&
__has_builtin(__builtin_popcountl))
+}
#endif // __BASE_BITFIELD_HH__
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