From 29c86bb03ab6d9d9bb51173cdd01dd5c5e380ac0 Mon Sep 17 00:00:00 2001
From: Anthony Pesch <inolen@gmail.com>
Date: Wed, 18 Oct 2017 17:37:10 -0400
Subject: [PATCH] avoid using popcnt intrinsics to support older processors
 such as the core 2 duo which are plenty fast enough to support

---
 src/core/math.h | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/src/core/math.h b/src/core/math.h
index 0fb5ac85..9af7604f 100644
--- a/src/core/math.h
+++ b/src/core/math.h
@@ -17,14 +17,21 @@ static inline uint32_t bswap24(uint32_t v) {
   return ((v & 0xff) << 16) | (v & 0x00ff00) | ((v & 0xff0000) >> 16);
 }
 
+static inline int popcnt32(uint32_t v) {
+  /* avoid using popcnt intrinsics to support older processors such as the
+     core 2 duo which are plenty fast enough to support */
+  v = (v & 0x55555555) + ((v >> 1) & 0x55555555);
+  v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
+  v = (v & 0x0f0f0f0f) + ((v >> 4) & 0x0f0f0f0f);
+  v = (v & 0x00ff00ff) + ((v >> 8) & 0x00ff00ff);
+  v = (v & 0x0000ffff) + ((v >> 16) & 0x0000ffff);
+  return (int)v;
+}
+
 #if COMPILER_MSVC
 
 #include <intrin.h>
 
-static inline int popcnt32(uint32_t v) {
-  return __popcnt(v);
-}
-
 static inline int clz32(uint32_t v) {
   unsigned long r = 0;
   _BitScanReverse(&r, v);
@@ -55,10 +62,6 @@ static inline uint32_t bswap32(uint32_t v) {
 
 #else
 
-static inline int popcnt32(uint32_t v) {
-  return __builtin_popcount(v);
-}
-
 static inline int clz32(uint32_t v) {
   return __builtin_clz(v);
 }