From 3ff9e900b9d52f154cba77d71f83da32dc5111ea Mon Sep 17 00:00:00 2001
From: Julian Fang <julianisabear@gmail.com>
Date: Fri, 5 Jul 2019 01:49:42 +0800
Subject: [PATCH] Implement find first set for reducing division

---
 src/heap.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/src/heap.c b/src/heap.c
index f474b1a0..017b0c5f 100644
--- a/src/heap.c
+++ b/src/heap.c
@@ -386,6 +386,31 @@ bool mi_check_owned(const void* p) {
   return mi_heap_check_owned(mi_get_default_heap(), p);
 }
 
+// find first set: return the index of the lowest set bit.
+static inline uint8_t mi_ffs32(uint32_t x);
+
+#if defined(_MSC_VER)
+#include <intrin.h>
+static inline uint8_t mi_ffs32(uint32_t x) {
+  uint32_t idx;
+  if (_BitScanForward(&idx, x) != 0)return idx;
+
+  return 0;
+}
+#elif defined(__GNUC__) || defined(__clang__)
+static inline uint8_t mi_ffs32(uint32_t x) {
+  return (__builtin_ffs(x))-1;
+}
+#else
+static inline uint8_t mi_ffs32(uint32_t x) {
+  static const uint8_t debruijn[32] = {
+  0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 
+  31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
+};
+  return debruijn[((uint32_t)(x&-x)*0x077CB531U) >> 27];
+}
+#endif
+
 /* -----------------------------------------------------------
   Visit all heap blocks and areas
   Todo: enable visiting abandoned pages, and