mood: Speed up int_sqrt().

[paraslash.git] / mood.c

diff --git a/mood.c b/mood.c
index 54d7b85fa411ef65031d716b6a6d8bb7818b9720..c24b3d8b9da25c011e237b40e4508b0b61894598 100644 (file)
--- a/mood.c
+++ b/mood.c
@@ -84,6 +84,42 @@ struct mood {
  */
 static struct mood *current_mood;
 
+/*
+ * Find the position of the most-significant set bit.
+ *
+ * Copied and slightly adapted from the linux source tree, version 4.9.39
+ * (2017-07).
+ */
+__a_const static uint32_t fls64(uint64_t v)
+{
+       int n = 63;
+       const uint64_t ones = ~(uint64_t)0U;
+
+       if ((v & (ones << 32)) == 0) {
+               n -= 32;
+               v <<= 32;
+       }
+       if ((v & (ones << (64 - 16))) == 0) {
+               n -= 16;
+               v <<= 16;
+       }
+       if ((v & (ones << (64 - 8))) == 0) {
+               n -= 8;
+               v <<= 8;
+       }
+       if ((v & (ones << (64 - 4))) == 0) {
+               n -= 4;
+               v <<= 4;
+       }
+       if ((v & (ones << (64 - 2))) == 0) {
+               n -= 2;
+               v <<= 2;
+       }
+       if ((v & (ones << (64 - 1))) == 0)
+               n -= 1;
+       return n;
+}
+
 /*
  * Rough approximation to sqrt.
  *
@@ -96,10 +132,7 @@ __a_const static uint64_t int_sqrt(uint64_t x)
        op = x;
        res = 0;
 
-       one = one << 62;
-       while (one > op)
-               one >>= 2;
-
+       one = one << (fls64(x) & ~one);
        while (one != 0) {
                if (op >= res + one) {
                        op = op - (res + one);