mesa: Add a _mesa_fls() function to find the last bit set in a word.

ffs() finds the least significant bit set; _mesa_fls() finds the /most/
significant bit.

v2: Make it an inline function in imports.h, per Brian's suggestion.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Brian Paul <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

1 files changed, 22 insertions, 0 deletions

diff --git a/src/mesa/main/imports.h b/src/mesa/main/imports.h
index abf216c99ac..81da5104717 100644
--- a/src/mesa/main/imports.h
+++ b/src/mesa/main/imports.h
@@ -520,6 +520,28 @@ extern unsigned int
 _mesa_bitcount_64(uint64_t n);
 #endif
 
+/**
+ * Find the last (most significant) bit set in a word.
+ *
+ * Essentially ffs() in the reverse direction.
+ */
+static inline unsigned int
+_mesa_fls(unsigned int n)
+{
+#if defined(__GNUC__) && ((__GNUC__ * 100 + __GNUC_MINOR__) >= 304)
+   return n == 0 ? 0 : 32 - __builtin_clz(n);
+#else
+   unsigned int v = 1;
+
+   if (n == 0)
+      return 0;
+
+   while (n >>= 1)
+       v++;
+
+   return v;
+#endif
+}
 
 extern GLhalfARB
 _mesa_float_to_half(float f);