[Mesa-dev] [PATCH 1/2] mesa: Add a _mesa_fls() function to find the last bit set in a word.

[Kenneth Graunke]

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

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/main/imports.c | 22 ++++++++++++++++++++++
 src/mesa/main/imports.h |  2 ++
 2 files changed, 24 insertions(+)

Wow.  NAK my last patch which used ffs().  Totally backwards and just
happened to work for the one test because it only used limited values.

diff --git a/src/mesa/main/imports.c b/src/mesa/main/imports.c
index 934a2d0..eac8e21 100644
--- a/src/mesa/main/imports.c
+++ b/src/mesa/main/imports.c
@@ -316,6 +316,28 @@ _mesa_bitcount_64(uint64_t n)
 }
 #endif
 
+/**
+ * Find the last (most significant) bit set in a word.
+ *
+ * Essentially ffs() in the reverse direction.
+ */
+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
+}
 
 /**
  * Convert a 4-byte float to a 2-byte half float.
diff --git a/src/mesa/main/imports.h b/src/mesa/main/imports.h
index abf216c..ebe128e 100644
--- a/src/mesa/main/imports.h
+++ b/src/mesa/main/imports.h
@@ -520,6 +520,8 @@ extern unsigned int
 _mesa_bitcount_64(uint64_t n);
 #endif
 
+extern unsigned int
+_mesa_fls(unsigned int n);
 
 extern GLhalfARB
 _mesa_float_to_half(float f);
-- 
1.7.11.4