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--- chess:programming:msb_most_significant_bit [2021/10/30 12:21] – peter
+++ chess:programming:msb_most_significant_bit [2021/10/30 12:29] (current) – [Using Built-in for GCC] peter
@@ Line 41: / Line 41: @@
 <code cpp>
-__builtin_clz
+unsigned BSR64(uint64_t x)
-__builtin_clzl
+{
+  return 63-__builtin_clzll(x);
+}
 </code>
 <WRAP info>
-**NOTE:**  **<nowiki>__builtin_clz</nowiki>** and **<nowiki>__builtin_clzl</nowiki>** are undefined for 0 inputs.
+**NOTE:**  Depending on architecture, the built-in functions include:
+  * <nowiki>__builtin_clz</nowiki>
+  * <nowiki>__builtin_clzl</nowiki>
+  * <nowiki>__builtin_clzll</nowiki>
-  * See https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
+  * The returned value is undefined for 0 inputs.
+    * See https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
 </WRAP>
@@ Line 78: / Line 86: @@
 ===== Using de Bruijn for 32-bit =====
+<code cpp>
 u32 msbDeBruijn32(u32 v)
 {
@@ Line 137: / Line 146: @@
 <code cpp>
 #include <intrin.h>
+unsigned BSR32(unsigned long x)
+{
+  bsr_idx_t idx;
+  _BitScanReverse(&idx, x); // ignore bool retval
+  return idx;
+}
-unsigned char _BitScanForward(
+unsigned BSR64(uint64_t x)
-   unsigned long * Index,
+{
-   unsigned long Mask
+  bsr_idx_t idx;
-);
+  _BitScanReverse64(&idx, x); // ignore bool retval
-unsigned char _BitScanForward64(
+  return idx;
-   unsigned long * Index,
+}
-   unsigned __int64 Mask
-);
 </code>