Revert "x86: use TZCNT (#113)"

Reverted after objections to its inclusion.

This reverts commit a7271104bf9a2d82dc6a69090c12442eacd2fd71.

diff --git a/arch/x86/ctzl.h b/arch/x86/ctzl.h
index 7482788bff32df39bc3ead0166efb71dbc71e57e..bc9e9bd5b5933a133102551c5ea47b38e2de79b3 100644 (file)
--- a/arch/x86/ctzl.h
+++ b/arch/x86/ctzl.h
@@ -2,32 +2,24 @@
 #define X86_CTZL_H
 
 #include <intrin.h>
+#ifdef X86_CPUID
+# include "x86.h"
+#endif
 
 #if defined(_MSC_VER) && !defined(__clang__)
-/* __builtin_ctzl
- *  - For 0, the result is undefined
- *  - On the x86 architecture, it is typically implemented using BSF
- *  - the equivalent intrinsic on MSC is _BitScanForward
- *
- * _tzcnt_u32
- *  - For 0, the result is the size of the operand 
- *  - On processors that do not support TZCNT, the instruction byte encoding is executed as BSF. In this case the result for 0
- *    is undefined.
- *  - Performance:
- *    + AMD: The reciprocal throughput for TZCNT is 2 vs 3 for BSF
- *    + Intel: On modern Intel CPUs (Haswell), the performance of TZCNT is equivalent to BSF
- *    Reference: http://www.agner.org/optimize/instruction_tables.pdf
-*/
-#if defined(_M_IX86) || defined(_M_AMD64)
-#define __builtin_ctzl _tzcnt_u32
-#else
+/* This is not a general purpose replacement for __builtin_ctzl. The function expects that value is != 0
+ * Because of that assumption trailing_zero is not initialized and the return value of _BitScanForward is not checked
+ */
 static __forceinline unsigned long __builtin_ctzl(unsigned long value)
 {
-    unsigned long trailing_zero;
-    _BitScanForward(&trailing_zero, value);
-    return trailing_zero;
-}
+#ifdef X86_CPUID
+       if (x86_cpu_has_tzcnt)
+               return _tzcnt_u32(value);
 #endif
+       unsigned long trailing_zero;
+       _BitScanForward(&trailing_zero, value);
+       return trailing_zero;
+}
 #endif
 
 #endif