From: Hans Kristian Rosbach <hk-git@circlestorm.org>
Date: Thu, 24 Aug 2017 10:32:18 +0000 (+0200)
Subject: Revert "x86: use TZCNT (#113)"
X-Git-Tag: 1.9.9-b1~660^2~1
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=79a262f7a4da3b1ebb2d543df65ca8420c623a3f;p=thirdparty%2Fzlib-ng.git

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 7482788bf..bc9e9bd5b 100644
--- 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