Bug#846996: The mini-if is really not needed

[Thomas Viehweger <patchesThomas.Vie@web.de>]

I just tested with the following test program. The result is as expected:
#include <x86intrin.h>
#include <cstdio>
#include <cstdint>

int main(void)
{
        printf("0 -> %d\n\n", 16U);
        puts("__tzcnt_u16:");
        printf("0x%x -> %d\n", 0, __tzcnt_u16(0));
        for (uint16_t i = 1; i > 0; i <<= 1)
        {
                printf("0x%x -> %d\n", i, __tzcnt_u16(i));
        }

        puts("\n__lzcnt16:");
        printf("0x%x -> %d\n", 0, __lzcnt16(0));
        for (uint16_t i = 1; i > 0; i <<= 1)
        {
                printf("0x%x -> %d\n", i, __lzcnt16(i));
        }

        return 0;
}

g++ -march=native -o intrins intrinsictest.cpp
./intrins

Attached is the patch for
/usr/include/x86_64-linux-gnu/qt5/QtCore/qalgorithms.h

--- qalgorithms.h.orig	2016-12-18 15:57:30.279325472 +0100
+++ qalgorithms.h	2016-12-18 23:09:33.129324260 +0100
@@ -47,6 +47,10 @@ QT_WARNING_PUSH
 QT_WARNING_DISABLE_GCC("-Wdeprecated-declarations")
 QT_WARNING_DISABLE_CLANG("-Wdeprecated-declarations")
 
+#if defined(__BMI__) && defined(__LZCNT__) && !QT_HAS_BUILTIN(__builtin_ctzs)
+#  include <x86intrin.h>
+#endif
+
 /*
     Warning: The contents of QAlgorithmsPrivate is not a part of the public Qt API
     and may be changed from version to version or even be completely removed.
@@ -626,8 +630,10 @@ Q_DECL_RELAXED_CONSTEXPR inline uint qCo
 Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint16 v) Q_DECL_NOTHROW
 {
 #if defined(Q_CC_GNU)
-#  if QT_HAS_BUILTIN(__builtin_ctzs) || (defined(__LZCNT__) && defined(__BMI__))
+#  if QT_HAS_BUILTIN(__builtin_ctzs)
     return v ? __builtin_ctzs(v) : 16U;
+#  elif (defined(__LZCNT__) && defined(__BMI__))
+    return __tzcnt_u16(v);
 #  else
     return v ? __builtin_ctz(v) : 16U;
 #  endif
@@ -689,8 +695,10 @@ Q_DECL_RELAXED_CONSTEXPR inline uint qCo
 Q_DECL_RELAXED_CONSTEXPR inline uint qCountLeadingZeroBits(quint16 v) Q_DECL_NOTHROW
 {
 #if defined(Q_CC_GNU)
-#  if QT_HAS_BUILTIN(__builtin_clzs) || (defined(__LZCNT__) && defined(__BMI__))
+#  if QT_HAS_BUILTIN(__builtin_clzs)
     return v ? __builtin_clzs(v) : 16U;
+#  elif (defined(__LZCNT__) && defined(__BMI__))
+    return __lzcnt16(v);
 #  else
     return v ? __builtin_clz(v)-16U : 16U;
 #  endif