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Use __builtin_clz/ctz when available.

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Nicely ask the compiler if it has a built-in for clz/ctz before
resorting to CPU specific brute force measurements.

Change-Id: Ifa992267ec4528219d7da14524af738316ceeaea
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Erik Verbruggen 2015-11-18 10:57:44 +01:00
parent bf7ce1ff84
commit 4e8f07cd0f
1 changed files with 17 additions and 1 deletions
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18
src/corelib/tools/qsimd_p.h
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@ -431,7 +431,23 @@ static inline quint64 qCpuFeatures()
#define qCpuHasFeature(feature) ((qCompilerCpuFeatures & (Q_UINT64_C(1) << CpuFeature ## feature)) \
|| (qCpuFeatures() & (Q_UINT64_C(1) << CpuFeature ## feature)))
#ifdef Q_PROCESSOR_X86
#if QT_HAS_BUILTIN(__builtin_clz) && QT_HAS_BUILTIN(__builtin_ctz) && defined(Q_CC_CLANG) && !defined(Q_CC_INTEL)
static Q_ALWAYS_INLINE unsigned _bit_scan_reverse(unsigned val)
{
Q_ASSERT(val != 0); // if val==0, the result is undefined.
unsigned result = static_cast<unsigned>(__builtin_clz(val)); // Count Leading Zeros
// Now Invert the result: clz will count *down* from the msb to the lsb, so the msb index is 31
// and the lsb inde is 0. The result for _bit_scan_reverse is expected to be the index when
// counting up: msb index is 0 (because it starts there), and the lsb index is 31.
result ^= sizeof(unsigned) * 8 - 1;
return result;
}
static Q_ALWAYS_INLINE unsigned _bit_scan_forward(unsigned val)
{
Q_ASSERT(val != 0); // if val==0, the result is undefined.
return static_cast<unsigned>(__builtin_ctz(val)); // Count Trailing Zeros
}
#elif defined(Q_PROCESSOR_X86)
// Bit scan functions for x86
# if defined(Q_CC_MSVC) && !defined(Q_OS_WINCE)
// MSVC calls it _BitScanReverse and returns the carry flag, which we don't need