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Improve qustrlen further by avoiding the alignment prologue

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This avoids looping prior to the main 16-byte loop, by performing one
load that may include bytes prior to the start of the string. This is
guaranteed not to fault, since str points to a valid character, but it
may cause Valgrind to print warnings.

Change-Id: I6e9274c1e7444ad48c81fffd14dcae854bba24b2
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Thiago Macieira 2017-08-20 15:31:03 -07:00
parent 49154acde3
commit f7350ac99e
2 changed files with 52 additions and 18 deletions
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31
src/corelib/tools/qstring.cpp
View File

@ -165,28 +165,37 @@ qssize_t qustrlen(const ushort *str) Q_DECL_NOTHROW
qssize_t result = 0;
#ifdef __SSE2__
// progress until we get an aligned pointer
const ushort *ptr = str;
while (*ptr && quintptr(ptr) % 16)
++ptr;
if (*ptr == 0)
return ptr - str;
// find the 16-byte alignment immediately prior or equal to str
quintptr misalignment = quintptr(str) & 0xf;
Q_ASSERT((misalignment & 1) == 0);
const ushort *ptr = str - (misalignment / 2);
// load 16 bytes and see if we have a null
// (aligned loads can never segfault)
int mask;
const __m128i zeroes = _mm_setzero_si128();
do {
__m128i data = _mm_load_si128(reinterpret_cast<const __m128i *>(ptr));
ptr += 8;
__m128i comparison = _mm_cmpeq_epi16(data, zeroes);
quint32 mask = _mm_movemask_epi8(comparison);
// ignore the result prior to the beginning of str
mask >>= misalignment;
// Have we found something in the first block? Need to handle it now
// because of the left shift above.
if (mask)
return qCountTrailingZeroBits(quint32(mask)) / 2;
do {
ptr += 8;
data = _mm_load_si128(reinterpret_cast<const __m128i *>(ptr));
comparison = _mm_cmpeq_epi16(data, zeroes);
mask = _mm_movemask_epi8(comparison);
} while (mask == 0);
// found a null
uint idx = qCountTrailingZeroBits(quint32(mask));
return ptr - str - 8 + idx / 2;
return ptr - str + idx / 2;
#endif
if (sizeof(wchar_t) == sizeof(ushort))

37
tests/auto/corelib/tools/qstringview/tst_qstringview.cpp
View File

@ -364,14 +364,21 @@ void tst_QStringView::basics() const
void tst_QStringView::literals() const
{
#if !defined(Q_OS_WIN) || defined(Q_COMPILER_UNICODE_STRINGS)
// the + ensures it's a pointer, not an array
QCOMPARE(QStringView(+u"Hello").size(), 5);
QStringView sv = u"Hello";
const char16_t hello[] = u"Hello";
const char16_t longhello[] =
u"Hello World. This is a much longer message, to exercise qustrlen.";
const char16_t withnull[] = u"a\0zzz";
#else // storage_type is wchar_t
// the + ensures it's a pointer, not an array
QCOMPARE(QStringView(+L"Hello").size(), 5);
QStringView sv = L"Hello";
const wchar_t hello[] = L"Hello";
const wchar_t longhello[] =
L"Hello World. This is a much longer message, to exercise qustrlen.";
const wchar_t withnull[] = L"a\0zzz";
#endif
Q_STATIC_ASSERT(sizeof(longhello) >= 16);
QCOMPARE(QStringView(hello).size(), 5);
QCOMPARE(QStringView(hello + 0).size(), 5); // forces decay to pointer
QStringView sv = hello;
QCOMPARE(sv.size(), 5);
QVERIFY(!sv.empty());
QVERIFY(!sv.isEmpty());
@ -390,6 +397,24 @@ void tst_QStringView::literals() const
QVERIFY(!sv2.isNull());
QVERIFY(!sv2.empty());
QCOMPARE(sv2.size(), 5);
QStringView sv3(longhello);
QCOMPARE(size_t(sv3.size()), sizeof(longhello)/sizeof(longhello[0]) - 1);
QCOMPARE(sv3.last(), QLatin1Char('.'));
sv3 = longhello;
QCOMPARE(size_t(sv3.size()), sizeof(longhello)/sizeof(longhello[0]) - 1);
for (int i = 0; i < sv3.size(); ++i) {
QStringView sv4(longhello + i);
QCOMPARE(size_t(sv4.size()), sizeof(longhello)/sizeof(longhello[0]) - 1 - i);
QCOMPARE(sv4.last(), QLatin1Char('.'));
sv4 = longhello + i;
QCOMPARE(size_t(sv4.size()), sizeof(longhello)/sizeof(longhello[0]) - 1 - i);
}
// these are different results
QCOMPARE(size_t(QStringView(withnull).size()), sizeof(withnull)/sizeof(withnull[0]) - 1);
QCOMPARE(QStringView(withnull + 0).size(), 1);
}
void tst_QStringView::at() const