diff options
Diffstat (limited to 'src/corelib/tools/qstring.cpp')
| -rw-r--r-- | src/corelib/tools/qstring.cpp | 63 |
1 files changed, 48 insertions, 15 deletions
diff --git a/src/corelib/tools/qstring.cpp b/src/corelib/tools/qstring.cpp index 45d6d89..120d971 100644 --- a/src/corelib/tools/qstring.cpp +++ b/src/corelib/tools/qstring.cpp @@ -173,7 +173,7 @@ static int ucstricmp(const ushort *a, const ushort *ae, const uchar *b) return 1; } -// Unicode case-insensitive comparison +// Unicode case-sensitive comparison static int ucstrcmp(const QChar *a, int alen, const QChar *b, int blen) { if (a == b && alen == blen) @@ -202,22 +202,55 @@ static int ucstrnicmp(const ushort *a, const ushort *b, int l) return ucstricmp(a, a + l, b, b + l); } +// Benchmarking indicates that doing memcmp is much slower than +// executing the comparison ourselves. +// +// The profiling was done on a population of calls to qMemEquals, generated +// during a run of the demo browser. The profile of the data (32-bit x86 +// Linux) was: +// +// total number of comparisons: 21353 +// longest string compared: 95 +// average comparison length: 14.8786 +// cache-line crosses: 5661 (13.3%) +// alignment histogram: +// 0xXXX0 = 512 (1.2%) strings, 0 (0.0%) of which same-aligned +// 0xXXX2 = 15087 (35.3%) strings, 5145 (34.1%) of which same-aligned +// 0xXXX4 = 525 (1.2%) strings, 0 (0.0%) of which same-aligned +// 0xXXX6 = 557 (1.3%) strings, 6 (1.1%) of which same-aligned +// 0xXXX8 = 509 (1.2%) strings, 0 (0.0%) of which same-aligned +// 0xXXXa = 24358 (57.0%) strings, 9901 (40.6%) of which same-aligned +// 0xXXXc = 557 (1.3%) strings, 0 (0.0%) of which same-aligned +// 0xXXXe = 601 (1.4%) strings, 15 (2.5%) of which same-aligned +// total = 42706 (100%) strings, 15067 (35.3%) of which same-aligned +// +// 92% of the strings have alignment of 2 or 10, which is due to malloc on +// 32-bit Linux returning values aligned to 8 bytes, and offsetof(array, QString::Data) == 18. +// +// The profile on 64-bit will be different since offsetof(array, QString::Data) == 26. +// +// The benchmark results were, for a Core-i7 @ 2.67 GHz 32-bit, compiled with -O3 -funroll-loops: +// 16-bit loads only: 872,301 CPU ticks [Qt 4.5 / memcmp] +// 32- and 16-bit loads: 773,362 CPU ticks [Qt 4.6] +// SSE2 "movdqu" 128-bit loads: 618,736 CPU ticks +// SSE3 "lddqu" 128-bit loads: 619,954 CPU ticks +// SSSE3 "palignr" corrections: 852,147 CPU ticks +// SSE4.2 "pcmpestrm": 738,702 CPU ticks +// +// The same benchmark on an Atom N450 @ 1.66 GHz, is: +// 16-bit loads only: 2,185,882 CPU ticks +// 32- and 16-bit loads: 1,805,060 CPU ticks +// SSE2 "movdqu" 128-bit loads: 2,529,843 CPU ticks +// SSE3 "lddqu" 128-bit loads: 2,514,858 CPU ticks +// SSSE3 "palignr" corrections: 2,160,325 CPU ticks +// SSE4.2 not available +// +// The conclusion we reach is that alignment the SSE2 unaligned code can gain +// 20% improvement in performance in some systems, but suffers a penalty due +// to the unaligned loads on others. + static bool qMemEquals(const quint16 *a, const quint16 *b, int length) { - // Benchmarking indicates that doing memcmp is much slower than - // executing the comparison ourselves. - // To make it even faster, we do a 32-bit comparison, comparing - // twice the amount of data as a normal word-by-word comparison. - // - // Benchmarking results on a 2.33 GHz Core2 Duo, with a 64-QChar - // block of data, with 4194304 iterations (per iteration): - // operation usec cpu ticks - // memcmp 330 710 - // 16-bit 79 167-171 - // 32-bit aligned 49 105-109 - // - // Testing also indicates that unaligned 32-bit loads are as - // performant as 32-bit aligned. if (a == b || !length) return true; |