diff options
author | Andrei Golubev <andrei.golubev@qt.io> | 2020-10-29 14:25:48 +0100 |
---|---|---|
committer | Lars Knoll <lars.knoll@qt.io> | 2020-11-04 11:21:12 +0100 |
commit | 7eed57511a786861b513af089aef48651fed7891 (patch) | |
tree | f6eeaa99bbea63af1207d7d3a64e6e87ce051c28 /src/corelib | |
parent | 12718e5ea289df1d3f113bf0ff4e64ea8961e20d (diff) |
QArrayDataPointer: redesign (and simplify) growth policy
It looks like we can drastically simplify the way QADP grows without
sacrificing much:
1. append-only use cases should have the same performance as before
2. prepend-only use cases should (with the help of other commits) get
additional performance speedup
3. mid-insertion is harder to reason about, but it is either unchanged
or benefits a bit as there's some free space at both ends now
4. mixed prepend/append cases are weird and would keep excess free
space around but this is less critical and overall less used AFAIK
Now, QList would actually start to feel like a double-ended container
instead of "it's QVector but with faster prepend". This commit should
help close the performance gap between 6.0 and 5.15 as well
As a drawback, we will most likely have more space allocated in mixed
and mid-insert cases. This needs to be checked
Task-number: QTBUG-86583
Change-Id: I7c6ede896144920fe01862b9fe789c8fdfc11f80
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Diffstat (limited to 'src/corelib')
-rw-r--r-- | src/corelib/text/qbytearray.cpp | 6 | ||||
-rw-r--r-- | src/corelib/text/qstring.cpp | 6 | ||||
-rw-r--r-- | src/corelib/tools/qarraydataops.h | 22 | ||||
-rw-r--r-- | src/corelib/tools/qarraydatapointer.h | 25 |
4 files changed, 27 insertions, 32 deletions
diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp index c6dbf4c7aa..e550224c4c 100644 --- a/src/corelib/text/qbytearray.cpp +++ b/src/corelib/text/qbytearray.cpp @@ -1728,7 +1728,11 @@ void QByteArray::reallocGrowData(qsizetype alloc, Data::ArrayOptions options) if (!alloc) // expected to always allocate alloc = 1; - if (d->needsDetach()) { + // when we're requested to grow backwards, it means we're in prepend-like + // case. realloc() is good, but we might actually get more free space at the + // beginning with a call to allocateGrow, so prefer that instead + const bool preferAllocateGrow = options & Data::GrowsBackwards; + if (d->needsDetach() || preferAllocateGrow) { const auto newSize = qMin(alloc, d.size); DataPointer dd(DataPointer::allocateGrow(d, alloc, newSize, options)); dd->copyAppend(d.data(), d.data() + newSize); diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp index 26d0a7acc8..d77ade226e 100644 --- a/src/corelib/text/qstring.cpp +++ b/src/corelib/text/qstring.cpp @@ -2525,7 +2525,11 @@ void QString::reallocGrowData(qsizetype alloc, Data::ArrayOptions options) if (!alloc) // expected to always allocate alloc = 1; - if (d->needsDetach()) { + // when we're requested to grow backwards, it means we're in prepend-like + // case. realloc() is good, but we might actually get more free space at the + // beginning with a call to allocateGrow, so prefer that instead + const bool preferAllocateGrow = options & Data::GrowsBackwards; + if (d->needsDetach() || preferAllocateGrow) { const auto newSize = qMin(alloc, d.size); DataPointer dd(DataPointer::allocateGrow(d, alloc, newSize, options)); dd->copyAppend(d.data(), d.data() + newSize); diff --git a/src/corelib/tools/qarraydataops.h b/src/corelib/tools/qarraydataops.h index 9626f4eeff..3b95d04fa8 100644 --- a/src/corelib/tools/qarraydataops.h +++ b/src/corelib/tools/qarraydataops.h @@ -1320,23 +1320,15 @@ public: const qsizetype freeAtBegin = this->freeSpaceAtBegin(); const qsizetype freeAtEnd = this->freeSpaceAtEnd(); - const qsizetype capacity = this->constAllocatedCapacity(); - if (this->size > 0 && where == this->begin()) { // prepend - // Qt5 QList in prepend: not enough space at begin && 33% full - // Now (below): - return freeAtBegin < n && (this->size >= (capacity / 3)); + // Idea: always reallocate when not enough space at the corresponding end + if (where == this->end()) { // append or size == 0 + return freeAtEnd < n; + } else if (where == this->begin()) { // prepend + return freeAtBegin < n; + } else { // general insert + return (freeAtBegin < n && freeAtEnd < n); } - - if (where == this->end()) { // append - // Qt5 QList in append: not enough space at end && less than 66% free space at front - // Now (below): - return freeAtEnd < n && !((freeAtBegin - n) >= (2 * capacity / 3)); - } - - // Qt5 QList in insert: no free space - // Now: no free space OR not enough space on either of the sides (bad perf. case) - return (freeAtBegin + freeAtEnd) < n || (freeAtBegin < n && freeAtEnd < n); } // using Base::truncate; diff --git a/src/corelib/tools/qarraydatapointer.h b/src/corelib/tools/qarraydatapointer.h index ad90b1d580..3cabeca649 100644 --- a/src/corelib/tools/qarraydatapointer.h +++ b/src/corelib/tools/qarraydatapointer.h @@ -222,21 +222,16 @@ public: if (!valid || !grows) return QArrayDataPointer(header, dataPtr); - // when growing, special rules apply to memory layout - - if (from.needsDetach()) { - // When detaching: the free space reservation is biased towards - // append as in Qt5 QList. If we're growing backwards, put the data - // in the middle instead of at the end - assuming that prepend is - // uncommon and even initial prepend will eventually be followed by - // at least some appends. - if (options & Data::GrowsBackwards) - dataPtr += (header->alloc - newSize) / 2; - } else { - // When not detaching: fake ::realloc() policy - preserve existing - // free space at beginning. - dataPtr += from.freeSpaceAtBegin(); - } + // must always hold true, as valid is the first condition we check and + // if-statement short-circuits + Q_ASSERT(valid); + + // Idea: * when growing backwards, adjust pointer to prepare free space at the beginning + // * when growing forward, adjust by the previous data pointer offset + + // TODO: what's with CapacityReserved? + dataPtr += (options & Data::GrowsBackwards) ? (header->alloc - newSize) / 2 + : from.freeSpaceAtBegin(); return QArrayDataPointer(header, dataPtr); } |