Emilio Cobos Álvarez
211fe2c78d
This reuses the existing mechanism we have for this, and causes fewer collisions / is better distributed. On the jQuery benchmark from bug 2051347, this gets the same perfect distribution both before and after the hash change. I filed bug 2057753 to investigate if MruCache (used from AtomCache / NS_AtomizeMainThread and co) should be using a similar pattern. Differential Revision: https://phabricator.services.mozilla.com/D314221
42 lines
1.5 KiB
C++
42 lines
1.5 KiB
C++
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#include "nsHtml5AtomTable.h"
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#include "mozilla/HashFunctions.h"
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#include "mozilla/MathAlgorithms.h"
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#include "nsHashKeys.h"
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#include "nsThreadUtils.h"
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nsHtml5AtomTable::nsHtml5AtomTable() {
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static_assert((kRecentlyUsedSize & (kRecentlyUsedSize - 1)) == 0,
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"must be a power of two");
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#ifdef DEBUG
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mPermittedLookupEventTarget = mozilla::GetCurrentSerialEventTarget();
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#endif
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}
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nsHtml5AtomTable::~nsHtml5AtomTable() = default;
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nsAtom* nsHtml5AtomTable::GetAtom(const nsAString& aKey) {
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MOZ_ASSERT(mPermittedLookupEventTarget->IsOnCurrentThread());
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// We index using the high bits of the scrambled hash rather than `hash %
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// size`. This stays well-distributed for the short, similar attribute values
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// we cache, so frequently-parsed strings don't evict each other.
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constexpr uint32_t kTableShift =
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mozilla::kHashNumberBits - mozilla::CeilingLog2(kRecentlyUsedSize);
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const uint32_t hash = mozilla::HashString(aKey);
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const uint32_t index = mozilla::ScrambleHashCode(hash) >> kTableShift;
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if (nsAtom* atom = mRecentlyUsedParserAtoms[index]) {
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if (atom->hash() == hash && atom->Equals(aKey)) {
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return atom;
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}
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}
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RefPtr<nsAtom> atom = NS_Atomize(aKey, hash);
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nsAtom* ret = atom.get();
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mRecentlyUsedParserAtoms[index] = std::move(atom);
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return ret;
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}
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