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Bug 868557. Explicitly unalias list in nsStyleContext::AddChild() r=dbaron

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Clang and GCC probably MSVC are not able to unalias list because they don't
realize that mEmptyChild and mChild can't alias mNextSibling and mPrevSibling.
If we explicitly dereference list we get better code, saving 3 instructions
with clang.
This commit is contained in:
Jeff Muizelaar 2013-05-06 11:05:44 -04:00
parent a541708f53
commit 8d12260f42
1 changed files with 10 additions and 7 deletions
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17
layout/style/nsStyleContext.cpp
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@ -110,17 +110,20 @@ void nsStyleContext::AddChild(nsStyleContext* aChild)
aChild->mNextSibling == aChild,
"child already in a child list");
nsStyleContext **list = aChild->mRuleNode->IsRoot() ? &mEmptyChild : &mChild;
nsStyleContext **listPtr = aChild->mRuleNode->IsRoot() ? &mEmptyChild : &mChild;
// Explicitly dereference listPtr so that compiler doesn't have to know that mNextSibling
// etc. don't alias with what ever listPtr points at.
nsStyleContext *list = *listPtr;
// Insert at the beginning of the list. See also FindChildWithRules.
if (*list) {
if (list) {
// Link into existing elements, if there are any.
aChild->mNextSibling = (*list);
aChild->mPrevSibling = (*list)->mPrevSibling;
(*list)->mPrevSibling->mNextSibling = aChild;
(*list)->mPrevSibling = aChild;
aChild->mNextSibling = list;
aChild->mPrevSibling = list->mPrevSibling;
list->mPrevSibling->mNextSibling = aChild;
list->mPrevSibling = aChild;
}
(*list) = aChild;
(*listPtr) = aChild;
}
void nsStyleContext::RemoveChild(nsStyleContext* aChild)