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shend authored
Groups inside ComputedStyle, such as StyleSurroundData, are stored as pointers. When we copy the StyleSurroundData of a ComputedStyle, instead of copying each member inside StyleSurroundData, we can copy the pointer itself and do a copy when we write to it (copy-on-write). Similarly, when we compare two StyleSurroundData pointers, we can first check if they're pointing to the same object. If so, then we know that the two StyleSurroundDatas are equal without having to compare each member inside StyleSurroundData. This patch introduces two macros: fieldwise_compare and fieldwise_copy, which generate the code to do this optimisation. These macros take a a list of fields to compare/copy and generate the cheapest set of code for that task, taking advantage of group sharing. This patch removes the perf cost of: https://codereview.chromium.org/2786883002 by generating the exact same code as what was originally handwritten. Diff of generated code: https://gist.github.com/darrnshn/fe97404e680016753030fc788fcf24ff/revisions BUG=628043 Review-Url: https://codereview.chromium.org/2826633002 Cr-Commit-Position: refs/heads/master@{#467243}
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