555435 - reduce memory overhead of preloading cmaps

Status: RESOLVED FIXED

(Core :: Layout: Text and Fonts, defect)

Description

[Jonathan Kew [:jfkthame]]

We preload the font cmap table into a gfxSparseBitSet in each font entry, for use in font fallback. Because gfxSparseBitSet uses nsTArray<> to manage its collection of Blocks, it will typically over-allocate array capacity during construction as the array grows.

However, as these arrays are never modified after the initial reading of the cmap, any residual over-allocation at that time represents permanently-wasted space.

I checked on my Mac with a fairly modest collection of fonts installed (around 200 families), and the total over-capacity of the cmap block arrays was around 800K. For a user with a very large number of fonts, it would be correspondingly greater.

We can reclaim this memory by simply calling Compact() on the block arrays after each cmap is loaded.

Comment 1

[Jonathan Kew [:jfkthame]]

Attachment: patch, v1: call mBlocks.Compact() after loading cmap

Comment 2

[Jonathan Kew [:jfkthame]]

http://hg.mozilla.org/mozilla-central/rev/e8fa8766e3d1