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[layout-cache] Cache lookahead, not input, classdef in ChainContextFormat2 

From the commit:

+    /* For ChainContextFormat2 we cache the LookaheadClassDef instead of InputClassDef.
+     * The reason is that most heavy fonts want to identify a glyph in context and apply
+     * a lookup to it. In this scenario, the length of the input sequence is one, whereas
+     * the lookahead / backtrack are typically longer.  The one glyph in input sequence is
+     * looked-up below and no input glyph is looked up in individual rules, whereas the
+     * lookahead and backtrack glyphs are tried.  Since we match lookahead before backtrack,
+     * we should cache lookahead.  This decisions showed a 20% improvement in shaping of
+     * the Gulzar font.

https://github.com/harfbuzz/harfbuzz/pull/3636
This commit is contained in:
Behdad Esfahbod 2022-06-07 11:41:05 -06:00
parent 39820af72f
commit bfee6839b0
1 changed files with 13 additions and 10 deletions
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21
src/hb-ot-layout-gsubgpos.hh
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@ -3330,23 +3330,26 @@ struct ChainContextFormat2
const ClassDef &input_class_def = this+inputClassDef;
const ClassDef &lookahead_class_def = this+lookaheadClassDef;
/* For ChainContextFormat2 we cache the LookaheadClassDef instead of InputClassDef.
* The reason is that most heavy fonts want to identify a glyph in context and apply
* a lookup to it. In this scenario, the length of the input sequence is one, whereas
* the lookahead / backtrack are typically longer. The one glyph in input sequence is
* looked-up below and no input glyph is looked up in individual rules, whereas the
* lookahead and backtrack glyphs are tried. Since we match lookahead before backtrack,
* we should cache lookahead. This decisions showed a 20% improvement in shaping of
* the Gulzar font.
*/
struct ChainContextApplyLookupContext lookup_context = {
{{cached && &backtrack_class_def == &input_class_def ? match_class_cached : match_class,
cached ? match_class_cached : match_class,
cached && &lookahead_class_def == &input_class_def ? match_class_cached : match_class}},
cached && &input_class_def == &lookahead_class_def ? match_class_cached : match_class,
cached ? match_class_cached : match_class}},
{&backtrack_class_def,
&input_class_def,
&lookahead_class_def}
};
if (cached && c->buffer->cur().syllable() < 255)
index = c->buffer->cur().syllable ();
else
{
index = input_class_def.get_class (c->buffer->cur().codepoint);
if (cached && index < 255)
c->buffer->cur().syllable() = index;
}
const ChainRuleSet &rule_set = this+ruleSet[index];
return_trace (rule_set.apply (c, lookup_context));
}