Bug 921858 - check for spaces in textrun before skipping word cache. r=jfkthame

gfx/thebes/gfxFont.cpp

@@ -2346,7 +2346,7 @@ gfxFont::HasSubstitutionRulesWithSpaceLookups(int32_t aRunScript)
 }
 
 bool
-gfxFont::BypassShapedWordCache(int32_t aRunScript)
+gfxFont::SpaceMayParticipateInShaping(int32_t aRunScript)
 {
     // We record the presence of space-dependent features in the font entry
     // so that subsequent instantiations for the same font face won't
@@ -3690,6 +3690,21 @@ gfxFont::ShapeTextWithoutWordCache(gfxContext *aContext,
 inline static bool IsChar8Bit(uint8_t /*aCh*/) { return true; }
 inline static bool IsChar8Bit(char16_t aCh) { return aCh < 0x100; }
 
+inline static bool HasSpaces(const uint8_t *aString, uint32_t aLen)
+{
+    return memchr(aString, 0x20, aLen) != nullptr;
+}
+
+inline static bool HasSpaces(const char16_t *aString, uint32_t aLen)
+{
+    for (const char16_t *ch = aString; ch < aString + aLen; ch++) {
+        if (*ch == 0x20) {
+            return true;
+        }
+    }
+    return false;
+}
+
 template<typename T>
 bool
 gfxFont::SplitAndInitTextRun(gfxContext *aContext,
@@ -3720,16 +3735,24 @@ gfxFont::SplitAndInitTextRun(gfxContext *aContext,
     }
 #endif
 
-    if (BypassShapedWordCache(aRunScript)) {
+    uint32_t wordCacheCharLimit =
+        gfxPlatform::GetPlatform()->WordCacheCharLimit();
+
+    // If spaces can participate in shaping (e.g. within lookups for automatic
+    // fractions), need to shape without using the word cache which segments
+    // textruns on space boundaries. Word cache can be used if the textrun
+    // is short enough to fit in the word cache and it lacks spaces.
+    if (SpaceMayParticipateInShaping(aRunScript)) {
+        if (aRunLength > wordCacheCharLimit ||
+            HasSpaces(aString + aRunStart, aRunLength)) {
             TEXT_PERF_INCR(tp, wordCacheSpaceRules);
             return ShapeTextWithoutWordCache(aContext, aString + aRunStart,
                                              aRunStart, aRunLength, aRunScript,
                                              aTextRun);
         }
+    }
 
     InitWordCache();
-    uint32_t wordCacheCharLimit =
-        gfxPlatform::GetPlatform()->WordCacheCharLimit();
 
     // the only flags we care about for ShapedWord construction/caching
     uint32_t flags = aTextRun->GetFlags();

gfx/thebes/gfxFont.h

@@ -1807,8 +1807,8 @@ protected:
     // whether font contains substitution lookups containing spaces
     bool HasSubstitutionRulesWithSpaceLookups(int32_t aRunScript);
 
-    // whether to use word cache or not
-    bool BypassShapedWordCache(int32_t aRunScript);
+    // do spaces participate in shaping rules? if so, can't used word cache
+    bool SpaceMayParticipateInShaping(int32_t aRunScript);
 
     // For 8-bit text, expand to 16-bit and then call the following method.
     bool ShapeText(gfxContext    *aContext,