Bug 1820504 - Optimize GraphemeClusterBreakIteratorUtf16::Next by making the...

Bug 1820504 - Optimize GraphemeClusterBreakIteratorUtf16::Next by making the evaluation of baseIsEmoji lazy. r=TYLin

This doesn't change behavior, but takes the emoji property lookup out of the hot path.

Credit to the Betterbird project for noting this in their profiling work, as recorded in
https://bugzilla.mozilla.org/show_bug.cgi?id=1812386#c7

Differential Revision: https://phabricator.services.mozilla.com/D171698

intl/lwbrk/Segmenter.cpp

@@ -130,12 +130,39 @@ Maybe<uint32_t> GraphemeClusterBreakIteratorUtf16::Next() {
   const uint32_t kFitzpatrickLowFirst = 0xDFFB;
   const uint32_t kFitzpatrickLowLast = 0xDFFF;
 
-  bool baseIsEmoji = (GetEmojiPresentation(ch) == EmojiDefault) ||
-                     (GetEmojiPresentation(ch) == TextDefault &&
-                      ((mPos < len && mText[mPos] == kVS16) ||
-                       (mPos + 1 < len && mText[mPos] == kFitzpatrickHigh &&
-                        mText[mPos + 1] >= kFitzpatrickLowFirst &&
-                        mText[mPos + 1] <= kFitzpatrickLowLast)));
+  // Checking the emoji-presentation property of the base character is a bit
+  // expensive, so we do it lazily.
+  enum class EmojiStatus : uint8_t {
+    No,
+    Yes,
+    Unknown,
+  } baseIsEmojiStatus = EmojiStatus::Unknown;
+
+  // Remember the base character and the position of the next, in case we need
+  // to evaluate its emoji status.
+  uint32_t baseCh = ch;
+  uint32_t afterBase = mPos;
+
+  auto isFitzpatrickModifierAt = [&](uint32_t aPos) -> bool {
+    return aPos + 1 < len && mText[aPos] == kFitzpatrickHigh &&
+           mText[aPos + 1] >= kFitzpatrickLowFirst &&
+           mText[aPos + 1] <= kFitzpatrickLowLast;
+  };
+
+  auto baseIsEmoji = [&]() -> bool {
+    if (baseIsEmojiStatus == EmojiStatus::Unknown) {
+      auto basePresentation = GetEmojiPresentation(baseCh);
+      baseIsEmojiStatus =
+          basePresentation == EmojiDefault ||
+                  (basePresentation == TextDefault &&
+                   ((afterBase < len && mText[afterBase] == kVS16) ||
+                    isFitzpatrickModifierAt(afterBase)))
+              ? EmojiStatus::Yes
+              : EmojiStatus::No;
+    }
+    return baseIsEmojiStatus == EmojiStatus::Yes;
+  };
+
   bool prevWasZwj = false;
 
   while (mPos < len) {
@@ -152,7 +179,7 @@ Maybe<uint32_t> GraphemeClusterBreakIteratorUtf16::Next() {
 
     bool extendCluster =
         IsClusterExtender(ch) ||
-        (baseIsEmoji && prevWasZwj &&
+        (prevWasZwj && baseIsEmoji() &&
          ((GetEmojiPresentation(ch) == EmojiDefault) ||
           (GetEmojiPresentation(ch) == TextDefault && mPos + chLen < len &&
            mText[mPos + chLen] == kVS16)));