Bug 1393348 - part3: Converting SelectionType to index of Selection array in nsFrameSelection should use array instead of switch statement r=smaug

GetIndexFromSelectionType() in nsFrameSelection.cpp or
nsFrameSelection::GetSelection() appears in profile of attachment 8848015.  So,
it should not use switch statement due to really hot path.

With the previous patch, we can make it use array to retrieve the index from
SelectionType with static_cast<int8_t>.

MozReview-Commit-ID: 8jvIF5buTyT

--HG--
extra : rebase_source : ccb9c855caa6688e45097f67de3160860339d281

layout/generic/nsFrameSelection.cpp

@@ -144,34 +144,31 @@ nsPeekOffsetStruct::nsPeekOffsetStruct(nsSelectionAmount aAmount,
 {
 }
 
-static int8_t
+// Array which contains index of each SelecionType in Selection::mDOMSelections.
+// For avoiding using if nor switch to retrieve the index, this needs to have
+// -1 for SelectionTypes which won't be created its Selection instance.
+static const int8_t kIndexOfSelections[] = {
+  -1,    // SelectionType::eInvalid
+  -1,    // SelectionType::eNone
+  0,     // SelectionType::eNormal
+  1,     // SelectionType::eSpellCheck
+  2,     // SelectionType::eIMERawClause
+  3,     // SelectionType::eIMESelectedRawClause
+  4,     // SelectionType::eIMEConvertedClause
+  5,     // SelectionType::eIMESelectedClause
+  6,     // SelectionType::eAccessibility
+  7,     // SelectionType::eFind
+  8,     // SelectionType::eURLSecondary
+  9,     // SelectionType::eURLStrikeout
+};
+
+inline int8_t
 GetIndexFromSelectionType(SelectionType aSelectionType)
 {
-  switch (aSelectionType) {
-    case SelectionType::eNormal:
-      return 0;
-    case SelectionType::eSpellCheck:
-      return 1;
-    case SelectionType::eIMERawClause:
-      return 2;
-    case SelectionType::eIMESelectedRawClause:
-      return 3;
-    case SelectionType::eIMEConvertedClause:
-      return 4;
-    case SelectionType::eIMESelectedClause:
-      return 5;
-    case SelectionType::eAccessibility:
-      return 6;
-    case SelectionType::eFind:
-      return 7;
-    case SelectionType::eURLSecondary:
-      return 8;
-    case SelectionType::eURLStrikeout:
-      return 9;
-    default:
-      return -1;
-  }
-  /* NOTREACHED */
+  // The enum value of eInvalid is -1 and the others are sequential value
+  // starting from 0.  Therefore, |SelectionType + 1| is the index of
+  // kIndexOfSelections.
+  return kIndexOfSelections[static_cast<int8_t>(aSelectionType) + 1];
 }
 
 /*