Bug 1682006 - Don't call Truncate() on the return string in GetProper…

…tyValue. r=xidorn

Turns out this causes a bit of overhead when appending to an AutoString,
because Truncate() calls SetToEmptyBuffer(), which makes it go from
inline-string to buffer string, and next time we append we need to turn
the string from a buffer-based string to an inline string again, which
is some minor, but measurable, work, while profiling some
micro-benchmarks.

Probably we should make Truncate() a no-op for a zero-length string, but
I'll send that separately.

Differential Revision: https://phabricator.services.mozilla.com/D99493

editor/libeditor/CSSEditUtils.cpp

@@ -1248,9 +1248,9 @@ bool CSSEditUtils::DoStyledElementsHaveSameStyle(
     return true;
   }
 
-  nsAutoCString propertyNameString;
-  nsAutoString firstValue, otherValue;
   for (uint32_t i = 0; i < firstLength; i++) {
+    nsAutoString firstValue, otherValue;
+    nsAutoCString propertyNameString;
     firstCSSDecl->Item(i, propertyNameString);
     DebugOnly<nsresult> rvIgnored =
         firstCSSDecl->GetPropertyValue(propertyNameString, firstValue);
@@ -1266,6 +1266,8 @@ bool CSSEditUtils::DoStyledElementsHaveSameStyle(
     }
   }
   for (uint32_t i = 0; i < otherLength; i++) {
+    nsAutoString firstValue, otherValue;
+    nsAutoCString propertyNameString;
     otherCSSDecl->Item(i, propertyNameString);
     DebugOnly<nsresult> rvIgnored =
         otherCSSDecl->GetPropertyValue(propertyNameString, otherValue);

layout/style/nsComputedDOMStyle.cpp

@@ -404,7 +404,7 @@ css::Rule* nsComputedDOMStyle::GetParentRule() { return nullptr; }
 NS_IMETHODIMP
 nsComputedDOMStyle::GetPropertyValue(const nsACString& aPropertyName,
                                      nsAString& aReturn) {
-  aReturn.Truncate();
+  MOZ_ASSERT(aReturn.IsEmpty());
 
   nsCSSPropertyID prop = nsCSSProps::LookupProperty(aPropertyName);
 

layout/style/nsDOMCSSDeclaration.cpp

@@ -37,8 +37,8 @@ nsresult nsDOMCSSDeclaration::GetPropertyValue(const nsCSSPropertyID aPropID,
                                                nsAString& aValue) {
   MOZ_ASSERT(aPropID != eCSSProperty_UNKNOWN,
              "Should never pass eCSSProperty_UNKNOWN around");
+  MOZ_ASSERT(aValue.IsEmpty());
 
-  aValue.Truncate();
   if (DeclarationBlock* decl =
           GetOrCreateCSSDeclaration(eOperation_Read, nullptr)) {
     decl->GetPropertyValueByID(aPropID, aValue);
@@ -91,10 +91,9 @@ void nsDOMCSSDeclaration::SetPropertyValue(const nsCSSPropertyID aPropID,
 }
 
 void nsDOMCSSDeclaration::GetCssText(nsAString& aCssText) {
-  DeclarationBlock* decl = GetOrCreateCSSDeclaration(eOperation_Read, nullptr);
-  aCssText.Truncate();
+  MOZ_ASSERT(aCssText.IsEmpty());
 
-  if (decl) {
+  if (auto* decl = GetOrCreateCSSDeclaration(eOperation_Read, nullptr)) {
     decl->ToString(aCssText);
   }
 }
@@ -165,19 +164,17 @@ void nsDOMCSSDeclaration::IndexedGetter(uint32_t aIndex, bool& aFound,
 NS_IMETHODIMP
 nsDOMCSSDeclaration::GetPropertyValue(const nsACString& aPropertyName,
                                       nsAString& aReturn) {
-  aReturn.Truncate();
-  if (DeclarationBlock* decl =
-          GetOrCreateCSSDeclaration(eOperation_Read, nullptr)) {
+  MOZ_ASSERT(aReturn.IsEmpty());
+  if (auto* decl = GetOrCreateCSSDeclaration(eOperation_Read, nullptr)) {
     decl->GetPropertyValue(aPropertyName, aReturn);
   }
   return NS_OK;
 }
 
 void nsDOMCSSDeclaration::GetPropertyPriority(const nsACString& aPropertyName,
                                               nsAString& aPriority) {
+  MOZ_ASSERT(aPriority.IsEmpty());
   DeclarationBlock* decl = GetOrCreateCSSDeclaration(eOperation_Read, nullptr);
-
-  aPriority.Truncate();
   if (decl && decl->GetPropertyIsImportant(aPropertyName)) {
     aPriority.AssignLiteral("important");
   }