Bug 1370070 - Avoid unnecessary string creation. r=smaug

MozReview-Commit-ID: LLQQrCpK630

dom/base/nsContentUtils.cpp

@@ -4493,9 +4493,15 @@ nsContentUtils::GetSubdocumentWithOuterWindowId(nsIDocument *aDocument,
 /* static */
 nsresult
 nsContentUtils::ConvertStringFromEncoding(const nsACString& aEncoding,
-                                          const nsACString& aInput,
+                                          const char* aInput,
+                                          uint32_t aInputLen,
                                           nsAString& aOutput)
 {
+  CheckedInt32 len = aInputLen;
+  if (!len.isValid()) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
   nsAutoCString encoding;
   if (aEncoding.IsEmpty()) {
     encoding.AssignLiteral("UTF-8");
@@ -4507,7 +4513,7 @@ nsContentUtils::ConvertStringFromEncoding(const nsACString& aEncoding,
   nsAutoPtr<TextDecoder> decoder(new TextDecoder());
   decoder->InitWithEncoding(encoding, false);
 
-  decoder->Decode(aInput.BeginReading(), aInput.Length(), false,
+  decoder->Decode(aInput, len.value(), false,
                   aOutput, rv);
   return rv.StealNSResult();
 }

dom/base/nsContentUtils.h

@@ -624,9 +624,17 @@ public:
    *                  string (meaning UTF-8)
    */
   static nsresult ConvertStringFromEncoding(const nsACString& aEncoding,
-                                            const nsACString& aInput,
+                                            const char* aInput,
+                                            uint32_t aInputLen,
                                             nsAString& aOutput);
 
+  static nsresult ConvertStringFromEncoding(const nsACString& aEncoding,
+                                            const nsACString& aInput,
+                                            nsAString& aOutput) {
+    return ConvertStringFromEncoding(
+        aEncoding, aInput.BeginReading(), aInput.Length(), aOutput);
+  }
+
   /**
    * Determine whether a buffer begins with a BOM for UTF-8, UTF-16LE,
    * UTF-16BE

dom/file/FileReader.cpp

@@ -480,8 +480,8 @@ FileReader::GetAsText(Blob *aBlob,
     }
   }
 
-  nsDependentCSubstring data(aFileData, aDataLen);
-  return nsContentUtils::ConvertStringFromEncoding(encoding, data, aResult);
+  return nsContentUtils::ConvertStringFromEncoding(
+      encoding, aFileData, aDataLen, aResult);
 }
 
 nsresult