From 4470f78754246bbd134b65cad2e0a9638418677f Mon Sep 17 00:00:00 2001
From: wolfbeast <mcwerewolf@gmail.com>
Date: Wed, 7 Feb 2018 11:13:44 +0100
Subject: Avoid unnecessary string creation.

---
 dom/base/FileReader.cpp     |  4 ++--
 dom/base/nsContentUtils.cpp | 10 ++++++++--
 dom/base/nsContentUtils.h   | 10 +++++++++-
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/dom/base/FileReader.cpp b/dom/base/FileReader.cpp
index 003edc61f2..63a0ef2eee 100644
--- a/dom/base/FileReader.cpp
+++ b/dom/base/FileReader.cpp
@@ -452,8 +452,8 @@ FileReader::GetAsText(Blob *aBlob,
     }
   }
 
-  nsDependentCSubstring data(aFileData, aDataLen);
-  return nsContentUtils::ConvertStringFromEncoding(encoding, data, aResult);
+  return nsContentUtils::ConvertStringFromEncoding(
+      encoding, aFileData, aDataLen, aResult);
 }
 
 nsresult
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 29d28f8cea..321a61f18c 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -4128,9 +4128,15 @@ 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");
@@ -4142,7 +4148,7 @@ 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();
 }
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 278fbd0083..f688eeecf5 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -554,9 +554,17 @@ 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
-- 
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