QFsFileEngine: avoid triple(quadruple) lookup of the same key

Instead of contains(), 1-2x operator[](), and remove(), equalling 3-4
separate lookups, use find() + erase(), which does just one
lookup.

Since our erase() function is C++11-compliant these days and takes
const_iterator instead of (mutable) iterator, we can use the const
find() overload to delay a detach (attempt) until we actually erase().

Change-Id: I8e67a48e221e548528049fa093ab7ef2f1802f7e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 9 insertions, 7 deletions

diff --git a/src/corelib/io/qfsfileengine_unix.cpp b/src/corelib/io/qfsfileengine_unix.cpp
index a57d924c12..0b3e401be8 100644
--- a/src/corelib/io/qfsfileengine_unix.cpp
+++ b/src/corelib/io/qfsfileengine_unix.cpp
@@ -646,18 +646,19 @@ bool QFSFileEnginePrivate::unmap(uchar *ptr)
 {
 #if !defined(Q_OS_INTEGRITY)
     Q_Q(QFSFileEngine);
-    if (!maps.contains(ptr)) {
+    const auto it = std::as_const(maps).find(ptr);
+    if (it == maps.cend()) {
         q->setError(QFile::PermissionsError, qt_error_string(EACCES));
         return false;
     }
 
-    uchar *start = ptr - maps[ptr].first;
-    size_t len = maps[ptr].second;
+    uchar *start = ptr - it->first;
+    size_t len = it->second;
     if (-1 == munmap(start, len)) {
         q->setError(QFile::UnspecifiedError, qt_error_string(errno));
         return false;
     }
-    maps.remove(ptr);
+    maps.erase(it);
     return true;
 #else
     return false;
diff --git a/src/corelib/io/qfsfileengine_win.cpp b/src/corelib/io/qfsfileengine_win.cpp
index e95eef225b..a0af2cf176 100644
--- a/src/corelib/io/qfsfileengine_win.cpp
+++ b/src/corelib/io/qfsfileengine_win.cpp
@@ -871,17 +871,18 @@ uchar *QFSFileEnginePrivate::map(qint64 offset, qint64 size,
 bool QFSFileEnginePrivate::unmap(uchar *ptr)
 {
     Q_Q(QFSFileEngine);
-    if (!maps.contains(ptr)) {
+    const auto it = std::as_const(maps).find(ptr);
+    if (it == maps.cend()) {
         q->setError(QFile::PermissionsError, qt_error_string(ERROR_ACCESS_DENIED));
         return false;
     }
-    uchar *start = ptr - maps[ptr];
+    uchar *start = ptr - *it;
     if (!UnmapViewOfFile(start)) {
         q->setError(QFile::PermissionsError, qt_error_string());
         return false;
     }
 
-    maps.remove(ptr);
+    maps.erase(it);
     if (maps.isEmpty()) {
         ::CloseHandle(mapHandle);
         mapHandle = NULL;