Index: lld/trunk/COFF/Driver.cpp
===================================================================
--- lld/trunk/COFF/Driver.cpp
+++ lld/trunk/COFF/Driver.cpp
@@ -818,9 +818,18 @@
     Config->ManifestID = 2;
   }
 
-  // Handle /fixed
-  if (Args.hasArg(OPT_fixed)) {
-    if (Args.hasArg(OPT_dynamicbase)) {
+  // Handle /dynamicbase and /fixed. We can't use hasFlag for /dynamicbase
+  // because we need to explicitly check whether that option or its inverse was
+  // present in the argument list in order to handle /fixed.
+  auto *DynamicBaseArg = Args.getLastArg(OPT_dynamicbase, OPT_dynamicbase_no);
+  if (DynamicBaseArg &&
+      DynamicBaseArg->getOption().getID() == OPT_dynamicbase_no)
+    Config->DynamicBase = false;
+
+  bool Fixed = Args.hasFlag(OPT_fixed, OPT_fixed_no, false);
+  if (Fixed) {
+    if (DynamicBaseArg &&
+        DynamicBaseArg->getOption().getID() == OPT_dynamicbase) {
       error("/fixed must not be specified with /dynamicbase");
     } else {
       Config->Relocatable = false;
@@ -828,8 +837,9 @@
     }
   }
 
-  if (Args.hasArg(OPT_appcontainer))
-    Config->AppContainer = true;
+  // Handle /appcontainer
+  Config->AppContainer =
+      Args.hasFlag(OPT_appcontainer, OPT_appcontainer_no, false);
 
   // Handle /machine
   if (auto *Arg = Args.getLastArg(OPT_machine))
@@ -984,16 +994,11 @@
   }
 
   // Handle miscellaneous boolean flags.
-  if (Args.hasArg(OPT_allowbind_no))
-    Config->AllowBind = false;
-  if (Args.hasArg(OPT_allowisolation_no))
-    Config->AllowIsolation = false;
-  if (Args.hasArg(OPT_dynamicbase_no))
-    Config->DynamicBase = false;
-  if (Args.hasArg(OPT_nxcompat_no))
-    Config->NxCompat = false;
-  if (Args.hasArg(OPT_tsaware_no))
-    Config->TerminalServerAware = false;
+  Config->AllowBind = Args.hasFlag(OPT_allowbind, OPT_allowbind_no, true);
+  Config->AllowIsolation =
+      Args.hasFlag(OPT_allowisolation, OPT_allowisolation_no, true);
+  Config->NxCompat = Args.hasFlag(OPT_nxcompat, OPT_nxcompat_no, true);
+  Config->TerminalServerAware = Args.hasFlag(OPT_tsaware, OPT_tsaware_no, true);
   if (Args.hasArg(OPT_nosymtab))
     Config->WriteSymtab = false;
 
@@ -1048,12 +1053,13 @@
                                    ArrayRef<StringRef>(SearchPaths).slice(1)));
 
   // Handle /largeaddressaware
-  if (Config->is64() || Args.hasArg(OPT_largeaddressaware))
-    Config->LargeAddressAware = true;
+  Config->LargeAddressAware = Args.hasFlag(
+      OPT_largeaddressaware, OPT_largeaddressaware_no, Config->is64());
 
   // Handle /highentropyva
-  if (Config->is64() && !Args.hasArg(OPT_highentropyva_no))
-    Config->HighEntropyVA = true;
+  Config->HighEntropyVA =
+      Config->is64() &&
+      Args.hasFlag(OPT_highentropyva, OPT_highentropyva_no, true);
 
   // Handle /entry and /dll
   if (auto *Arg = Args.getLastArg(OPT_entry)) {
@@ -1208,7 +1214,7 @@
   }
 
   // Handle /safeseh.
-  if (Args.hasArg(OPT_safeseh)) {
+  if (Args.hasFlag(OPT_safeseh, OPT_safeseh_no, false)) {
     for (ObjFile *File : ObjFile::Instances)
       if (!File->SEHCompat)
         error("/safeseh: " + File->getName() + " is not compatible with SEH");
Index: lld/trunk/COFF/Options.td
===================================================================
--- lld/trunk/COFF/Options.td
+++ lld/trunk/COFF/Options.td
@@ -9,10 +9,11 @@
 class P<string name, string help> :
       Joined<["/", "-", "-?"], name#":">, HelpText<help>;
 
-// Boolean flag suffixed by ":no".
-multiclass B<string name, string help> {
-  def "" : F<name>;
-  def _no : F<name#":no">, HelpText<help>;
+// Boolean flag which can be suffixed by ":no". Using it unsuffixed turns the
+// flag on and using it suffixed by ":no" turns it off.
+multiclass B<string name, string help_on, string help_off> {
+  def "" : F<name>, HelpText<help_on>;
+  def _no : F<name#":no">, HelpText<help_off>;
 }
 
 def align   : P<"align", "Section alignment">;
@@ -85,18 +86,31 @@
     HelpText<"Allow undefined symbols when creating executables">;
 def force_unresolved : F<"force:unresolved">;
 
-defm allowbind: B<"allowbind", "Disable DLL binding">;
-defm allowisolation : B<"allowisolation", "Set NO_ISOLATION bit">;
+defm allowbind : B<"allowbind", "Enable DLL binding (default)",
+                   "Disable DLL binding">;
+defm allowisolation : B<"allowisolation", "Enable DLL isolation (default)",
+                        "Disable DLL isolation">;
 defm appcontainer : B<"appcontainer",
-                      "Image can only be run in an app container">;
-defm dynamicbase : B<"dynamicbase",
-                     "Disable address space layout randomization">;
-defm fixed    : B<"fixed", "Enable base relocations">;
-defm highentropyva : B<"highentropyva", "Set HIGH_ENTROPY_VA bit">;
-defm largeaddressaware : B<"largeaddressaware", "Disable large addresses">;
-defm nxcompat : B<"nxcompat", "Disable data execution provention">;
-defm safeseh : B<"safeseh", "Produce an image with Safe Exception Handler">;
-defm tsaware  : B<"tsaware", "Create non-Terminal Server aware executable">;
+                      "Image can only be run in an app container",
+                      "Image can run outside an app container (default)">;
+defm dynamicbase : B<"dynamicbase", "Enable ASLR (default unless /fixed)",
+                     "Disable ASLR (default when /fixed)">;
+defm fixed : B<"fixed", "Disable base relocations",
+               "Enable base relocations (default)">;
+defm highentropyva : B<"highentropyva",
+                       "Enable 64-bit ASLR (default on 64-bit)",
+                       "Disable 64-bit ASLR">;
+defm largeaddressaware : B<"largeaddressaware",
+                           "Enable large addresses (default on 64-bit)",
+                           "Disable large addresses (default on 32-bit)">;
+defm nxcompat : B<"nxcompat", "Enable data execution prevention (default)",
+                  "Disable data execution provention">;
+defm safeseh : B<"safeseh",
+                 "Produce an image with Safe Exception Handler (only for x86)",
+                 "Don't produce an image with Safe Exception Handler">;
+defm tsaware  : B<"tsaware",
+                  "Create Terminal Server aware executable (default)",
+                  "Create non-Terminal Server aware executable">;
 
 def help : F<"help">;
 def help_q : Flag<["/?", "-?"], "">, Alias<help>;