[PATCH] Enable -fsanitize=use-after-return by default under -fsanitize=address

[Kostya Serebryany]

Hi samsonov,

We enable ASAN's use-after-return instrumentation at compile-time,
but still keep it disabled at run-time.
This enables the users to flip the flag at run-time using environment variable
ASAN_OPTIONS=detect_stack_use_after_return=1 instead of using a separate build.
If UAR detection is disabled at run-time, this extra compile-time instrumentation
costs very small slowdown. On SPEC 2006 14 tests are not affected at all,
4 tests get ~ 1% slowdown and 453.povray gets 4%.

http://llvm-reviews.chandlerc.com/D1741

Files:
  lib/Driver/SanitizerArgs.cpp

Index: lib/Driver/SanitizerArgs.cpp
===================================================================
--- lib/Driver/SanitizerArgs.cpp
+++ lib/Driver/SanitizerArgs.cpp
@@ -211,11 +211,11 @@
 #define SANITIZER_GROUP(NAME, ID, ALIAS) .Case(NAME, ID)
 #include "clang/Basic/Sanitizers.def"
     .Default(SanitizeKind());
-  // Assume -fsanitize=address implies -fsanitize=init-order.
+  // Assume -fsanitize=address implies -fsanitize=init-order,use-after-return.
   // FIXME: This should be either specified in Sanitizers.def, or go away when
-  // we get rid of "-fsanitize=init-order" flag at all.
+  // we get rid of "-fsanitize=init-order,use-after-return" flags at all.
   if (ParsedKind & Address)
-    ParsedKind |= InitOrder;
+    ParsedKind |= InitOrder | UseAfterReturn;
   return ParsedKind;
 }
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