This patch hopefully improves the documentation of the option in
question.
Applying to ubsan branch.
diff --git a/gcc/ChangeLog.ubsan b/gcc/ChangeLog.ubsan
index ac584ff..d7932c5 100644
--- a/gcc/ChangeLog.ubsan
+++ b/gcc/ChangeLog.ubsan
@@ -1,3 +1,7 @@
+2013-07-31 Marek Polacek <pola...@redhat.com>
+
+ * doc/invoke.texi: Improve documentation of -fsanitize=undefined.
+
2013-07-30 Marek Polacek <pola...@redhat.com>
* config/rs6000/rs6000.h (FRAME_GROWS_DOWNWARD): Use flag_sanitize
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 5dd9a62..652220b 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -5150,8 +5150,11 @@ See
@uref{http://code.google.com/p/data-race-test/wiki/ThreadSanitizer} for
@item -fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector
-Various computations will be instrumented to detect
-undefined behavior, e.g.@: division by zero or various overflows.
+Various computations will be instrumented to detect undefined behavior
+at runtime, e.g.@: division by zero or various overflows.
+While @option{-ftrapv} causes traps for signed overflows to be emitted,
+@option{-fsanitize=undefined} gives a diagnostic message.
+This currently works only for the C family of languages.
@item -fdump-final-insns@r{[}=@var{file}@r{]}
@opindex fdump-final-insns
Marek
