[PATCH] clarify documentation of -Q --help=optimizers

[Martin Sebor <msebor@gmail.com>]

[Sending to the right list this time]

The documentation of the -Q --help=optimizers options leads some
to expect that when options are reported as enabled imply the
corresponding optimization will take place.  (See the following
question on gcc-help:
https://gcc.gnu.org/ml/gcc-help/2015-10/msg00133.html)

The patch below tries to make it clear that that's not always
the case.

Martin

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 4f89fa1..337639e 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -1506,6 +1506,16 @@ options, an indication is given as to whether the
option is enabled,
  disabled or set to a specific value (assuming that the compiler
  knows this at the point where the @option{--help=} option is used).

+It's important to note that when a given optimization option is
+enabled, either explicitly on the command line or implicitly, whether
+or not the optimization it controls will be performed during an
+invocation of the compiler may depend on other options.  This is
+so because many options control various finer aspects of other more
+general optimizations that must be enabled in order for the former
+option to have any effect.  There is currently no command line
+interface to determine such dependencies between optimization
+options.
+
  Here is a truncated example from the ARM port of @command{gcc}:

  @smallexample