On 11/03/2015 03:19 AM, Alexander Monakov wrote:
On Thu, 22 Oct 2015, Martin Sebor wrote:
[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.
Hi,
The issue is due to optimization passes being skipped at -O0, and yet
corresponding optimization options not explicitely disabled. The effect of -O
is an old source of confusion, and now the intro to "Optimization Options"
says,
Most optimizations are only enabled if an -O level is set on the command
line. Otherwise they are disabled, even if individual optimization flags
are specified.
(added with this patch:
https://gcc.gnu.org/ml/gcc-patches/2009-10/msg00739.html )
Thanks for the reference! Despite the improvement, this continues
to be a recurring problem. Users tend to miss the added text, maybe
because it's on a different HTML page than the --help option. That
certainly seemed to be the case in this post:
https://gcc.gnu.org/ml/gcc-help/2015-10/msg00135.html
It's also possible that it's because the caveat is mentioned in
a context that doesn't match their use case. In the originally
referenced thread, the user wasn't looking to enable additional
optimizations. Rather, they were trying to see what (if any)
optimizations are enabled by default, with -O0.
As we observe, it's not visible enough, and I'm not sure saying that again in
the documentation (in a different section) is a good way to go. Maybe we'd
warn for attempts to enable optimizations at -O0, but that's not trivial.
Perhaps go with Richard's suggestion in the end of this mail ("Thus, at the
end of --help-optimizers print ...")?
https://gcc.gnu.org/ml/gcc-patches/2012-05/msg00113.html
Improving the compiler output is a good idea. The attached patch
prints "[disabled by -O0]" instead of "[enabled]" when an optimization
option is enabled by default but when optimization (i.e., -O1 or
greater) is not enabled.
The patch also further clarifies the wording in the documentation
to help users avoid falling into the trap of assuming, based on
an incomplete reading of the manual, that some optimizations are
performed even at -O0.
Martin
gcc/ChangeLog:
2015-11-04 Martin Sebor <mse...@redhat.com>
* opts.c (print_filtered_help): Indicate when an optimization option
is disabled as a result of -O0.
* doc/invoke.texi: Further clarify the effect of -O options
on individual optimization options.
gcc/testsuite/ChangeLog:
2015-11-04 Martin Sebor <mse...@redhat.com>
* testsuite/gcc.misc-tests/help.exp: Verify that optimization options
are printed as disabled when -O0 is specified and enabled otherwise.
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi (revision 229716)
+++ doc/invoke.texi (working copy)
@@ -1509,6 +1509,14 @@
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 remember 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, most commonly
+one of the @option{-O} options. This is 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.
+
Here is a truncated example from the ARM port of @command{gcc}:
@smallexample
@@ -1524,7 +1532,7 @@
are enabled at @option{-O2} by using:
@smallexample
--Q -O2 --help=optimizers
+ % gcc -Q -O2 --help=optimizers
@end smallexample
Alternatively you can discover which binary optimizations are enabled
@@ -1531,11 +1539,22 @@
by @option{-O3} by using:
@smallexample
-gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
-gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
-diff /tmp/O2-opts /tmp/O3-opts | grep enabled
+ % gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
+ % gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
+ % diff /tmp/O2-opts /tmp/O3-opts | grep enabled
@end smallexample
+Finally, the following example shows the difference in output for
+an option that, while technically enabled, is disabled as a consequence
+of the implicit @option{-O0} option, and for one that is disabled by
+default. This distinction is typically only of interest to GCC developers.
+@smallexample
+ % gcc -Q --help=optimizers
+ The following options control optimizations:
+ -faggressive-loop-optimizations [disabled by -O0]
+ -falign-functions [disabled]
+@end smallexample
+
@item -no-canonical-prefixes
@opindex no-canonical-prefixes
Do not expand any symbolic links, resolve references to @samp{/../}
@@ -7678,9 +7697,9 @@
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
-Most optimizations are only enabled if an @option{-O} level is set on
-the command line. Otherwise they are disabled, even if individual
-optimization flags are specified.
+Most optimizations are only performed when an @option{-O} option other
+than @option{-O0} is specified on the command line. Otherwise they are
+not performed, even if individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different
set of optimizations may be enabled at each @option{-O} level than
Index: opts.c
===================================================================
--- opts.c (revision 229716)
+++ opts.c (working copy)
@@ -1190,6 +1190,15 @@
sprintf (new_help + strlen (new_help),
"%#x", * (int *) flag_var);
}
+ else if (option->flags & CL_OPTIMIZATION && !optimize)
+ {
+ /* Many options controlling optimization are enabled
+ but ineffective at -O0. Make that clear in the output
+ to avoid misleading users to expect that the respective
+ optimizations may take place. */
+ strcat (new_help, option_enabled (i, opts)
+ ? _("[disabled by -O0]") : _("[disabled]"));
+ }
else
strcat (new_help, option_enabled (i, opts)
? _("[enabled]") : _("[disabled]"));
Index: testsuite/gcc.misc-tests/help.exp
===================================================================
--- testsuite/gcc.misc-tests/help.exp (revision 229716)
+++ testsuite/gcc.misc-tests/help.exp (working copy)
@@ -89,6 +89,18 @@
-O
} "" ""
+# Verify that options that are enabled by default but ineffective
+# without -O0 are printed as "disabled by -O0" when -O0 is specified,
+# and as "enabled" otherwise.
+check_for_options c "-Q -O0 --help=optimizers" {
+-O
+-finline[^-][^\n]*disabled by -O0
+} "" ""
+check_for_options c "-Q -O1 --help=optimizers" {
+-O
+-finline[^-][^\n]*enabled
+} "" ""
+
# Ensure PR 37805 is fixed.
# Specify patterns (arguments 3 and later) that match option names
# at the beginning of the line and not when they are referenced by
