Re: Weird optimization flags behavior

[Richard Guenther <richard dot guenther at gmail dot com>]

On Mon, Aug 6, 2012 at 5:20 AM, Parang Saraf <parang.saraf@gmail.com> wrote:
> Hi,
>
> I am facing some weird issues with the optimization flags. I tried
> posting this in the gcc-help but got no response. Here is my query:
>
> I can see the flags enabled for each optimization level using "gcc -Q
> -On -help=optimizers" (flags enabled)
> Also, when I compile a program for a particular optimization level, I
> can see the flags used by gcc using, "gcc -Q -v -On program.c" (flags
> used).
> I am working with gcc 4.4.5. I found a few anomalies, which are mentioned below:
>
> 1. There are a few flags which are documented in O1 but are enabled
> for O0. Example: -funit-at-a-time. This flag is both enabled and used
> at O0
> 2. There are a few flags, which are documented in O1 and are not
> enabled for O0 (which is correct), but are still used for O0. Example:
> -fauto-inc-dec flag is documented in O1 and is not enabled for O0 but
> is still used by O0.
> 3. Flags which are enabled but are not used. Example: flags -fdce and
> -fdse are enabled for O0, O1 but is never used by the compiler.
> 4. Here is the complete list of flags distribution:
> O0: documented 0; enabled 35; used 43
> O1: documented 26; enabled 56; used 66
> O2: documented 55; enabled 80; used 91
> O3: documented 61; enabled 86; used 97
>
> I understand that there are a few flags which are used internally by
> the gcc. But my question is, if some of the flags are shown as enabled
> for a particular optimization level, why are they are not used during
> compilation?
>
> Also, ideally if I am using -O0, i should not be able to add/enable
> optimization flags. But when i enable some of the O1 optimization
> flags at O0, I could see gcc using them. For example: "gcc -Q -v -O0
> -ftree-fre program.c". Here -ftree-fre is an optimization flag from
> O1, but still it gets enabled at -O0.
>
> Most importantly: How can I make my gcc 4.5.4 adhere strictly with the
> 4.5.4 documentation? If I have to recompile gcc and make some changes
> to the config files, then please let me know.
>
> I will greatly appreciate your response in this regard.

It is a common misunderstanding that -On is just an alias for a set of
-fX options.  It does not work this way.  In case of -funit-at-a-time this
is a bug in the documentation.

Note that GCC 4.5.x is no longer maintained.

Richard.

> Regards
> Parang