Regarding -fno optimization options

[Parang Saraf]

Hi,

I have been digging further into this.

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.

Regards
Parang

On Wed, Jul 25, 2012 at 11:24 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On 25 July 2012 07:34, Parang Saraf wrote:
> > Hi,
> >
> > I have few very basic questions:
> >
> > 1. I want to turn-on only some of the optimization flags for O1. Is
> > doing this enough?:
> >
> >       " gcc -O0 -fcprop-registers -fdefer-pop -fif-conversion [...] -o
> > program program.c "
> >
> >     I tried with a bunch of options, generated an assembly file and
> > compared the assembly file with the assembly generated from just -O0.
> > Ideally, there should be a difference. Right?
>
> With -O0 there is no optimisation done, you can't enable individual
> optimisation passes if no optmisation is done.
>
> You probably want to use -O1 and selectively turn off the passes you don't want.
>
> > 2. Secondly, if I can switch-on flags like this, then what is the need
> > for -fno optimization options? Are -fno options meant just to reduce
> > the number of command line options, ie. say if for O1 I want most of
> > the flags except a few, then I can do "-O1 -fno-flag" ?
>
> Yes.  It's easier to say "I want -O2 without X" rather than "I want
> -O1 with A and B and C and D and ..."
>
> > If that's the
> > case, then why not all the flags have a -fno switch.
>
> It is the case, and I think they do all have -fno switches.
>
> > 3. In the gcc 4.7.1 optimization options documentation, there is a line:
> >
> >      " 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. "
> >
> >     What exactly is the meaning of second line?
>
> What I said above. Without -O1 or higher most optimisations will not
> run, even if you say -fxxx.