question about knowing when optimization options are used

question about knowing when optimization options are used

[Shrader, David Lee]

Hello,

The gcc man page 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.

The man page says to use `-Q --help=optimizers` to find out what is turned on for the different -O levels as configure-time options can make things different from the defaults shown later in that man page. However, I have found that, even when using -O0, I can turn on optimization options, at least according to the help output:

$> gcc -Q --help=optimizers | grep "\-fmove-loop-invariants"
  -fmove-loop-invariants                [disabled]
$> gcc -Q --help=optimizers -fmove-loop-invariants | grep "\-fmove-loop-invariants"
  -fmove-loop-invariants                [enabled]

I get the same result when using "-O0". Given the line from the gcc man page, I would have expected this option to still be disabled even when using it explicitly on the command line. Have I misunderstood the man page line? Or I am misunderstanding the help output? Or is something else altogether going on?

Thank for any and all help,
David

Re: question about knowing when optimization options are used

[Segher Boessenkool]

On Mon, Mar 16, 2020 at 11:14:25PM +0000, Shrader, David Lee via Gcc-help wrote:
> 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.

Yes.

> The man page says to use `-Q --help=optimizers` to find out what is turned on for the different -O levels as configure-time options can make things different from the defaults shown later in that man page. However, I have found that, even when using -O0, I can turn on optimization options, at least according to the help output:

You can enable the *flags*, sure, but with -O0 those flags do not do
anything.  Exactly as the text above says?  The "even if" part.


Segher

Re: [EXTERNAL] Re: question about knowing when optimization options are used

[Shrader, David Lee]

Hello,


Thank you for the reply. So, what you're saying is that the output of `gcc -Q --help=optimizers` doesn't necessarily reflect what optimization gcc is going to do when -O0 is being used, correct? This is because no (or very little) optimization is actually allowed to be done when -O0 is use.


Thank you again for the help!

David


________________________________
From: Segher Boessenkool <segher@kernel.crashing.org>
Sent: Monday, March 16, 2020 5:34 PM
To: Shrader, David Lee
Cc: gcc-help@gcc.gnu.org
Subject: [EXTERNAL] Re: question about knowing when optimization options are used

On Mon, Mar 16, 2020 at 11:14:25PM +0000, Shrader, David Lee via Gcc-help wrote:
> 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.

Yes.

> The man page says to use `-Q --help=optimizers` to find out what is turned on for the different -O levels as configure-time options can make things different from the defaults shown later in that man page. However, I have found that, even when using -O0, I can turn on optimization options, at least according to the help output:

You can enable the *flags*, sure, but with -O0 those flags do not do
anything.  Exactly as the text above says?  The "even if" part.


Segher

Re: [EXTERNAL] Re: question about knowing when optimization options are used

[Segher Boessenkool]

Hi!

On Mon, Mar 16, 2020 at 11:55:01PM +0000, Shrader, David Lee wrote:
> Thank you for the reply. So, what you're saying is that the output of `gcc -Q --help=optimizers` doesn't necessarily reflect what optimization gcc is going to do when -O0 is being used, correct? This is because no (or very little) optimization is actually allowed to be done when -O0 is use.

Yes.  Some of the documentation isn't clear on this, but in context it
should be clear?


Segher

Re: [EXTERNAL] Re: question about knowing when optimization options are used

[Jeffrey Walton]

On Mon, Mar 16, 2020 at 7:55 PM Shrader, David Lee via Gcc-help
<gcc-help@gcc.gnu.org> wrote:
>
> Thank you for the reply. So, what you're saying is that the output of `gcc -Q --help=optimizers` doesn't necessarily reflect what optimization gcc is going to do when -O0 is being used, correct? This is because no (or very little) optimization is actually allowed to be done when -O0 is use.

If you are interested in debugging+optimizations, then you might also
take a look at -Og. It is a relatively new option that is intended to
apply some optimizations while preserving the debugging experience. I
think it showed up around GCC 7, but the first mention in the change
log is GCC 8. Also see
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html .

$ gcc -Og -Q --help=optimizers | grep "\-fmove-loop-invariants"
  -fmove-loop-invariants              [disabled]
$ gcc -O3 -Q --help=optimizers | grep "\-fmove-loop-invariants"
  -fmove-loop-invariants              [enabled]

Jeff

RE: [EXTERNAL] Re: question about knowing when optimization options are used

[Shrader, David Lee]

It’s clear to me now. After reading through the optimization section again, I can see more of the meaning; perhaps because I am looking for it. Thank you for taking the time to answer.

David


From: Segher Boessenkool <segher@kernel.crashing.org<mailto:segher@kernel.crashing.org>>
Date: Monday, Mar 16, 2020, 6:36 PM
To: Shrader, David Lee <dshrader@lanl.gov<mailto:dshrader@lanl.gov>>
Cc: gcc-help@gcc.gnu.org <gcc-help@gcc.gnu.org<mailto:gcc-help@gcc.gnu.org>>
Subject: Re: [EXTERNAL] Re: question about knowing when optimization options are used

Hi!

On Mon, Mar 16, 2020 at 11:55:01PM +0000, Shrader, David Lee wrote:
> Thank you for the reply. So, what you're saying is that the output of `gcc -Q --help=optimizers` doesn't necessarily reflect what optimization gcc is going to do when -O0 is being used, correct? This is because no (or very little) optimization is actually allowed to be done when -O0 is use.

Yes.  Some of the documentation isn't clear on this, but in context it
should be clear?


Segher

RE: [EXTERNAL] Re: question about knowing when optimization options are used

[Shrader, David Lee]

Actually, I am interested in looking in to that. I end up fielding a lot of questions about what to do for adequate debugging, and I wasn’t aware of that flag. Thank you for pointing it out to me.

David


From: Jeffrey Walton via Gcc-help <gcc-help@gcc.gnu.org<mailto:gcc-help@gcc.gnu.org>>
Date: Monday, Mar 16, 2020, 7:35 PM
To: gcc-help <gcc-help@gcc.gnu.org<mailto:gcc-help@gcc.gnu.org>>
Subject: Re: [EXTERNAL] Re: question about knowing when optimization options are used

On Mon, Mar 16, 2020 at 7:55 PM Shrader, David Lee via Gcc-help
<gcc-help@gcc.gnu.org> wrote:
>
> Thank you for the reply. So, what you're saying is that the output of `gcc -Q --help=optimizers` doesn't necessarily reflect what optimization gcc is going to do when -O0 is being used, correct? This is because no (or very little) optimization is actually allowed to be done when -O0 is use.

If you are interested in debugging+optimizations, then you might also
take a look at -Og. It is a relatively new option that is intended to
apply some optimizations while preserving the debugging experience. I
think it showed up around GCC 7, but the first mention in the change
log is GCC 8. Also see
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html .

$ gcc -Og -Q --help=optimizers | grep "\-fmove-loop-invariants"
  -fmove-loop-invariants              [disabled]
$ gcc -O3 -Q --help=optimizers | grep "\-fmove-loop-invariants"
  -fmove-loop-invariants              [enabled]

Jeff

Re: [EXTERNAL] Re: question about knowing when optimization options are used

[Segher Boessenkool]

On Tue, Mar 17, 2020 at 01:44:48AM +0000, Shrader, David Lee via Gcc-help wrote:
> Actually, I am interested in looking in to that. I end up fielding a lot of questions about what to do for adequate debugging, and I wasn’t aware of that flag. Thank you for pointing it out to me.

-Og was introduced in GCC 4.8, seven years ago.


Segher

Re: question about knowing when optimization options are used

[Richard Sandiford]

Segher Boessenkool <segher@kernel.crashing.org> writes:
> On Mon, Mar 16, 2020 at 11:14:25PM +0000, Shrader, David Lee via Gcc-help wrote:
>> 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.
>
> Yes.
>
>> The man page says to use `-Q --help=optimizers` to find out what is turned on for the different -O levels as configure-time options can make things different from the defaults shown later in that man page. However, I have found that, even when using -O0, I can turn on optimization options, at least according to the help output:
>
> You can enable the *flags*, sure, but with -O0 those flags do not do
> anything.  Exactly as the text above says?  The "even if" part.

I think David's the right that the wording isn't clear though.
The documentation sets up a contrast between an option being "enabled"
or merely "specified", with "enabled" meaning something like "have an
effect".  But the quoted output then undermines that by using "enabled"
for what the documentation calls "specified":

$> gcc -Q --help=optimizers -fmove-loop-invariants | grep "\-fmove-loop-invariants"
  -fmove-loop-invariants                [enabled]

I can definitely see why this is confusing.

Richard

Re: question about knowing when optimization options are used

[mark_at_yahoo]

On 3/16/20 4:34 PM, Segher Boessenkool wrote:
> You can enable the *flags*, sure, but with -O0 those flags do not do
> anything.  Exactly as the text above says?  The "even if" part.

(Meta-response: I tried sending the following via nabble.com approx. 40 
hours ago and it seems not to have gotten through. Apologies if this 
eventually shows up as a duplicate.)

Sorry to question this, but my experience has been otherwise. I haven't 
tried it in a while (probably last did on an older release) but at least 
with gcc-arm, doing:

     -O0 \
     -fbranch-count-reg \
     -fcombine-stack-adjustments \
     -fcompare-elim \
     -fcprop-registers \
     <long list of other flags deleted>

*did* do something different than just "-O0".

The above was part of my semi-successful attempt to solve the following 
problem:
1) I'm compiling ARM code for an embedded microcontroller with very 
limited memory
2) My code includes header files with literally thousands (no 
exaggeration) of "static const int ..."s
3) The code (intentionally) uses only a handful of the consts
4) If compiled "-O1", no memory is allocated for the thousands of consts 
(not even the few that are used)
5) If compiled "-O0" memory is allocated for each const -- far more 
memory than is available
6) "-O0" executables are much easier to debug at the machine instruction 
level (sometimes necessary on microcontrollers)

Can anyone suggest a minimal set of "-f<options>"s to add  to -O0 which 
will do what I want, i.e. no code optimizations, but also no memory 
allocated for the consts?

Re: question about knowing when optimization options are used

[Segher Boessenkool]

On Tue, Mar 17, 2020 at 12:22:07PM +0000, Richard Sandiford wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > On Mon, Mar 16, 2020 at 11:14:25PM +0000, Shrader, David Lee via Gcc-help wrote:
> >> 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.
> >
> > Yes.
> >
> >> The man page says to use `-Q --help=optimizers` to find out what is turned on for the different -O levels as configure-time options can make things different from the defaults shown later in that man page. However, I have found that, even when using -O0, I can turn on optimization options, at least according to the help output:
> >
> > You can enable the *flags*, sure, but with -O0 those flags do not do
> > anything.  Exactly as the text above says?  The "even if" part.
> 
> I think David's the right that the wording isn't clear though.

Oh yes, I didn't mean to suggest it is perfect as is, sorry.

> The documentation sets up a contrast between an option being "enabled"
> or merely "specified", with "enabled" meaning something like "have an
> effect".  But the quoted output then undermines that by using "enabled"
> for what the documentation calls "specified":
> 
> $> gcc -Q --help=optimizers -fmove-loop-invariants | grep "\-fmove-loop-invariants"
>   -fmove-loop-invariants                [enabled]
> 
> I can definitely see why this is confusing.

Yeah.  Help output is not documentation, but if they aren't in agreement,
what is a user to do?  :-(

Does anyone have proposed improved wording, etc.?


Segher

Re: question about knowing when optimization options are used

[Segher Boessenkool]

On Wed, Mar 18, 2020 at 10:44:09AM -0700, mark_at_yahoo via Gcc-help wrote:
> On 3/16/20 4:34 PM, Segher Boessenkool wrote:
> >You can enable the *flags*, sure, but with -O0 those flags do not do
> >anything.  Exactly as the text above says?  The "even if" part.

> Sorry to question this, but my experience has been otherwise.

From the manual again:
  Most optimizations are completely disabled at '-O0' or if an '-O' level
  is not set on the command line, even if individual optimization flags
  are specified.  Similarly, '-Og' suppresses many optimization passes.

> I haven't 
> tried it in a while (probably last did on an older release) but at least 
> with gcc-arm, doing:
> 
>     -O0 \
>     -fbranch-count-reg \
>     -fcombine-stack-adjustments \
>     -fcompare-elim \
>     -fcprop-registers \
>     <long list of other flags deleted>
> 
> *did* do something different than just "-O0".

It might be interesting to see which compiler flag(s) you see any
differences with.

> 2) My code includes header files with literally thousands (no 
> exaggeration) of "static const int ..."s
> 3) The code (intentionally) uses only a handful of the consts
> 4) If compiled "-O1", no memory is allocated for the thousands of consts 
> (not even the few that are used)
> 5) If compiled "-O0" memory is allocated for each const -- far more 
> memory than is available
> 6) "-O0" executables are much easier to debug at the machine instruction 
> level (sometimes necessary on microcontrollers)
> 
> Can anyone suggest a minimal set of "-f<options>"s to add  to -O0 which 
> will do what I want, i.e. no code optimizations, but also no memory 
> allocated for the consts?

Try -Og instead?  It is mostly like -O1, but almost all debugging should
work fine with it (as the option name itself suggests :-) )


Segher

Re: question about knowing when optimization options are used

[Jonathan Wakely]

On Mon, 16 Mar 2020 at 23:35, Segher Boessenkool wrote:
>
> On Mon, Mar 16, 2020 at 11:14:25PM +0000, Shrader, David Lee via Gcc-help wrote:
> > 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.
>
> Yes.
>
> > The man page says to use `-Q --help=optimizers` to find out what is turned on for the different -O levels as configure-time options can make things different from the defaults shown later in that man page. However, I have found that, even when using -O0, I can turn on optimization options, at least according to the help output:
>
> You can enable the *flags*, sure, but with -O0 those flags do not do
> anything.  Exactly as the text above says?  The "even if" part.

See also https://gcc.gnu.org/wiki/FAQ#optimization-options