Re: Have "gcc -v" print information about slow run-time checking?

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

On Fri, Oct 20, 2017 at 12:40 PM, Thomas Schwinge
<thomas@codesourcery.com> wrote:
> Hi!
>
> In <https://gcc.gnu.org/PR77620>, we got a user wonder about released vs.
> trunk GCC execution times, not knowing that the latter has more run-time
> checking enabled.  Once that got clarified, the discussion proceeded:
>
> (In reply to petschy from comment #6)
>> Would it be sensible to put an extra line to the output of 'gcc/g++ -v' if
>> the slow checks are enabled, which just states this fact / warns about
>> (possibly mentioning the use of --enable-checking=release at configure)?
>> Future tickets like this might be avoided this way.
>
> (In reply to Andrew Pinski from comment #7)
>> We already output one if you use -ftime-report.
>
> (In reply to Jonathan Wakely from comment #8)
>> We get reports like this every few months, and nobody ever uses
>> -ftime-report before filing a bug. I think something in the -v output would
>> be useful.
>
> We kind-of already report that by means of "--enable-checking=[...]" as
> part of the "Configured with:" line:
>
>     $ gcc-6 -v 2>&1 | sed -n '/^Configured with: /s%.*--enable-checking=\([^ ]*\).*%\1%p'
>     release
>
>     $ build-gcc/gcc/xgcc -v 2>&1 | sed -n '/^Configured with: /s%.*--enable-checking=\([^ ]*\).*%\1%p'
>     yes,df,fold,extra,rtl
>
> Though, that only gets displayed if "--enable-checking=[...]" has been
> specified explicitly, I think?
>
>
> Is that is generally considered useful, should I look into adding a
> separate "Run-time checking: [...]" line?  Should that just print the
> configured checks, or also some "slow!" notice, as suggested?  The latter
> only for the really slow checks?  Are we able to identify these
> generally?

All checking slows down the compiler.  I guess adding stuff to -v output might
break scripts out in the wild...  but yes, we could output the same boiler-plate
as with -ftime-report.

>
> Also, from reading the documentation, I can't tell if (it's the idea
> that) running checking-enabled GCC with "-fno-checking" would indeed get
> rid of *all* the checking's run-time overhead?  (Basically, if all
> "ENABLE_*_CHECKING" usage is guarded by "flag_checking", too?  Not yet
> verified.)

No, for example all the tree and rtl macro checking isn't guarded by
flag_checking
nor are gcc_checking_assert()s, etc.  The flag wasn't intended to be used in the
negative form but to allow better debugging of problems in a release checking
compiler so when you get a random ICE you can run with -fchecking and
immediately pinpoint a more useful place where things go wrong (you mostly
get IL checking that way).

Richard.

>
> Grüße
>  Thomas