Re: [PATCH V4] gprofng: a new GNU profiler

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Wed, 2 Feb 2022, Vladimir Mezentsev wrote:
>
>> > > > Woudn't it be better to skip building gprofng in case no --enable-shared
>> > > > is specified at `configure' time?
>> > >   Is it actually necessary to require `--enable-shared' in the first place
>> > > for the program to work?
>> > I think so.  AFAIK the gprofng collector works as an interposed shared
>> > object.
>> 
>> yes.
>> We use the gprofng library in LD_PRELOAD. This library should be a shared
>> library.
>> 
>> 
>> I am trying to set  extra_configure_flags='--enable-shared' for gprofng build
>> only.
>> But gprofng depens on libopcodes and this library should build with -fPIC.
>> I haven't yet found how to solve this build issue.
>
>  Forcing `--enable-shared' in gprofng/ unconditionally could be confusing 
> to the user who might want to avoid shared libraries for a reason.
>
>  I think perhaps the best solution would be using AC_ENABLE_SHARED in 
> gprofng/ or the equivalent LT_INIT option (I think it's the default 
> anyway) and bail out with a suitable message such as:
>
> "Building in gprofng is unsupported with shared libraries disabled.
> Please reconfigure with `--enable-shared=gprofng' or `--disable-gprofng'."
>
> or suchlike.

I think this warning could be implemented in the top-level configure.ac,
setting `notsupp' for gprofng if `enable_shared' is "no".

Then, if the user builds binutils without --enable-shared, or in a host
or target for which gprofng is not supported, you get the familiar
message:

*** This configuration is not supported in the following subdirectories:
    gprofng
    (Any other directories should still work fine.)

>  Then in libopcodes/ you probably want the `pic-only' option to LT_INIT 
> (or use `--with-pic') unless `--enable-shared' has been also used for the 
> library, though I think it would best be conditionalised on gprofng having 
> been enabled if possible as there's a performance hit from using PIC code.  
> As there's a `--without-pic' option too the user can give this all may 
> have to be coordinated at the top level rather than within gprofng/.
>
>  Perhaps the way how `extra_host_libiberty_configure_flags' are handled 
> could serve as an example (though I think it should also use `--with-pic' 
> rather than `--enable-shared' unless `--enable-shared' has been explicitly 
> given already).
>
>  I realise what I have outlined is not a complete solution, but I hope it 
> can help regardless.
>
>   Maciej