Re: [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply defined symbols, and MSFT/GCC tool chains.

["Gao, Liming" <liming.gao@intel.com>]

If platform decides not to fix them in platform code, they can redefine /D GLOBAL_REMOVE_IF_UNREFERENCED =_declspec(selectany) in [BuildOptions] section of Platform.dsc. 

Thanks
Liming
> -----Original Message-----
> From: Kinney, Michael D
> Sent: Tuesday, March 28, 2017 1:07 AM
> To: Felix Poludov <Felixp@ami.com>; Gao, Liming <liming.gao@intel.com>; edk2-devel@lists.01.org; Kinney, Michael D
> <michael.d.kinney@intel.com>
> Subject: RE: [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply defined symbols, and MSFT/GCC tool chains.
> 
> Felix,
> 
> We need to find all instances of that usage and fix them.
> 
> We can make sure this issue is fixed for all open source packages
> and open source platforms before this change is committed to master.
> 
> I recommend you generate a new version of the patch based on feedback
> from this thread and ask all package and platform owners to verify that
> their packages and platforms build with that change.
> 
> Perhaps give 1-2 weeks for verification so the package and platform
> owners can resolve any issues found.
> 
> Other opinions?
> 
> Thanks,
> 
> Mike
> 
> 
> > -----Original Message-----
> > From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Felix Poludov
> > Sent: Monday, March 27, 2017 9:17 AM
> > To: Kinney, Michael D <michael.d.kinney@intel.com>; Gao, Liming
> > <liming.gao@intel.com>; edk2-devel@lists.01.org
> > Subject: Re: [edk2] [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply defined symbols, and
> > MSFT/GCC tool chains.
> >
> > Mike,
> >
> > I completely agree.
> > As far as code that breaks, the most typical problem I've seen is variable or constant
> > defined in a header file included by more than one C file.
> >
> > Are you going to make these modifications or do you want me to submit a patch?
> >
> > -----Original Message-----
> > From: Kinney, Michael D [mailto:michael.d.kinney@intel.com]
> > Sent: Monday, March 27, 2017 12:07 PM
> > To: Felix Poludov; Gao, Liming; edk2-devel@lists.01.org; Kinney, Michael D
> > Subject: RE: [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply defined symbols, and
> > MSFT/GCC tool chains.
> >
> > Felix,
> >
> > What is the condition that will fail if /Gw is set and GLOBAL_REMOVE_IF_UNREFERENCED
> > is defined to nothing?
> >
> > If these are real bugs, then I think we should identify those bugs and fix them and
> > then apply this strong policy for newer VS compilers.
> >
> > Mike
> >
> > > -----Original Message-----
> > > From: Felix Poludov [mailto:Felixp@ami.com]
> > > Sent: Monday, March 27, 2017 8:59 AM
> > > To: Kinney, Michael D <michael.d.kinney@intel.com>; Gao, Liming
> > > <liming.gao@intel.com>; edk2-devel@lists.01.org
> > > Subject: RE: [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply defined
> > > symbols, and MSFT/GCC tool chains.
> > >
> > > Mike,
> > >
> > > What do you think about defining GLOBAL_REMOVE_IF_UNREFERENCED in the
> > > tool chain definition file as an empty macro for a newer VS compilers?
> > > If this is done, as Liming pointed out, some code that compiles today may break.
> > > If this is not done, variables declared with
> > > GLOBAL_REMOVE_IF_UNREFERENCED are not subject to the default policy of
> > > breaking the build if multiple defined symbols are detected when MSFT tool chain is
> > used.
> > >
> > > -----Original Message-----
> > > From: Kinney, Michael D [mailto:michael.d.kinney@intel.com]
> > > Sent: Monday, March 27, 2017 11:35 AM
> > > To: Felix Poludov; Gao, Liming; edk2-devel@lists.01.org; Kinney,
> > > Michael D
> > > Subject: RE: [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply defined
> > > symbols, and MSFT/GCC tool chains.
> > >
> > > Felix,
> > >
> > > I prefer the default policy to break the build if multiple defined
> > > symbols are detected.
> > >
> > > Exceptions should only be allowed to support a specific compiler or a
> > > specific level of compiler optimizations.
> > >
> > > I do like the addition of the /Gw switch to the newer VS compilers.
> > > Adding the current GLOBAL_REMOVE_IF_UNREFERENCED macro to global
> > > variable declarations is a manual process that usually requires
> > > inspection of PE/COFF images to notice that data that should have been optimized
> > away is still present.
> > >
> > > Adding the #ifndef also looks like a good way to adopt the /Gw switch
> > > in newer VS Compilers and preserve backwards compatibility with older VS compilers.
> > >
> > > Thanks,
> > >
> > > Mike
> > >
> > >
> > > > -----Original Message-----
> > > > From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf
> > > > Of Felix Poludov
> > > > Sent: Monday, March 27, 2017 7:59 AM
> > > > To: Gao, Liming <liming.gao@intel.com>; edk2-devel@lists.01.org
> > > > Subject: Re: [edk2] [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply
> > > > defined symbols, and MSFT/GCC tool chains.
> > > >
> > > > Liming,
> > > >
> > > > Yes surrounding GLOBAL_REMOVE_IF_UNREFERENCED with #ifndef would be an
> > improvement.
> > > > Can you make this change?
> > > >
> > > > On the other note, don't you think that EDKII should have a generic
> > > > policy regarding multiply defined symbols (whether they are allowed or not)?
> > > > Today, they may or may not work depending on the compiler used.
> > > >
> > > > -----Original Message-----
> > > > From: Gao, Liming [mailto:liming.gao@intel.com]
> > > > Sent: Monday, March 27, 2017 12:49 AM
> > > > To: Felix Poludov; edk2-devel@lists.01.org
> > > > Cc: Gao, Liming
> > > > Subject: RE: [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply defined
> > > > symbols, and MSFT/GCC tool chains.
> > > >
> > > > Felix:
> > > >   This changes the default MSFT build behavior. It will impact all
> > > > platforms even if this platform has no requirement to pass GCC build.
> > > > I suggest to update platform DSC to enable it in MSFT tool chain if
> > > > this platform
> > > needs to support MSFT and GCC both.
> > > >
> > > > In Base.h: I agree to define GLOBAL_REMOVE_IF_UNREFERENCED only when
> > > > it is not defined. Then, Platform.dsc can append compiler option /D
> > > > GLOBAL_REMOVE_IF_UNREFERE to the different value in [BuildOptions] section.
> > > >
> > > > #ifndef GLOBAL_REMOVE_IF_UNREFERENCED
> > > >   #define GLOBAL_REMOVE_IF_UNREFERENCED __declspec(selectany) #endif
> > > >
> > > > Thanks
> > > > Liming
> > > > >-----Original Message-----
> > > > >From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf
> > > > >Of Felix Poludov
> > > > >Sent: Friday, March 24, 2017 8:53 PM
> > > > >To: edk2-devel@lists.01.org
> > > > >Subject: [edk2] [RFC] GLOBAL_REMOVE_IF_UNREFERENCED, multiply
> > > > >defined symbols, and MSFT/GCC tool chains.
> > > > >
> > > > >Trying to add GCC support to projects based on MSFT tool chain, I'm
> > > > >keep stumbling into multiply defined symbol errors reported by GCC linker.
> > > > >An attempt to understand why the errors are not reported by the
> > > > >Microsoft linker lead me to GLOBAL_REMOVE_IF_UNREFERENCED macro.
> > > > >The purpose of the macro is to enable link time optimization of
> > > > >global variables.
> > > > >However, the way it's defined for MSFT tool chain
> > > > >(__declspec(selectany) ) has a side effect of explicitly allowing
> > > > >multiple instances of a symbol defined with GLOBAL_REMOVE_IF_UNREFERENCED.
> > > > >For a while usage of the macro was the only option to enable global
> > > > >variable optimization.
> > > > >Starting from VS2013 compiler supports /Gw flag that enables global
> > > > >variable optimization without a special declarator.
> > > > >
> > > > >I propose to make the following modifications:
> > > > >
> > > > >1.      Change GLOBAL_REMOVE_IF_UNREFERENCED definition to an empty
> > > > >macro.
> > > > >
> > > > >Or more specifically, update macro definition in Base.h as follows:
> > > > >
> > > > >#ifndef GLOBAL_REMOVE_IF_UNREFERENCED
> > > > >
> > > > >#define GLOBAL_REMOVE_IF_UNREFERENCED
> > > > >
> > > > >#endif
> > > > >
> > > > >2.      Update VS2013 and VS2015 compiler flags to add /Gw option
> > > > >
> > > > >3.      Update compiler flags for older MSFT tool chains to define
> > > > >GLOBAL_REMOVE_IF_UNREFERENCED in a backward compatible manner for
> > > > >targets that enable optimization.
> > > > >
> > > > >/D GLOBAL_REMOVE_IF_UNREFERENCED =_declspec(selectany)
> > > > >
> > > > >
> > > > >The advantages of these modifications are:
> > > > >
> > > > >-        Better detection of on potential errors by breaking the build when
> > > > >symbol is defined more than once.
> > > > >
> > > > >-        Improved consistency between MSFT and GCC tool chains
> > > > >
> > > > >-        Improved link time optimization with VS2013 and newer MSFT tool chains.
> > > > >
> > > > >For example, mGaugeData in
> > > > >MdeModulePkg/Library/DxeCorePerformanceLib/DxeCorePerformanceLib.c
> > > > >is not declared as GLOBAL_REMOVE_IF_UNREFERENCED, so
> > > > >
> > > > >today performance library is linked with DXE Core even when
> > > > >performance measurements are disabled.
> > > > >
> > > > >The alternative option is to enable support of multiply defined
> > > > >symbols on GCC tool chain.
> > > > >One way to do it is by defining the macro as #define
> > > > >GLOBAL_REMOVE_IF_UNREFERENCED __attribute__((weak))
> > > > >
> > > > >However, I'm not sure that embracing multiple symbol definitions is
> > > > >a good idea.
> > > > >For example, see Ard's arguments in this commit comment
> > > > >https://github.com/tianocore/edk2/commit/214a3b79417f64bf2faae74af4
> > > > >2c
> > > > >1
> > > > >b9d23f50dc8
> > > > >
> > > > >Thanks
> > > > >Felix
> > > > >
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