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

["Kinney, Michael D" <michael.d.kinney@intel.com>]

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/214a3b79417f64bf2faae74af42c1
> >b9d23f50dc8
> >
> >Thanks
> >Felix
> >
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