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

[Felix Poludov <Felixp@ami.com>]

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/214a3b79417f64bf2faae74af42c1b9d23f50dc8

Thanks
Felix

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