Re: Does __attribute__ ((selectany)) make sense now for GCC?

[Andrew Fish <afish@apple.com>]

Tim,

Looks like that is only available on Windows versions of GCC, and is more about compatible behavior. 

selectany
The selectany attribute causes an initialized global variable to have link-once semantics. When multiple definitions of the variable are encountered by the linker, the first is selected and the remainder are discarded. Following usage by the Microsoft compiler, the linker is told not to warn about size or content differences of the multiple definitions.
Although the primary usage of this attribute is for POD types, the attribute can also be applied to global C++ objects that are initialized by a constructor. In this case, the static initialization and destruction code for the object is emitted in each translation defining the object, but the calls to the constructor and destructor are protected by a link-once guard variable.

The selectany attribute is only available on Microsoft Windows targets. You can use __declspec (selectany) as a synonym for __attribute__ ((selectany)) for compatibility with other compilers.


What I've noticed with clang/LLVM is the unreferenced globals get removed when you enable link time optimizations. 

I'd actually ask the opposite question. Does __declspec(selectany) impact dead code removal on current versions of VC++? 

Thanks,

Andrew Fish

> On May 18, 2018, at 3:22 PM, Tim Lewis <tim.lewis@insyde.com> wrote:
> 
> In Visual Studio we have __declspec(selectany) to limit the impact of unused
> data.
> 
> 
> 
> I see that GCC for Windows has __attribute__ ((selectany)).
> 
> 
> 
> Should we me using this for GLOBAL_REMOVE_IF_UNREFERENCED in
> MdePkg\Include\Base.h?
> 
> 
> 
> Tim
> 
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