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

["Tim Lewis" <tim.lewis@insyde.com>]

Andrew -

 

I know for Visual Studio it does the same thing for data that /Gy does for
code.

 

That is, if the data is unreferenced, then it doesn't get brought it.

 

Maybe link-time code generation has the same effect. That's what I'm curious
about. 

 

Tim

 

From: afish@apple.com <afish@apple.com> 
Sent: Friday, May 18, 2018 3:39 PM
To: Tim Lewis <tim.lewis@insyde.com>
Cc: edk2-devel@lists.01.org
Subject: Re: [edk2] Does __attribute__ ((selectany)) make sense now for GCC?

 

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
<mailto: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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