Re: [PATCH] MdePkg: move to 'hidden' visibility for all symbols under GCC/X64

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

Reviewed-by: Liming Gao <liming.gao@intel.com>

> -----Original Message-----
> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
> Sent: Monday, August 01, 2016 2:57 PM
> To: Shi, Steven <steven.shi@intel.com>; Zhu, Yonghong
> <yonghong.zhu@intel.com>; Gao, Liming <liming.gao@intel.com>; Justen,
> Jordan L <jordan.l.justen@intel.com>; edk2-devel@lists.01.org
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Subject: [PATCH] MdePkg: move to 'hidden' visibility for all symbols under
> GCC/X64
> 
> When using GCC to build for X64, we switched to the position independent
> small code model, which is much more efficient in terms of code generation
> and runtime relocation footprint, and produces binaries that can execute
> correctly from any offset.
> 
> However, the PIC routines are by default geared towards hosted binaries
> containing symbol references that may resolve to definitions in other
> dynamic objects, and for this reason, external symbol references are
> indirected via a GOT entry by default (which also results in a .reloc fixup
> entry) unless we annotate them.
> 
> For this reason, we introduced the 'protected' visibility annotation for
> all symbol definitions and references, by setting the GCC visibility
> pragma. However, as it turns out, this is not sufficient for all versions
> of GCC, and in some cases (GCC 5.x using the GCC49 toolchain tag), may
> still result in GOT based relocations.
> 
> So switch to 'hidden' visibility instead, which is slightly stronger, and
> fixes this issue for the versions of GCC that exhibit the problem.
> 
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  MdePkg/Include/X64/ProcessorBind.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/MdePkg/Include/X64/ProcessorBind.h
> b/MdePkg/Include/X64/ProcessorBind.h
> index a4aad3e524e8..666cc8e8bd16 100644
> --- a/MdePkg/Include/X64/ProcessorBind.h
> +++ b/MdePkg/Include/X64/ProcessorBind.h
> @@ -29,12 +29,12 @@
> 
>  #if defined(__GNUC__) && defined(__pic__)
>  //
> -// Mark all symbol declarations and references as protected, meaning they
> will
> +// Mark all symbol declarations and references as hidden, meaning they will
>  // not be subject to symbol preemption. This allows the compiler to refer to
>  // symbols directly using relative references rather than via the GOT, which
>  // contains absolute symbol addresses that are subject to runtime relocation.
>  //
> -#pragma GCC visibility push (protected)
> +#pragma GCC visibility push (hidden)
>  #endif
> 
>  #if defined(__INTEL_COMPILER)
> --
> 2.7.4