Re: [edk2-devel] [PATCH] Support GLOBAL_REMOVE_IF_UNREFERENCED in GCC5/11

["Pedro Falcato" <pedro.falcato@gmail.com>]

On Fri, Apr 28, 2023 at 3:47 AM JoeX Lu <pen-chunx.lu@intel.com> wrote:
>
> CC: Michael D Kinney <michael.d.kinney@intel.com>
> CC: Liming Gao <gaoliming@byosoft.com.cn>
> CC: Zhiguang Liu <zhiguang.liu@intel.com>
> Signed-off-by: JoeX Lu <pen-chunx.lu@intel.com>
> ---
>  MdePkg/Include/Base.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/MdePkg/Include/Base.h b/MdePkg/Include/Base.h
> index 6597e441a6..951fce43ee 100644
> --- a/MdePkg/Include/Base.h
> +++ b/MdePkg/Include/Base.h
> @@ -45,8 +45,12 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
>  ///  to it after all compiler and linker optimizations have been performed.
>  ///
>  ///
> +#if defined (__GNUC__)
> +#define GLOBAL_REMOVE_IF_UNREFERENCED __attribute__((unused))

According to GCC and LLVM docs, unused does not affect codegen nor
linking, and only silences unused variable/member/function warnings
(-Wunused).
So this is not the attribute you want. This is only doable using LTO
or static, and LTO already does GLOBAL_REMOVE_IF_UNREFERENCED by
default (to *keep* unreferenced symbols, __attribute__((used)) *does*
work).

Note that it also does the right thing if it's a static variable and
it can verify that the current TU will not use it
(https://godbolt.org/z/GsP1TPb39).

-- 
Pedro