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[profile] Add __attribute__((used)) to zero size dummy sections

Authored by MaskRay on Feb 17 2021, 7:22 PM.

Description

[profile] Add attribute((used)) to zero size dummy sections

D14468 added these dummy sections. This patch adds __attribute__((used)) so
that when compiled by GCC>=11 or (expected, D96838) Clang>=13 on some ELF platforms,
these sections will get SHF_GNU_RETAIN to make sure they will not be discarded
by ld --gc-sections.

We are trying to get rid of LLD's "start_/stop_ references retain C identifier name sections" rule.
If LLD drops the rule in the future (we will retain compatibility for __llvm_prf_* for a while),
__llvm_prf_* will need to have the SHF_GNU_RETAIN flag, otherwise:

// __llvm_prf_cnts/__llvm_prf_data usually exist, but {names,vnds} may not exist.
// Such diagnostics will happen with {cnts,data} as well if no input object file is instrumented.
% clang++ -fprofile-generate a.cc -fuse-ld=lld -Wl,--gc-sections
ld.lld: error: undefined hidden symbol: __start___llvm_prf_names
>>> referenced by InstrProfilingPlatformLinux.c
>>>               InstrProfilingPlatformLinux.c.o:(__llvm_profile_begin_names) in archive /tmp/RelA/lib/clang/13.0.0/lib/linux/libclang_rt.profile-x86_64.a
...

Differential Revision: https://reviews.llvm.org/D96902

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