Backport coverage fixes#108
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Nashenas88 wants to merge 3 commits intorust-lang:rustc/12.0-2021-04-15from
Closed
Backport coverage fixes#108Nashenas88 wants to merge 3 commits intorust-lang:rustc/12.0-2021-04-15from
Nashenas88 wants to merge 3 commits intorust-lang:rustc/12.0-2021-04-15from
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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
To make a kind of metadata section usage work, we want to drop the
`__start_/__stop_ references retain C identifier name sections` rule from LLD (see D96914).
If an application has no `__llvm_prf_data` input section surviving --gc-sections,
LLD will error for undefined hidden `{__start_,__stop_}__llvm_prf_*` from `libclang_rt.profile-*`.
Other `__llvm_prf_*` sections have similar issues.
Making the references weak can address the problem.
This probably enables the opportunity to drop zero size dummy sections in `InstrProfilingPlatformLinux.c`.
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D96936
They were added so that if no metadata section is present, `__start_llvm_prf_*` references would not cause "undefined symbol" errors. By switching to undefined weak symbols in D96936, the dummy sections are not needed. This patch is also needed to work around https://sourceware.org/bugzilla/show_bug.cgi?id=27490 Differential Revision: https://reviews.llvm.org/D97648
Member
Can you clarify which conflicting versions are in play here? |
Member
|
The compiler-rt in Rust right now vs lld @ main. It turns out there are more backports to do so we can enable linker GC with coverage enabled, which helps fix the very high RAM usage. Doing all of them may not be worth it. I'm working on finding out if this change is worth it on its own.. |
Author
|
Got feedback from @tmandry and @petrhosek that this isn't going to be sufficient on its own. Going to close this out for the time being. |
vext01
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Dec 11, 2023
Add pass for splitting blocks after calls.
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Cherry pick of three changes that fix an incompatibility between recent lld versions and compiler-rt runtime used by rust that was causing
ld.lld: error: undefined hidden symbol: __start__llvm_prf_dataerrors.cc @petrhosek @tmandry