In the textual format, `noduplicates` means no COMDAT/section group
deduplication is performed. Therefore, if both sets of sections are retained, and
they happen to define strong external symbols with the same names,
there will be a duplicate definition linker error.
In PE/COFF, the selection kind lowers to `IMAGE_COMDAT_SELECT_NODUPLICATES`.
The name describes the corollary instead of the immediate semantics. The name
can cause confusion to other binary formats (ELF, wasm) which have implemented/
want to implement the "no deduplication" selection kind. Rename it to be clearer.
Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D106319
In SanitizerCoverage, the metadata sections (`__sancov_guards`,
`__sancov_cntrs`, `__sancov_bools`) are referenced by functions. After
inlining, such a `__sancov_*` section can be referenced by more than one
functions, but its sh_link still refers to the original function's section.
(Note: a SHF_LINK_ORDER section referenced by a section other than its linked-to
section violates the invariant.)
If the original function's section is discarded (e.g. LTO internalization +
`ld.lld --gc-sections`), ld.lld may report a `sh_link points to discarded section` error.
This above reasoning means that `!associated` is not appropriate to be called by
an inlinable function. Non-interposable functions are inline candidates, so we
have to drop `!associated`. A `__sancov_pcs` is not referenced by other sections
but is expected to parallel a metadata section, so we have to make sure the two
sections are retained or discarded at the same time. A section group does the
trick. (Note: we have a module ctor, so `getUniqueModuleId` guarantees to
return a non-empty string, and `GetOrCreateFunctionComdat` guarantees to return
non-null.)
For interposable functions, we could keep using `!associated`, but
LTO can change the linkage to `internal` and allow such functions to be inlinable,
so we have to drop `!associated`, too. To not interfere with section
group resolution, we need to use the `noduplicates` variant (section group flag 0).
(This allows us to get rid of the ModuleID parameter.)
In -fno-pie and -fpie code (mostly dso_local), instrumented interposable
functions have WeakAny/LinkOnceAny linkages, which are rare. So the
section group header overload should be low.
This patch does not change the object file output for COFF (where `!associated` is ignored).
Reviewed By: morehouse, rnk, vitalybuka
Differential Revision: https://reviews.llvm.org/D97430
Arthur Eubanks