MaskRay added a comment.
In D96838#2582965 <https://reviews.llvm.org/D96838#2582965>, @probinson wrote:
>> For ELF, `used` does not retain the entity, regardless of this patch.
>
> That statement does not match my experience.
You probably used C identifier name sections with `__attribute__((used,
section("foo")))`.
The `__start_foo` reference from a live section retains all input sections
`foo`:
https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order#c-identifier-name-sections
> Note that there are two separate variables documented in the LangRef:
> `llvm.used` (which corresponds to source attribute `used` and is documented
> as affecting linker behavior) and `llvm.compiler.used` (which is a rare
> construct with no corresponding source attribute, and does not affect linker
> behavior). So, according to LangRef, source attribute `used` maps to
> `llvm.used` which is documented as affecting linker behavior, and that's
> exactly the behavior I am seeing in my project.
As the description of this patch suggests, `llvm.used`has linker behavior
effects on macOS with all symbols, and Windows with non-local-linkage symbols,
but not effect on ELF.
The behavior you observed is due to `__start_`/`__stop_` references from live
input sections. (D96914 <https://reviews.llvm.org/D96914>
https://lists.llvm.org/pipermail/llvm-dev/2021-February/148682.html)
> But from a user perspective, the IR representation is irrelevant, I don't
> actually care what the description of `llvm.used` says. I put attribute
> `used` on my data, and it is still there in the final executable. That's how
> things have worked for a long time, and I am concerned that you are about to
> break it.
You probably meant the LLD patch D96914 <https://reviews.llvm.org/D96914>. It
does not change the default now.
> Note that I am not using groups or comdats or anything else funky; I'm
> declaring static variables in a custom section, adding attribute `used` and
> the necessary `__start_/__stop_` symbols, and everything works exactly as I
> want.
If you use a non-C-identifier-name section, then `__start_` does not work.
`__attribute__((used))` does not retain it on ELF.
> Maybe there is a different patch that more directly relates to my concern,
> and probably we should be having this conversation on the llvm-dev thread
> instead of in a patch that not many people are paying attention to. I'm
> very happy to move the discussion elsewhere if that would help.
So GCC folks' opinions is that `used` is orthogonal to `retain`. From this
viewpoint, `__attribute__((used))` will be better represented as
`llvm.compiler.used`, but LangRef currently discourages this usage.
I am a bit busy currently, but if you move the discussion to llvm-dev, I'll be
happy to discuss it there.
Repository:
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