[llvm-dev] Embedding LLD version to executables

[Mehdi Amini via llvm-dev]

> On Oct 18, 2016, at 8:30 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> ----- Original Message -----
>> From: "Mehdi Amini via llvm-dev" <llvm-dev at lists.llvm.org>
>> To: "Rui Ueyama" <ruiu at google.com>
>> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
>> Sent: Tuesday, October 18, 2016 10:22:00 PM
>> Subject: Re: [llvm-dev] Embedding LLD version to executables
>> 
>> 
>>> On Oct 18, 2016, at 6:44 PM, Rui Ueyama via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> 
>>> I'd like to make LLD embed version information so that we can
>>> determine if an executable was created by LLD and if that's the
>>> case which version of LLD.
>> 
>> Pardon my ignorance, but what’s the motivation?
>> 
>> We don’t embed the clang version in every binary clang generates for
>> instance.
> 
> We do. Clang outputs an "ident" comment with its version information, and that ends up in the object files (at least on Linux).

Interesting, we don’t on Darwin:

$ echo "int main() {}" | clang -x c -o - - -S  | grep clang
$ echo "int main() {}" | clang -x c -o - - -S -target x86_64-pc-linux-gnu | grep clang
	.ident	"Apple LLVM version 8.0.0 (clang-800.0.42.1)”

Dos it show up in the final binary? If yes, how does it behave when you mix-and-match versions in different .o?

Also I’m still not sure why doing this?

— 
Mehdi

> 
>> 
>>> 
>>> ld.bfd doesn't seem to embed any information, so we cannot tell
>>> whether an executable was linked by ld.bfd or not easily.
>>> 
>>> ld.gold embeds a string "GNU gold <version>" as
>>> ".note.gnu.gold-version" section contents.
>>> 
>>> I'm wondering what we should do for LLD. The gold's way seems
>>> almost right, but I think we don't want to use the same section
>>> name because it contains "gold" as part of the section name.
>>> However, at the same time, I don't believe we want to create
>>> ".note.gnu.lld-version", because if we do, all programs that
>>> determine linker version need to look at
>>> ".note.gnu.<linker-name>-version" for all known linkers. That's
>>> absurd. (Also, our product is not GNU, so ".gnu" part is probably
>>> irrelevant.)
>>> 
>>> I'm leaning towards ".note.linker-version" in hope that other
>>> linkers follow it.
>> 
>> At least that would look much better than a .gnu.xxxx
> 
> I agree.
> 
> -Hal
> 
>> 
>> —
>> Mehdi
>> 
>> 
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>> 
> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory

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