[PATCH] D14382: [ELF2] - Basic implementation of -r/--relocatable

[Rafael Espíndola via llvm-commits]

I think there is enough evidence to say that we will probably have to
implement -r. The remaining question IMHO is when. Since this involves
relocations, my preference would be to do it once we are happy with
the way we handle relocations in a regular link. In particular, we
have to find a clean way of implementing relocation optimizations like
"load from got" -> lea.

Rui, do you have an opinion on it?

Cheers,
Rafael


On 16 February 2016 at 16:20, Ed Maste via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> I'd like to pick up the discussion about -r (relocatable output) support in lld.
>
> On 18 November 2015 at 17:01, Rui Ueyama <ruiu at google.com> wrote:
>>
>> Thanks for the info. I saw some need for -r, and I agree that this is just
>> one case and there would be other use cases in the wild. At the same time, I
>> feel like they do that way since the linker supports that feature, which
>> could have been done in a different way. They could update the code to
>> remove dependency to the -r functionality. (I'm happy to contribute such
>> patch to a project that uses -r.) It is much harder to remove a feature than
>> adding, so I'd want to be conservative now.
>
> I came back to look at this now because I've started trying to build
> and test llvm/clang/lldb/lld linked with lld again. check-llvm fails
> because Bindings/Go/go.test uses -r:
>
> FAIL: LLVM :: Bindings/Go/go.test (2514 of 15889)
> ******************** TEST 'LLVM :: Bindings/Go/go.test' FAILED
> ********************
> [...]
> Command Output (stderr):
> --
> # llvm.org/llvm/bindings/go/llvm
> CC: warning: argument unused during compilation: '-pthread'
> -r option is not supported. Use 'ar' command instead.
> CC: error: linker command failed with exit code 1 (use -v to see invocation)
>
> The "Use 'ar' command instead." suggestion isn't sufficient as there
> are valid uses for -r that are not addressed by ar.
>
> In FreeBSD we have:
> 1. Static libpam, using a linker set to simulate loadable modules
> 2. crunchgen(1) to build the rescue(8) binary
> 3. i386 C startup (csu)
> 4. i386 boot components (kgzldr & btx)
> 5. usr.sbin/uathload (basically using ld -r for objcopy)
>
> ar isn't usable for any of these, and all but #5 rely on ld -r. The
> objcopy case in #5 isn't something we want to support in lld I think.
> #2, #3 and #4 all combine multiple objects into a single .o and rely
> on symbol resolution between those objects to avoid conflicts when
> further linking the output with other objects.
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