[llvm-dev] Wildcard patterns in `--undefined` linker option

Peter Smith via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 11 06:54:38 PDT 2019

On Tue, 11 Jun 2019 at 14:31, Rui Ueyama via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi,
> I got a feature request from an internal customer of lld, but I don't know whether we should implement it or not, so I'd like to get opinions from people on this mailing list.
> The feature request is to allow wildcard patterns in the `--undefined` option. `--undefined foo` (or `-u foo` for short) makes the linker to pull out an object file from a static library if the file defines symbol foo. So, by allowing wildcard patterns, you can pull out all object files defining some JNI symbols (which start with "Java_") from static archives by specifying `-u "Java_*"`, for example.
> This seems mildly useful to me, but it comes with a cost. Currently, `-u` is literally as fast as a single hash lookup. If you allow wildcard patterns in `-u`, you have to attempt a wildcard pattern match against all symbols in the symbol table, which can be expensive.
> I'm also not sure how useful it will actually be. The above JNI case is somewhat convincing, but that's just one use case, and if there's only one use case, adding a new feature for that particular case is probably not a very good idea.
> Does anyone have any opinion on whether we should support this or not?

Arm's proprietary linker supported a similar wildcard feature for the
various section and symbol commands. My opinion was that it was very
useful for a small number of cases. Usually where a customer's set of
symbols to operate on was changing frequently during development and
with a simple naming convention and a wildcard they could save quite a
bit of effort. Their alternative was to either manually maintain the
list of symbols or write a tool to generate it.

We went down the approach of a fast path when there where no wildcards
and a slow path when there was. Aside from the check that there was a
wildcard it wasn't too much extra overhead for the fast path.


> Thanks,
> Rui
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