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

[Rui Ueyama via llvm-dev]

I created a patch for review to allow wildcard patterns in `-u`.

https://reviews.llvm.org/D63244

On Wed, Jun 12, 2019 at 5:35 PM Peter Smith <peter.smith at linaro.org> wrote:

> On Wed, 12 Jun 2019 at 04:48, Rui Ueyama <ruiu at google.com> wrote:
> >
> > On Tue, Jun 11, 2019 at 10:54 PM Peter Smith <peter.smith at linaro.org>
> wrote:
> >>
> >> 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.
> >
> >
> > Thank you for the info. Does the ARM proprietary linker support `*`? Is
> there any way to escape `*`? I think `*` is not usually used in a symbol
> name, so treating `*` as a metacharacter should be fine, but I'm wondering
> how the ARM linker works.
> >
>
> No it didn't have an escape character. The way the '*' was defined
> (match 0 or more characters) would have matched a symbol name with a *
> in it, but would also have matched more than it should have if the
> intention was to just match the symbol with the '*' character. There
> was an another wildcard '?' that matched any one character which could
> have been used to try and match the '*' character a bit more
> precisely. As you say '*' and '?' are not common in symbol names, I
> don't think I've ever seen it happen in practice so the lack of
> escaping didn't turn out to be a problem.
>
> >> 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.
> >
> >
> > That's true. The usual use case won't be penalized by adding a wildcard
> support.
> >
> >>
> >> Peter
> >>
> >> > Thanks,
> >> > Rui
> >> > _______________________________________________
> >> > LLVM Developers mailing list
> >> > llvm-dev at lists.llvm.org
> >> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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