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

Peter Smith via llvm-dev llvm-dev at lists.llvm.org
Wed Jun 12 01:35:43 PDT 2019

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
>> > _______________________________________________
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>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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