[llvm-dev] [lld] Reject some lld specific one-dash long options

[Fangrui Song via llvm-dev]

On 2020-07-29, David Blaikie wrote:
>This email and the review didn't seem to mention a clause included in
>the final commit:
>
>"Some changed options are also used by gold, but I haven't seen their
>one-dash use cases outside of lld's testsuite."
>
>& I think there's a couple of issues with this particular patch, FWIW
>- backwards compatibility with lld is still a thing. Deprecation
>might've been more suitable - giving folks time to opt-out (usually
>it's easier for a build system to add a flag (such as from global
>LD_FLAGS, etc) to turn off warnings temporarily than it is to change
>the flags used inside a build) for any existing spellings. While
>certainly having a policy for all new flags going forward seems great.
>
>But compatibility with gold seems valuable too & wasn't mentioned &
>wasn't in the review - but added post-review. That seems like a
>slightly different discussion (I'd say three groups: New flags,
>existing lld-only flags, existing cross-linker flags (& I guess you've
>split that in two groups - bfd+lld flags and gold+lld flags, which is
>a fair point - gold, I believe, has fewer users than bfd))
>
>(just came across this when I went to use a newer lld and had to
>change my -Wl,-gdb-index to -Wl,--gdb-index)

I'll not interpret gold's support of --gdb-index that way.  The documented form
and the form used in projects is --gdb-index, never -gdb-index.  gold accepting
-gdb-index is merely an accidental implementation detail, partly due to how it
mimicked GNU ld's lax option parsing.
(If you remove some characters from --gdb-index, GNU ld will happily allow
-gdndex. That is the grouped short options syntax.)

The decision not having a deprecation warning step was a practical choice, with
consideration of usage. FWIW I cannot find any occurrence "Wl,-gdb-index". My
employer has a few temporary use cases of -Wl,-lto-*. We fixed it.

Additionally, ld.lld has a suggestion feature of a misspelled option:

   ld.lld: error: unknown argument '-gdb-index', did you mean '--gdb-index'
   ld.lld: error: unknown argument '-thinlto-jobs=1', did you mean '--thinlto-jobs=1'

You felt inconvenienced by the change. I feel sorry about that, but that is it.
I don't think there was/is more action item we need to do.

>On Tue, May 5, 2020 at 4:02 PM Fangrui Song via llvm-dev
><llvm-dev at lists.llvm.org> wrote:
>>
>> GNU ld supports most long options with either one or two dashes.  For
>> compatibility, lld has to support both.
>>
>> For newer and lld specific options, we don't have such compatibility
>> problem.  I'd suggest we reject one-dash long options to avoid collision
>> with short options. For example,
>>
>> * -lto-emit-obj can be read as -l to-emit-obj
>> * -thinlto-cache-dir= means -t -h inlto-cache-dir= in GNU ld
>>
>> I created a patch https://reviews.llvm.org/D79371 to disallow some
>> one-dash long options. These options are carefully chosen: I can't find
>> anything depending on their one-dash forms.
>>
>> lld has a spell corrector for option names so a misspelled -lto-emit-obj can be
>> identified quickly.
>>
>>
>> Thoughts?
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