[llvm-dev] Binary utilities: switch command line parsing from llvm::cl to OptTable (byproduct: drop -long-option?)

[David Blaikie via llvm-dev]

On Fri, Jul 2, 2021 at 10:15 AM Fāng-ruì Sòng via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> llvm/tools/ include some binary utilities used as replacement for GNU
> binutils, e.g. llvm-objcopy, llvm-symbolizer, llvm-nm.
> In some old threads people discussed some drawbacks of using cl::opt for
> user-facing utilities (I cannot find them now).
>

Would be good to describe some of the known drawbacks/expected benefits.

One potential one (though I don't recall it being discussed recently) would
be that maybe this addresses the issue of global ctors in cl::opt? Does
OptTable avoid/not use global constructors? That would be nice - it's an
ongoing issue that LLVM library users pay for command line argument support
they have no need for in the form of global ctor execution time.


> Switching to OptTable is an appealing solution. I have prepared two
> patches for two binary utilities: llvm-nm and llvm-strings.
>
> * llvm-strings https://reviews.llvm.org/D104889
> * llvm-nm https://reviews.llvm.org/D105330
>
> llvm-symbolizer was switched last year. llvm-objdump was switched by
> thakis earlier this year.
>
> The switch can fix some corners with lib/Support/CommandLine.cpp. Here is
> a summary:
>
> * -t=d is removed (equal sign after a short option). Use -t d instead.
> * --demangle=0 (=0 to disable a boolean option) is removed. Omit the
> option or use --no-demangle instead.
> * To support boolean options (e.g. --demangle --no-demangle), we don't
> need to compare their positions (if (NoDemangle.getPosition() >
> Demangle.getPosition()) , see llvm-nm.cpp)
> * grouped short options can be specified with one line
> `setGroupedShortOptions`, instead of adding cl::Grouping to every short
> options.
> * We don't need to add cl::cat to every option and call
> `HideUnrelatedOptions` to hide unrelated options from --help. The issue
> would happen with cl::opt tools if linker garbage collection is disabled or
> libLLVM-13git.so is used. (See https://reviews.llvm.org/D104363)
> * If we decide to support binary utility multiplexting (
> https://reviews.llvm.org/D104686), we will not get conflicting options.
> An option may have different meanings in different utilities (especially
> for one-letter options).
>
> *I expect that most users will not observe any difference.*
>
> There is a related topic whether we should disallow the single-dash
> `-long-option` form.
>
(Discussed in 2019:
> https://lists.llvm.org/pipermail/llvm-dev/2019-April/131786.html Accept
> --long-option but not -long-option for llvm binary utilities)
> *I'd like to disallow -long-option but may want to do this in a separate
> change.*
>

I'd say definitely do this as a separate change. I expect there'd be a long
tail of users after this change ships in an LLVM release, etc, such that we
may want to undo some amount of it a long time after the change is made.


>
> The main point is that (1) grouped short options have syntax conflict with
> one-dash long options. (2) the GNU getopt_long style two-dash long option
> is much more popular.
>
> I can think of potential pushback for some Mach-O specific options, e.g.
> nm -arch
> http://www.manpagez.com/man/1/nm/osx-10.12.6.php says `-arch` has one
> dash.
> If such options may have problems, we can keep supporting one dash forms.
> With OptTable, allowing one-dash forms for a specific option is easy.
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