[llvm-dev] [lld] Flavour option purpose

N via llvm-dev
Mon, 13 Nov 2017 12:31:45 -0000

> For example, MSVC link.exe doesn't have --start-group and --end-group
options because their symbol resolution semantics are different than Unix.

Yes, but this is specific for a linker, not for a target. For example, GNU ld
supports this option for all binary formats. A linker with universal CLI would
then
support this flag for all targets as well (or drop the support for all targets
if recursive dependency resolution can be implemented without that flag).

> link.exe on the other hand doesn't have --init or --fini options
because that's ELF-only concept.

Yes, so a universal CLI would hide this flag when linking non-ELF binaries, as
well as any other option not supported by the target currently linking for.
(This is not the same as using a separate flavour for that target.)

> Linker scripts exist only in GNU.


My reasoning is the same as for `start-group'. In fact, lld does not achieve
feature parity with GNU ld configured for mach-o here:
the latter can use scripts when linking a mach-o binary, but lld cannot use
scripts at all unless GNU flavor is used, and then mach-o is
unsupported.

> Also, I think if we try to create a unified interface, I guess it would end
up with this situation -- https://xkcd.com/927/. We have three standards -- GNU,
Darwin and MSVC. I don't know if we want another one.

This depends on whether the flavoured CLIs are removed upon merge. (Of course,
it is probably impractical to remove them all at once.)

In my opinion, GNU ld has CLI closest to my description, so it is a good basis
for a universal interface. In fact, GNU flavour can be universal if support
for more targets is added there, but then the other flavours will likely be
redundant. One difference would be that a universal CLI need not maintain
compatibility
with GNU ld CLI.

> On 1 Nov 2017, at 20:36, Rui Ueyama <ruiu at google.com> wrote:
>
> Most command line options in GNU, macOS and MSVC are not just different in
terms of notion but different in terms of semantics. For example, MSVC link.exe
doesn't have --start-group and --end-group options because their symbol
resolution semantics are different than Unix. link.exe on the other hand
doesn't have --init or --fini options because that's ELF-only concept.
Linker scripts exist only in GNU. You cannot hide these differences even if you
create a unified driver, which makes a unified driver less attractive to me.
>
> Also, I think if we try to create a unified interface, I guess it would end
up with this situation -- https://xkcd.com/927/. We have three standards -- GNU,
Darwin and MSVC. I don't know if we want another one.
>
> On Wed, Nov 1, 2017 at 12:16 PM, N via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> I am aware of these flags, but this is not my point. I am talking about
Darwin and Link lld drivers
> not being on par with the GNU driver. While using clang for linking helps,
as it provides some flags
> to the linker, as soon as you want to do something clang does not
automatically do, you still have
> the issue of different flags for different drivers/targets.
>
> > On 1 Nov 2017, at 20:08, Jonathan Roelofs <jonathan at
codesourcery.com> wrote:
> >
> >
> >
> > On 11/1/17 10:29 AM, N via llvm-dev wrote:
> >> Thank you for your answers.
> >> Indeed, Clang is universal, but only until one needs to pass some
commands to the linker which are not
> >> automatically passed by the driver. Then we are back to the same
problem.
> >
> > Clang and GCC both support the `-Wl,` and `-Xlinker` flags...
> >
> >
> > Jon
> >
> >> In my opinion, it would be great if lld had a unified, GNU-like
interface. Of course, if nobody else is interested,
> >> it's probably not even worth it drafting an RFC. :(
> >>> On 27 Oct 2017, at 01:09, James Y Knight <jyknight at
google.com> wrote:
> >>>
> >>> I mean, there is such a universal driver -- it's called
"clang".
> >>>
> >>>
> >>> On Oct 26, 2017 5:31 PM, "Rui Ueyama via llvm-dev"
<llvm-dev at lists.llvm.org> wrote:
> >>> Martell recently added "ld64.lld" as a name for the
Darwin driver.
> >>>
> >>> As to why there's no driver that provides a unified
command line arguments, I can't speak for other people. But no one seems to
have been interested in it enough to actually invent and implement a set of
unified command line arguments.
> >>>
> >>> On Thu, Oct 26, 2017 at 2:20 PM, N <scandium at me.com>
wrote:
> >>>> I think using ld.lld or lld-link is preferred way over
"lld -flavor gnu" or "lld -flavor link".
> >>>
> >>> -flavor seems to be still actively used in Clang… By the the
way, there seems to be no special
> >>> command name for Darwin targets, so ld.lld (incorrectly)
invokes the ELF linker there
> >>> (see https://bugs.llvm.org/show_bug.cgi?id=34792#c1)
> >>>
> >>> Regardless, I would be interested in hearing the answer to the
rest of the questions.
> >>>
> >>>> On 26 Oct 2017, at 23:10, Rui Ueyama <ruiu at
google.com> wrote:
> >>>>
> >>>> The -flavor option exists mostly for historical reasons. I
think using ld.lld or lld-link is preferred way over "lld -flavor gnu"
or "lld -flavor link".
> >>>>
> >>>> On Thu, Oct 26, 2017 at 2:06 PM, N via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> >>>> Hi all,
> >>>>
> >>>> According to lld/docs/Driver.rst, Flavor command line
option determines the style of lld command-line interface when invoked.
> >>>>
> >>>> However, it looks like this option also determines the set
of supported targets we are linking for. For example, lld -flavor gnu
> >>>> cannot link mach-o binaries, and could not link PE
binaries either (well, not until rL312926).
> >>>>
> >>>> Is this really intended by the design of lld? It looks the
flavours are merely legacy compatibility shims, but then why is there no
> >>>> universal lld driver that is able to link binary for any
platform using a unified CLI?
> >>>> _______________________________________________
> >>>> LLVM Developers mailing list
> >>>> llvm-dev at lists.llvm.org
> >>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>>
> >>>
> >>>
> >>>
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> llvm-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>
> >>>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> > --
> > Jon Roelofs
> > jonathan at codesourcery.com
> > CodeSourcery / Mentor Embedded / Siemens
>
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