[llvm-dev] Thoughts on the LLVM linker

Mikael Egevig via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 26 00:34:20 PST 2019

Okay, I understand.  Thanks for your explanation :-)

Den tir. 26. nov. 2019 kl. 08.23 skrev Rui Ueyama <ruiu at google.com>:

> The reason why we have different command line interfaces for different
> platforms is simply because they are actually different. It is not because
> lld is Windows-centric (it's not.)
> The native linkers for these major platforms (e.g. link.exe or
> /usr/bin/ld) are quite different in terms of behavior. It's because native
> linkers have evolved separately in their environment for dozens of years.
> Each linker has evolved their own vocabulary (e.g. command line parameters)
> over the course of their history, sometimes borrowing concepts and command
> line flags from others, but they rarely made dead-copy features. Instead,
> they adjusted concepts to fit to their platform. So even if two command
> line options have the same or very similar names, they are not quite the
> same. So, if you create a unified command line interface, you'd likely end
> up with -unix-foo, -windows-foo, -mac-foo to support option -foo. You
> cannot merge them to option -foo by ignoring subtle differences.
> WebAssembly is quite new, but they are pretty different from other binary
> formats because it is not a native binary format. For example, functions
> don't have addresses in wasm because they live outside of the wasm
> execution environment. As a result, even though wasm-ld borrows many
> options from Unix, it naturally has many wasm-specific command line
> options, and the options don't make sense for non-wasm targets.
> The other problem of an imaginary unified driver is that it allows to use
> impossible combinations of features. Most command line options make sense
> for only one target, but if your linker accepts all options of all
> platforms, you would have to define some reasonable behaviors for all
> unprecedented combinations of features. That is actually a very difficult
> problem to solve. (Of course you can solve the issue by disallowing all
> combinations of command line flags of different targets, but that nullifies
> the point of creating a unified driver.)
> So, yeah, it may look a bit silly to have different command line
> interfaces for all these targets, but I think it technically makes sense.
> On Tue, Nov 26, 2019 at 2:49 PM Mikael Egevig via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Hi,
>> I am using LLVM as the backend in a small hobby compiler project I'm
>> working on.  I plan to support all platforms that LLVM supports (limited by
>> my available hardware only).
>> As far as I can tell, there are different linkers for the various host
>> systems out there (the output of 'lld' if you invoke it on Arch Linux):
>>     lld is a generic driver.
>>     Invoke ld.lld (Unix), ld64.lld (macOS), lld-link (Windows), wasm-ld
>> (WebAssembly) instead
>> I am wondering if there are any plans to make a single, standard,
>> cross-platform link tool so that people who are not locked into the
>> Microsoft ecosystem (Visual Studio and command-line tools) can use the LLVM
>> linker without having to maintain three driver interfaces.  If I am not
>> mistaken, you today have to use three or four different interfaces,
>> depending on the host platform, just to link a few files.
>> The way I see it, a single LLVM-linker with the same options and syntax
>> on all platforms would be highly beneficial to those who use LLVM as their
>> backend through generating .ll files as input to the LLVM tool chain.
>> P.S. I am aware that I can use Clang as a gigantic driver for the linking
>> process, but I am trying to get rid of all Clang dependencies because it is
>> not needed by my project in any way.
>> Thank you,
>> Mikael Egevig
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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