[cfe-dev] [RFC] Recognize -fuse-ld={bfd, gold, lld} but don't prepend "ld." or "ld64." for other values

[David Blaikie via cfe-dev]

On Wed, May 20, 2020 at 12:10 PM Fangrui Song via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> In https://reviews.llvm.org/D80225 I intended to update the semantics of
> -fuse-ld=
> Some context (from my archaeology) about the option
>
> * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55470 added support for
> -fuse-ld=bfd to mean ld.bfd and -fuse-ld=gold to mean ld.gold
> * rL194328 ported the feature and made -fuse-ld= available for other
> values (with the ld. prefix)
> * D78290 changed the prefix to ld64. on Darwin.
> * GCC added support for -fuse-ld=lld. No other value than bfd,gold,lld is
> allowed (so our
>    behavior change here will not be incompatible with GCC).
>
> Our existing behavior is cumbersome:
>
> * Inconsistency between an absolute
>    path (which does not get a ld.  prefix) and a relative path (which gets
>    a ld. prefix) For a relative path, -fuse-ld=dir/foo currently tries to
>    access x86_64-unknown-linux-gnu-ld.dir/foo (if
>    LLVM_DEFAULT_TARGET_TRIPLE is x86_64-unknown-linux-gnu).
> * lld's new Mach-O port needs to pick an executable name. It would be
>    nice if -fuse-ld= simply accepts a program name, not necessarily
> prefixed by `ld64.`
>

To clarify (it seems like the link between "Mach-O lld needs to pick an
executable name" and "it would be nice if -fuse-ld accepts a program name"
isn't quite spelled out): By making this change to -fuse-ld it would enable
the Mach-O version of lld to be called "lld" rather than "ld64.lld"? Is
that the goal?

Is Mach-O lld likely to need different command line compatibility than ELF
or COFF lld? It seems like having a distinct name to enable detection of
what command line support/object file format assumptions/other behavior
changes compared to ELF/COFF may be useful, and "ld64.lld" is probably as
good a name as any, if that functionality is desirable?


>
> I suggest we just hard code the currently used values which intend to get
> a prefix: bfd, gold, lld. For all other values, don't add a prefix.
> (PS4 seems to use -fuse-ld=ps4 but it overrides ToolChain::GetLinkerPath,
> thus unaffected)
>
> Thoughts?
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