[PATCH] Support ld.mold linker.

[Richard Biener]

On Mon, Jan 3, 2022 at 4:23 PM Martin Liška <mliska@suse.cz> wrote:
>
> On 1/3/22 15:48, Richard Biener wrote:
> > On Tue, Dec 28, 2021 at 2:10 PM Martin Liška <mliska@suse.cz> wrote:
> >>
> >> Hello.
> >>
> >> The mold linker is getting quite popular and I think we should support it:
> >> https://github.com/rui314/mold
> >
> > Does it support the gold plugin API/ABI and thus proper LTO?
>
> No, but it's planned to be added:
> https://github.com/rui314/mold/issues/181
>
> > If not
> > I'm not sure we should encourage use.   For example using
> > -flto -fuse-ld=lld will report strange
> >
> > ld.lld: error: undefined symbol: main
> >>>> referenced by start.S:104 (../sysdeps/x86_64/start.S:104)
> >>>>                /usr/lib/../lib64/crt1.o:(_start)
> > collect2: error: ld returned 1 exit status
> >
> > on a simple test with a main() unless I manually add -fno-use-linker-plugin.
>
> Yep, that's consequence of the fact a compiler is not built with such linker.
> Maybe we can add a documentation note, what do you think?

Since we have a fixed set of supported linkers and we know their implementation
status with respect to linker plugin support we can maybe auto-add
-fno-use-linker-plugin
(via specs?) when -fuse-ld=lld or -fuse-ld=mold is used?  But yes,
adding a note to
the documentation that for lld and mold LTO support is severely
restricted compared to
GNU ld or gold on platforms that support the linker plugin and thus
their use is discouraged
there might be a good idea.

Richard.

>
> Cheers,
> Martin
>
> >
> > Richard.
> >
> >> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> >>
> >> Ready to be installed?
> >> Thanks,
> >> Martin
> >>
> >> ---
> >>    gcc/collect2.c | 10 +++++++---
> >>    gcc/common.opt |  4 ++++
> >>    gcc/gcc.c      |  4 ++++
> >>    gcc/opts.c     |  1 +
> >>    4 files changed, 16 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/gcc/collect2.c b/gcc/collect2.c
> >> index d47fe3f9195..b322527847c 100644
> >> --- a/gcc/collect2.c
> >> +++ b/gcc/collect2.c
> >> @@ -776,6 +776,7 @@ main (int argc, char **argv)
> >>          USE_GOLD_LD,
> >>          USE_BFD_LD,
> >>          USE_LLD_LD,
> >> +      USE_MOLD_LD,
> >>          USE_LD_MAX
> >>        } selected_linker = USE_DEFAULT_LD;
> >>      static const char *const ld_suffixes[USE_LD_MAX] =
> >> @@ -784,7 +785,8 @@ main (int argc, char **argv)
> >>          PLUGIN_LD_SUFFIX,
> >>          "ld.gold",
> >>          "ld.bfd",
> >> -      "ld.lld"
> >> +      "ld.lld",
> >> +      "ld.mold"
> >>        };
> >>      static const char *const real_ld_suffix = "real-ld";
> >>      static const char *const collect_ld_suffix = "collect-ld";
> >> @@ -957,6 +959,8 @@ main (int argc, char **argv)
> >>            selected_linker = USE_GOLD_LD;
> >>          else if (strcmp (argv[i], "-fuse-ld=lld") == 0)
> >>            selected_linker = USE_LLD_LD;
> >> +       else if (strcmp (argv[i], "-fuse-ld=mold") == 0)
> >> +         selected_linker = USE_MOLD_LD;
> >>          else if (startswith (argv[i], "-o"))
> >>            {
> >>              /* Parse the output filename if it's given so that we can make
> >> @@ -1048,7 +1052,7 @@ main (int argc, char **argv)
> >>      ld_file_name = 0;
> >>    #ifdef DEFAULT_LINKER
> >>      if (selected_linker == USE_BFD_LD || selected_linker == USE_GOLD_LD ||
> >> -      selected_linker == USE_LLD_LD)
> >> +      selected_linker == USE_LLD_LD || selected_linker == USE_MOLD_LD)
> >>        {
> >>          char *linker_name;
> >>    # ifdef HOST_EXECUTABLE_SUFFIX
> >> @@ -1283,7 +1287,7 @@ main (int argc, char **argv)
> >>                else if (!use_collect_ld
> >>                         && startswith (arg, "-fuse-ld="))
> >>                  {
> >> -                 /* Do not pass -fuse-ld={bfd|gold|lld} to the linker. */
> >> +                 /* Do not pass -fuse-ld={bfd|gold|lld|mold} to the linker. */
> >>                    ld1--;
> >>                    ld2--;
> >>                  }
> >> diff --git a/gcc/common.opt b/gcc/common.opt
> >> index 2ed818d6057..dba3fa886f9 100644
> >> --- a/gcc/common.opt
> >> +++ b/gcc/common.opt
> >> @@ -3046,6 +3046,10 @@ fuse-ld=lld
> >>    Common Driver Negative(fuse-ld=lld)
> >>    Use the lld LLVM linker instead of the default linker.
> >>
> >> +fuse-ld=mold
> >> +Common Driver Negative(fuse-ld=mold)
> >> +Use the Modern linker (MOLD) linker instead of the default linker.
> >> +
> >>    fuse-linker-plugin
> >>    Common Undocumented Var(flag_use_linker_plugin)
> >>
> >> diff --git a/gcc/gcc.c b/gcc/gcc.c
> >> index b75b50b87b2..06e18a75b52 100644
> >> --- a/gcc/gcc.c
> >> +++ b/gcc/gcc.c
> >> @@ -4282,6 +4282,10 @@ driver_handle_option (struct gcc_options *opts,
> >>           use_ld = ".gold";
> >>           break;
> >>
> >> +    case OPT_fuse_ld_mold:
> >> +       use_ld = ".mold";
> >> +       break;
> >> +
> >>        case OPT_fcompare_debug_second:
> >>          compare_debug_second = 1;
> >>          break;
> >> diff --git a/gcc/opts.c b/gcc/opts.c
> >> index cdd6463e49b..60f1cf045c9 100644
> >> --- a/gcc/opts.c
> >> +++ b/gcc/opts.c
> >> @@ -3094,6 +3094,7 @@ common_handle_option (struct gcc_options *opts,
> >>        case OPT_fuse_ld_bfd:
> >>        case OPT_fuse_ld_gold:
> >>        case OPT_fuse_ld_lld:
> >> +    case OPT_fuse_ld_mold:
> >>        case OPT_fuse_linker_plugin:
> >>          /* No-op. Used by the driver and passed to us because it starts with f.*/
> >>          break;
> >> --
> >> 2.34.1
> >>
>