GCC 12 Adds Support For Using The Mold Linker

[cl333r]

No. GNU ld AKA the BFD linker AKA ld.bfd is different from the Gold linker AKA ld.gold. An operating system typically has an ld symlink to the linker (.e.g. ld.bfd, ld.gold, ld.lld. ld.mold, etc.)

The /usr/bin/ld symlink chain on my Ubuntu PC:

Screenshot_20211230_044415.png
So, ld => x86_64-linux-gnu-ld => x86_64-linux-gnu-ld.bfd

PS: is there any phoronix option to avoid the downsizing the uploaded image like this?

[brad0]

The /usr/bin/ld symlink chain on my Ubuntu PC:

Screenshot_20211230_044415.png
So, ld => x86_64-linux-gnu-ld => x86_64-linux-gnu-ld.bfd

PS: is there any phoronix option to avoid the downsizing the uploaded image like this?

Ya, the odd ball naming of the linker binary is because unlike lld the GNU linker only handles one arch at a time. So the naming is such so they'll co-exist and people might have other linkers for cross-compilation. There are a number of legacy design decisions with GCC and binutils regarding arch handling and cross-compilation that makes it ugly and a mess.

[CommunityMember]

Does the use of different linkers influence the performance of the resulting binary when not using LTO?

Primarily the difference is link speed, which can matter a lot for CI functionality(*), and nearly nothing for the users of the resulting binary. There are, of course, edge cases where two linkers that happen to combine certain things in differnet orders may influence the performance on specific processor implementations, but those cases tend to be in the noise.

(*) And for some projects, that is a lot of linking (every commit is built and tested across dozens of platforms), which can use lots of resources even before first release/deployment, and delay feedback to developers (and developers hate waiting for feedback, except for those wanting to slack off and do some chair sword play: https://xkcd.com/303/ )

[ptr1337]

Here is a patch, which works for GCC 11, for those who want to use a stable GCC.

--- gcc/collect2.c
+++ 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 (strncmp (argv[i], "-o", 2) == 0)
      {
        /* 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--;
         }
--- gcc/common.opt
+++ 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)
 

--- gcc/gcc.c
+++ 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;
--- gcc/opts.c
+++ 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;

[PerformanceExpert]

Primarily the difference is link speed, which can matter a lot for CI functionality(*), and nearly nothing for the users of the resulting binary. There are, of course, edge cases where two linkers that happen to combine certain things in differnet orders may influence the performance on specific processor implementations, but those cases tend to be in the noise

Fast linking for CI and development is extremely useful, but you still want the binary to be as optimized as possible. Linkers perform many optimizations, for example compressing dynamic relocations, relaxing relocations into simpler ones (eg. remove a GOT indirection). These optimizations can make significant improvements in binary size, load time and run time. Linkers also insert various veneers to extend branches that are out of range on many ISAs and even scan code for and workaround CPU bugs (I laugh when people bring up the "smart format, dumb linker" thing, that's so wrong...). So the question is whether this new linker supports all that, or whether it is fast by removing all these optimizations and features?

[khanich]

So the question is whether this new linker supports all that, or whether it is fast by removing all these optimizations and features?

From what I know about mold, it's mostly better parallelization

[F.Ultra]

Is there a runtime performance difference or is it all buildtime?

build time only

[geearf]

build time only

Thank you!

[linuxgeex]

Is there a runtime performance difference or is it all buildtime?

I haven't seen comparison benchmarks of post-linker performance, so I think any answer you get on this will be premature. However, you can be pretty sure that the resulting binary will be slower than a BOLT or LTO binary built with LDD, in the ballpark of 2-14% slower.

Maybe if enough people clamour for it, phoronix will do a article.

[geearf]

I haven't seen comparison benchmarks of post-linker performance, so I think any answer you get on this will be premature. However, you can be pretty sure that the resulting binary will be slower than a BOLT or LTO binary built with LDD, in the ballpark of 2-14% slower.

Maybe if enough people clamour for it, phoronix will do a article.

Fair point, thanks for checking!
Does this linker not support LTO?