Add "(compatible with the GNU linker)" to the -version output.

[ruiu]

Summary

Previous output:

$ ld.lld -version
LLD 5.0.0

New output:

$ ld.lld -versin
LLD 5.0.0 (compatible with the GNU linker)

[krytarowski]

looks reasonable

[silvas]

Lgtm with a some suggestions that I don't feel too strongly about.

lld/ELF/Driver.cpp

319

Maybe say "linkers" since there is bfd and gold?

[silvas]

Btw, I think this is one of the clearest proofs that LLD has crossed a big milestone and is now entering "production quality". You don't see this kind of hack in a non-production-quality program.

In a sense, this is an admission that LLD is reaching a point where it has an open-ended set of users "in the wild".

It is amazing how far we've come.

[ruiu]

Make it plural. By the way, this patch reminds me why all major
browsers are still claiming that they are "Mozilla/5.0" in their
user-agent strings.

[emaste]

I'll follow up to my Jan 2017 libtool list posting about this issue with a link to this review. I hope that we can get some traction, such that this may be a short-term workaround rather than permanent descent into craziness like browser user agents.

[emaste]

Although I'd rather not do this, I think it is a necessary workaround.

lld/test/ELF/driver.test

20

Suggested by @dim elsewhere, s/the //

[emaste]

It was suggested on the libtool mailing list that --with-gnu-ld to configure scripts might override libtool's tool --version string check and forcibly enable the GNU ld code path. I haven't yet tried this and won't be able to for a few days, but will take a look next week if nobody else does first.

lld/ELF/Driver.cpp

void LinkerDriver::main(ArrayRef<const char *> ArgsArr, bool CanExitEarly) {
   Config->ErrorLimit = getInteger(Args, OPT_error_limit, 20);
 
   // Handle -help
   if (Args.hasArg(OPT_help)) {
     printHelp(ArgsArr[0]);
     return;
   }
 
-  // GNU linkers disagree here. Though both -version and -v are mentioned
-  // in help to print the version information, GNU ld just normally exits,
-  // while gold can continue linking. We are compatible with ld.bfd here.
+  // Handle -v or -version.
+  //
+  // A note about "compatible with the GNU linker" message: this is a hack
+  // for scripts generated by GNU Libtool 2.4.6 (released in February 2014
+  // and still the newest version in March 2017) or earlier to recognize
+  // LLD as a GNU compatible linker. As long as an output for the -v
+  // option contains "GNU" or "BFD", they recognize us as GNU-compatible.
+  //
+  // This is somewhat ugly hack, but in reality, we had no choice other
+  // than doing this. Considering the very long release cycle of Libtool,
+  // it is not easy to improve it to recognize LLD as a GNU compatible
+  // linker in a timely manner. Even if we make it, there are still a lot
+  // of "configure" scripts that are generated by old version of
+  // Libtool. We cannot convince every software developer to migrate to
+  // the latest version and re-generate scripts. So we have this hack.
   if (Args.hasArg(OPT_version) || Args.hasArg(OPT_v))
-    message(getLLDVersion());
+    message(getLLDVersion() + " (compatible with the GNU linker)");
+
+  // ld.bfd always exits after printing out the version string.
+  // ld.gold proceeds if a given option is -v. Because gold's behavior
+  // is more permissive than ld.bfd, we chose what gold does here.
   if (Args.hasArg(OPT_version))
     return;
 
   Config->ExitEarly = CanExitEarly && !Args.hasArg(OPT_full_shutdown);
 
   if (const char *Path = getReproduceOption(Args)) {
     // Note that --reproduce is a debug option so you can ignore it
     // if you are trying to understand the whole picture of the code.

lld/test/ELF/driver.test

 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: not ld.lld %t -o /no/such/file 2>&1 | FileCheck -check-prefix=MISSING %s
 # MISSING: cannot open output file /no/such/file
 
 # RUN: ld.lld --help 2>&1 | FileCheck -check-prefix=HELP %s
 # HELP: USAGE:
 
 # RUN: ld.lld --version 2>&1 | FileCheck -check-prefix=VERSION %s
-# VERSION: LLD
+# VERSION: LLD {{.*}} (compatible with the GNU linker)
 
 # RUN: not ld.lld -v 2>&1 | FileCheck -check-prefix=VERSION %s
 
 ## Attempt to link DSO with -r
 # RUN: ld.lld -shared %t -o %t.so
 # RUN: not ld.lld -r %t.so %t -o %tfail 2>&1 | FileCheck -check-prefix=ERR %s
 # ERR: attempted static link of dynamic object