Clang: Add support for -fuse-ld= in the mingw driver

[martell]

Summary

We will still default to ld until such a time lld become a stable release
lld supports arm NT under the machine name "thumb2pe"

[martell]

lib/Driver/Tools.cpp

9034

lld is much better than gnu at linking libraries.
It searches previously passed libs to find missing functions
This is a flaw in binutils that lld have overcome for PECOFF

[rnk]

This uses the old LLD COFF port, which Rui has rewritten in lld-link2. The new linker is way faster, but 32-bit x86 support isn't ready yet and it doesn't understand the GNU linker command line arguments. At the end of the day, we'll want to use the new linker, since it's faster, supports LTO, and new development and bugfixes are happening there.

Give that this will be temporary, do you want to use the old COFF port in the mingw toolchain for now, or try to use the new one?

[martell]

In D11088#202752, @rnk wrote:

This uses the old LLD COFF port, which Rui has rewritten in lld-link2. The new linker is way faster, but 32-bit x86 support isn't ready yet and it doesn't understand the GNU linker command line arguments. At the end of the day, we'll want to use the new linker, since it's faster, supports LTO, and new development and bugfixes are happening there.

Give that this will be temporary, do you want to use the old COFF port in the mingw toolchain for now, or try to use the new one?

Hi rnk :)

Yes I would like to use the old linker for now as I have prepared a patch set for that to work with gnu mode.
Also the old COFF linker supports Windows on ARM which I think the new one is lacking .

I don't know what way Rui wants to do this but
I think when the new COFF linker supports everything the old one did maybe he can switch the names from within lld itself so that it becomes the default.
Rather than updating clang code to specify the new linker.

[martell]

Just a note.

Technically this patch doesn't actually specify which coff port to use because it specifies "gnu" mode
lld will just error out with unknown emulation mode atm for i386pe i386pep and thumb2pe.
This would be down to us from within lld to specify which one is actually used for each mode

We could if we wanted from within lld use the old coff for arm and the new one for x86 and x86_64

[rnk]

lgtm

[martell]

ping!
can someone commit this please? :)

[yaron.keren]

r242121

lib/Driver/Tools.cpp

void MinGW::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   // Silence warning for "clang -g foo.o -o foo"
   Args.ClaimAllArgs(options::OPT_g_Group);
   // and "clang -emit-llvm foo.o -o foo"
   Args.ClaimAllArgs(options::OPT_emit_llvm);
   // and for "clang -w foo.o -o foo". Other warning options are already
   // handled somewhere else.
   Args.ClaimAllArgs(options::OPT_w);
 
+  StringRef Linker = Args.getLastArgValue(options::OPT_fuse_ld_EQ, "ld");
+  if (Linker.equals_lower("lld")) {
+    CmdArgs.push_back("-flavor");
+    CmdArgs.push_back("gnu");
+  }
+
   if (!D.SysRoot.empty())
     CmdArgs.push_back(Args.MakeArgString("--sysroot=" + D.SysRoot));
 
   if (Args.hasArg(options::OPT_s))
     CmdArgs.push_back("-s");
 
   CmdArgs.push_back("-m");
   if (TC.getArch() == llvm::Triple::x86)
     CmdArgs.push_back("i386pe");
   if (TC.getArch() == llvm::Triple::x86_64)
     CmdArgs.push_back("i386pep");
+  if (TC.getArch() == llvm::Triple::arm)
+    CmdArgs.push_back("thumb2pe");
 
   if (Args.hasArg(options::OPT_mwindows)) {
     CmdArgs.push_back("--subsystem");
     CmdArgs.push_back("windows");
   } else if (Args.hasArg(options::OPT_mconsole)) {
     CmdArgs.push_back("--subsystem");
     CmdArgs.push_back("console");
   }
if (!Args.hasArg(options::OPT_nodefaultlibs)) {
       }
       CmdArgs.push_back("-ladvapi32");
       CmdArgs.push_back("-lshell32");
       CmdArgs.push_back("-luser32");
       CmdArgs.push_back("-lkernel32");
 
       if (Args.hasArg(options::OPT_static))
         CmdArgs.push_back("--end-group");
-      else
+      else if (!Linker.equals_lower("lld"))
         AddLibGCC(Args, CmdArgs);
     }
 
     if (!Args.hasArg(options::OPT_nostartfiles)) {
       // Add crtfastmath.o if available and fast math is enabled.
       TC.AddFastMathRuntimeIfAvailable(Args, CmdArgs);
 
       CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crtend.o")));
     }
   }
-  const char *Exec = Args.MakeArgString(TC.GetProgramPath("ld"));
+  const char *Exec = Args.MakeArgString(TC.GetProgramPath(Linker.data()));
   C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs));
 }
 
 /// XCore Tools
 // We pass assemble and link construction to the xcc tool.
 
 void XCore::Assembler::ConstructJob(Compilation &C, const JobAction &JA,
                                     const InputInfo &Output,