[clang] [MinGW] Fix libunwind extension

[mstorsjo]

So, using -l:libunwind.dll.a is definitely more correct than -l:libunwind.so on this platform, so in that sense this is good.

In my toolchains I don't use this option at all so far, because I have libunwind bundled into libc++.a/libc++.dll.a, but I guess it would be nice to move towards this setup and stop bundling them. But the toolchains can be configured to omit either the shared or the static version of the libraries, and if libunwind only exists in static form, this would fail.

If there's only a shared version, and building with -static-libgcc, I guess it's ok to look for explicitly libunwind.a and fail in that case, but for the default case, I think the normal library resolution logic would be best, i.e. just doing -lunwind.

The ideal version from my point of view would be this:

case ToolChain::UNW_CompilerRT:
  if (LGT == LibGccType::StaticLibGcc)
    CmdArgs.push_back("-l:libunwind.a");
  else if (TC.getTriple().isOSCygMing()) {
    if (LGT == LibGccType::SharedLibGcc)
      CmdArgs.push_back("-l:libunwind.dll.a");
    else
      CmdArgs.push_back("-lunwind"); // Let the linker choose between libunwind.dll.a and libunwind.a depending on what's available, and depending on the -static flag
  } else
    CmdArgs.push_back("-l:libunwind.so");
  break;

That's rather different than the other, non-mingw cases though, but I'm not familiar with the reasoning behind those.

Adding @rnk if he'd happen to have opinions.

[mati865]

Applied review comment and formatted.

[mati865]

@mstorsjo @rnk will be away since June

[mstorsjo]

In D79995#2054298, @mati865 wrote:

@mstorsjo @rnk will be away since June

Yeah, I've noticed - but was waiting to see if he'd comment on it before that.

Btw, would it be possible to add some test for this? (Sorry for not mentioning it earlier.) clang/test/Driver/compiler-rt-unwind.c seems to have existing testcases for this, either add more to that one or make a copy of the file and add new mingw specific ones there.

[mati865]

Added test.

[mstorsjo]

LGTM now - thanks! What's your preferred git author line for this project? I can probably push it tomorrow.

[mati865]

No worries.
Mateusz Mikuła <[e-mail]>

clang/lib/Driver/ToolChains/CommonArgs.cpp

if (LGT == LibGccType::StaticLibGcc)
       CmdArgs.push_back("-lgcc_eh");
     else
       CmdArgs.push_back("-lgcc_s");
     break;
   }
   case ToolChain::UNW_CompilerRT:
     if (LGT == LibGccType::StaticLibGcc)
       CmdArgs.push_back("-l:libunwind.a");
+    else if (TC.getTriple().isOSCygMing()) {
+      if (LGT == LibGccType::SharedLibGcc)
+        CmdArgs.push_back("-l:libunwind.dll.a");
       else
+        // Let the linker choose between libunwind.dll.a and libunwind.a
+        // depending on what's available, and depending on the -static flag
+        CmdArgs.push_back("-lunwind");
+    } else
       CmdArgs.push_back("-l:libunwind.so");
     break;
   }
 
   if (AsNeeded)
     CmdArgs.push_back("--no-as-needed");
 }