lld-link: Fix default output name with /dll flag.

[thakis]

Summary

If /dll is passed, the default output filename should be foo.dll, not foo.exe.

[ruiu]

LGTM. Thanks!

[thakis]

r266929, thanks!

[thakis]

The test is now failing on the bots: Adding /dll means that lld now calls lib.exe to make an import library, and lib.exe isn't on the path (and on non-Windows bots, not even available). Do you have a suggestion for how to fix this? Do I revert the change to the test file (and then maybe reland it once generating import libraries doesn't rely on lib.exe anymore)? Are there tests for import library generation? How do these handle that?

[thakis]

I added a "REQUIRES: winlib" in r266935 which will probably help; looks like that what other tests do.

[ruiu]

Yes, that's adding the REQUIRES is we do for testing features that depend on lib.exe.

COFF/Driver.cpp

bool link(llvm::ArrayRef<const char *> Args) {
   Configuration C;
   LinkerDriver D;
   Config = &C;
   Driver = &D;
   Driver->link(Args);
   return true;
 }
 
-// Drop directory components and replace extension with ".exe".
+// Drop directory components and replace extension with ".exe" or ".dll".
 static std::string getOutputPath(StringRef Path) {
   auto P = Path.find_last_of("\\/");
   StringRef S = (P == StringRef::npos) ? Path : Path.substr(P + 1);
-  return (S.substr(0, S.rfind('.')) + ".exe").str();
+  const char* E = Config->DLL ? ".dll" : ".exe";
+  return (S.substr(0, S.rfind('.')) + E).str();
 }
 
 // Opens a file. Path has to be resolved already.
 // Newly created memory buffers are owned by this driver.
 MemoryBufferRef LinkerDriver::openFile(StringRef Path) {
   auto MBOrErr = MemoryBuffer::getFile(Path);
   error(MBOrErr, Twine("Could not open ") + Path);
   std::unique_ptr<MemoryBuffer> &MB = *MBOrErr;

test/COFF/out.test

 # RUN: yaml2obj < %p/Inputs/ret42.yaml > %t.obj
 
 # RUN: mkdir -p %T/out/tmp
 # RUN: cp %t.obj %T/out/out1.obj
 # RUN: cp %t.obj %T/out/tmp/out2
 # RUN: cp %t.obj %T/out/tmp/out3.xyz
 
+# RUN: rm -f out1.exe out2.exe out3.exe out3.dll
 # RUN: lld-link /entry:main %T/out/out1.obj
 # RUN: lld-link /entry:main %T/out/tmp/out2
-# RUN: lld-link /entry:main %T/out/tmp/out3.xyz
+# RUN: lld-link /dll /entry:main %T/out/tmp/out3.xyz
 
 # RUN: llvm-readobj out1.exe | FileCheck %s
 # RUN: llvm-readobj out2.exe | FileCheck %s
-# RUN: llvm-readobj out3.exe | FileCheck %s
+# RUN: llvm-readobj out3.dll | FileCheck %s
 
 CHECK: File: