Fix llvm-config --link-shared --libs for mingw

[tkelman]

Summary

On mingw, the llvm shlib gets built without a prefix or version suffix as just LLVM.dll, see
https://github.com/llvm-mirror/llvm/blob/edd4534a45df348c4ae87cfa8d08e719ae0c7085/cmake/modules/AddLLVM.cmake#L453-L471

llvm-config looks for and fails to find LLVM-3.9.dll, giving
$ ./llvm-config.exe --link-shared --libs
llvm-config: error: LLVM-3.9.dll is missing

I've fixed this by modifying SharedVersionedExt (and moving where the "-" for the version gets set).
That makes llvm-config look for the right file name and find it, but because the
GetComponentLibraryNameSlice function only checks Lib.startswith("lib") to determine
whether or not to remove both prefix and extension, it would output -lLLVM.dll which the
mingw linker doesn't handle correctly. Fixed that by making GetComponentLibraryNameSlice
check Lib.startswith(SharedPrefix), so now the output is just -lLLVM which links correctly.

[mgorny]

Sorry, I know very little of Windows.

[beanz]

This looks reasonable, can you please add a test case?

[tkelman]

Didn't get notified about your review, whoops. Where would this get tested? Won't it need one of the buildbots to be building a dll with mingw?

[beanz]

I'm pretty confident we have buildbots testing mingw. You should be able to add a test case under tests/tools/llvm-config and have REQUIRES: mingw32 in it.

[tkelman]

With shared libs enabled? That's only worked for a couple of months, since https://reviews.llvm.org/D25865. I'll try to figure out how the tests work.

[beanz]

Ah... don't know if we have one with shared libs enabled, but adding a test is still good. You probably need a new available feature added to lit for checking the dylib. In case you're unsure how to do that I wrote it up real quick and put it in a paste (https://reviews.llvm.org/P7962).

Basically with that you can write a test that has REQUIRES: mingw32,libllvm and it should only run when the dll is generated on mingw build hosts. Hope that helps.

tools/llvm-config/llvm-config.cpp

#endif
   /// removed or, as in the case of CMake's `BUILD_SHARED_LIBS`, never present
   /// in the first place. This can't be done at configure/build time.
 
   StringRef SharedExt, SharedVersionedExt, SharedDir, SharedPrefix, StaticExt,
       StaticPrefix, StaticDir = "lib", DirSep = "/";
   const Triple HostTriple(Triple::normalize(LLVM_HOST_TRIPLE));
   if (HostTriple.isOSWindows()) {
     SharedExt = "dll";
-    SharedVersionedExt = LLVM_DYLIB_VERSION ".dll";
+    SharedVersionedExt = ".dll";
     if (HostTriple.isOSCygMing()) {
       StaticExt = "a";
       StaticPrefix = "lib";
     } else {
       StaticExt = "lib";
       DirSep = "\\";
       std::replace(ActiveObjRoot.begin(), ActiveObjRoot.end(), '/', '\\');
       std::replace(ActivePrefix.begin(), ActivePrefix.end(), '/', '\\');
       std::replace(ActiveBinDir.begin(), ActiveBinDir.end(), '/', '\\');
       std::replace(ActiveLibDir.begin(), ActiveLibDir.end(), '/', '\\');
       std::replace(ActiveCMakeDir.begin(), ActiveCMakeDir.end(), '/', '\\');
       std::replace(ActiveIncludeOption.begin(), ActiveIncludeOption.end(), '/',
                    '\\');
     }
     SharedDir = ActiveBinDir;
     StaticDir = ActiveLibDir;
   } else if (HostTriple.isOSDarwin()) {
     SharedExt = "dylib";
-    SharedVersionedExt = LLVM_DYLIB_VERSION ".dylib";
+    SharedVersionedExt = "-" LLVM_DYLIB_VERSION ".dylib";
     StaticExt = "a";
     StaticDir = SharedDir = ActiveLibDir;
     StaticPrefix = SharedPrefix = "lib";
   } else {
     // default to the unix values:
     SharedExt = "so";
-    SharedVersionedExt = LLVM_DYLIB_VERSION ".so";
+    SharedVersionedExt = "-" LLVM_DYLIB_VERSION ".so";
     StaticExt = "a";
     StaticDir = SharedDir = ActiveLibDir;
     StaticPrefix = SharedPrefix = "lib";
   }
 
   const bool BuiltDyLib = (std::strcmp(LLVM_ENABLE_DYLIB, "ON") == 0);
 
   /// CMake style shared libs, ie each component is in a shared library.
   const bool BuiltSharedLibs = std::strcmp(LLVM_ENABLE_SHARED, "ON") == 0;
 
   bool DyLibExists = false;
   const std::string DyLibName =
-      (SharedPrefix + "LLVM-" + SharedVersionedExt).str();
+      (SharedPrefix + "LLVM" + SharedVersionedExt).str();
 
   // If LLVM_LINK_DYLIB is ON, the single shared library will be returned
   // for "--libs", etc, if they exist. This behaviour can be overridden with
   // --link-static or --link-shared.
   bool LinkDyLib = (std::strcmp(LLVM_LINK_DYLIB, "ON") == 0);
 
   if (BuiltDyLib) {
     std::string path((SharedDir + DirSep + DyLibName).str());
#endif
   }
   LinkMode LinkMode =
       (LinkDyLib || BuiltSharedLibs) ? LinkModeShared : LinkModeAuto;
 
   /// Get the component's library name without the lib prefix and the
   /// extension. Returns true if Lib is in a recognized format.
   auto GetComponentLibraryNameSlice = [&](const StringRef &Lib,
                                           StringRef &Out) {
-    if (Lib.startswith("lib")) {
+    if (Lib.startswith(SharedPrefix)) {
       unsigned FromEnd;
       if (Lib.endswith(StaticExt)) {
         FromEnd = StaticExt.size() + 1;
       } else if (Lib.endswith(SharedExt)) {
         FromEnd = SharedExt.size() + 1;
       } else {
         FromEnd = 0;
       }
 
       if (FromEnd != 0) {
-        Out = Lib.slice(3, Lib.size() - FromEnd);
+        Out = Lib.slice(SharedPrefix.size(), Lib.size() - FromEnd);
         return true;
       }
     }
 
     return false;
   };
   /// Maps Unixizms to the host platform.
   auto GetComponentLibraryFileName = [&](const StringRef &Lib,