[LLD] [COFF] Look for libfoo.a if foo.lib is specified, for MinGW

[mstorsjo]

Summary

This allows using #pragma comment(lib, "foo") in MinGW built code for system libraries, if built with -fms-extensions.

[mstorsjo]

As long as this is mostly intended for system libraries, we can skip the .dll.a part, and don't need to think about any static flag.

[mstorsjo]

Simplified it to only care about the case of system libraries for now. As this feature isn't available in ld.bfd and therefore not in use in the general mingw ecosystem, it might be ok to limit it to this case for now, for use with sanitizers?

[rnk]

COFF/Driver.cpp

377 ↗

(On Diff #168757)

I guess this could be has_extension, probably with Style::windows, as well.

383–385 ↗

(On Diff #168757)

How about llvm::sys::path::replace_extension?

386 ↗

(On Diff #168757)

Eh, saving a library name for every -defaultlib: we find in an object file is kind of a waste of memory. They're everywhere. But, this is a pre-existing problem that needs fixing, so I won't push to fix it now.

[mstorsjo]

COFF/Driver.cpp

383–385 ↗

(On Diff #168757)

I actually just changed it to use that, in the version I updated a few minutes earlier.

[rnk]

lgtm

[ruiu]

COFF/Driver.cpp

380 ↗

(On Diff #168872)

Please add a comment to explain this mingw-specific behavior. It might be a good thing to separate this code as a new function.

[mstorsjo]

COFF/Driver.cpp

380 ↗

(On Diff #168872)

Ok, I added more comments and split this to a separate function, before pushing.

lld/trunk/COFF/Driver.h

private:
   // Opens a file. Path has to be resolved already.
   MemoryBufferRef openFile(StringRef Path);
 
   // Searches a file from search paths.
   Optional<StringRef> findFile(StringRef Filename);
   Optional<StringRef> findLib(StringRef Filename);
   StringRef doFindFile(StringRef Filename);
   StringRef doFindLib(StringRef Filename);
+  StringRef doFindLibMinGW(StringRef Filename);
 
   // Parses LIB environment which contains a list of search paths.
   void addLibSearchPaths();
 
   // Library search path. The first element is always "" (current directory).
   std::vector<StringRef> SearchPaths;
 
   // We don't want to add the same file more than once.

lld/trunk/COFF/Driver.cpp

if (Seen)
       return None;
   }
 
   if (Path.endswith_lower(".lib"))
     VisitedLibs.insert(sys::path::filename(Path));
   return Path;
 }
 
+// MinGW specific. If an embedded directive specified to link to
+// foo.lib, but it isn't found, try libfoo.a instead.
+StringRef LinkerDriver::doFindLibMinGW(StringRef Filename) {
+  if (Filename.contains('/') || Filename.contains('\\'))
+    return Filename;
+
+  SmallString<128> S = Filename;
+  sys::path::replace_extension(S, ".a");
+  StringRef LibName = Saver.save("lib" + S.str());
+  return doFindFile(LibName);
+}
+
 // Find library file from search path.
 StringRef LinkerDriver::doFindLib(StringRef Filename) {
   // Add ".lib" to Filename if that has no file extension.
   bool HasExt = Filename.contains('.');
   if (!HasExt)
     Filename = Saver.save(Filename + ".lib");
-  return doFindFile(Filename);
+  StringRef Ret = doFindFile(Filename);
+  // For MinGW, if the find above didn't turn up anything, try
+  // looking for a MinGW formatted library name.
+  if (Config->MinGW && Ret == Filename)
+    return doFindLibMinGW(Filename);
+  return Ret;
 }
 
 // Resolves a library path. /nodefaultlib options are taken into
 // consideration. This never returns the same path (in that case,
 // it returns None).
 Optional<StringRef> LinkerDriver::findLib(StringRef Filename) {
   if (Config->NoDefaultLibAll)
     return None;

lld/trunk/test/COFF/libname-mingw.test

+# RUN: mkdir -p %t/a
+# RUN: cp %p/Inputs/std64.lib %t/a/libstd64.a
+
+# RUN: lld-link /lldmingw /out:%t.exe /entry:main /verbose \
+# RUN:   /defaultlib:std64.lib /subsystem:console %p/Inputs/hello64.obj \
+# RUN:   /libpath:%t/a 2>&1 | FileCheck %s
+
+CHECK: a{{[/\\]}}libstd64.a