darwin: Unconditionally pass -lto_library, remove -Wliblto warning.

https://reviews.llvm.org/D25932 made it so that clang always checks if
libLTO.dylib is present on disk, even if -flto is not being used. The
motivation for that change was that if a dependency happens to contain bitcode,
ld64 will try to load libLTO without -flto explicitly being enabled. However,
the change had the undesirable side effect of warning if libLTO.dylib doesn't
exist even if it isn't needed.

Change things so that -lto_library is always passes, independent of if it
exists or not. ld64 only looks at this flag if it uses LTO. If the dylib
exists, all is well. If it doesn't, and LTO is not being used, all is well too.
If ld64 does end up using LTO and the dylib does not exist, ld64 will print
something like

    ld: could not process llvm bitcode object file, because foo/libLTO.dylib could not be loaded file 'test.o' for architecture x86_64

https://reviews.llvm.org/D26984

llvm-svn: 287685

1 files changed, 8 insertions, 8 deletions

diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 038f682..8760b85 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -8221,20 +8221,20 @@ void darwin::Linker::AddLinkArgs(Compilation &C, const ArgList &Args,
   }
 
   // Use -lto_library option to specify the libLTO.dylib path. Try to find
-  // it in clang installed libraries. If not found, the option is not used
-  // and 'ld' will use its default mechanism to search for libLTO.dylib.
+  // it in clang installed libraries. ld64 will only look at this argument
+  // when it actually uses LTO, so libLTO.dylib only needs to exist at link
+  // time if ld64 decides that it needs to use LTO.
+  // Since this is passed unconditionally, ld64 will never look for libLTO.dylib
+  // next to it. That's ok since ld64 using a libLTO.dylib not matching the
+  // clang version won't work anyways.
   if (Version[0] >= 133) {
     // Search for libLTO in <InstalledDir>/../lib/libLTO.dylib
     StringRef P = llvm::sys::path::parent_path(D.Dir);
     SmallString<128> LibLTOPath(P);
     llvm::sys::path::append(LibLTOPath, "lib");
     llvm::sys::path::append(LibLTOPath, "libLTO.dylib");
-    if (llvm::sys::fs::exists(LibLTOPath)) {
-      CmdArgs.push_back("-lto_library");
-      CmdArgs.push_back(C.getArgs().MakeArgString(LibLTOPath));
-    } else {
-      D.Diag(diag::warn_drv_lto_libpath);
-    }
+    CmdArgs.push_back("-lto_library");
+    CmdArgs.push_back(C.getArgs().MakeArgString(LibLTOPath));
   }
 
   // ld64 version 262 and above run the deduplicate pass by default.