Add lto support to clang-cl and lld-link

Enables lto & thinlto support for clang-cl when used with lld-link.

mesonbuild/compilers/mixins/visualstudio.py

@@ -16,6 +16,7 @@ from ... import mesonlib
 from ... import mlog
 from mesonbuild.compilers.compilers import CompileCheckMode
 from ...options import OptionKey
+from mesonbuild.linkers.linkers import ClangClDynamicLinker
 
 if T.TYPE_CHECKING:
     from ...environment import Environment
@@ -443,6 +444,10 @@ class ClangClCompiler(VisualStudioLikeCompiler):
     def __init__(self, target: str):
         super().__init__(target)
 
+        self.base_options.update(
+            {OptionKey('b_lto_threads'), OptionKey('b_lto'), OptionKey('b_lto_mode'), OptionKey('b_thinlto_cache'),
+             OptionKey('b_thinlto_cache_dir')})
+
         # Assembly
         self.can_compile_suffixes.add('s')
         self.can_compile_suffixes.add('sx')
@@ -494,3 +499,27 @@ class ClangClCompiler(VisualStudioLikeCompiler):
         if libs is None:
             raise mesonlib.MesonBugException('Could not find libomp')
         return super().openmp_link_flags(env) + libs
+
+    def get_lto_compile_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
+        args: T.List[str] = []
+        if mode == 'thin':
+            # LTO data generated by clang-cl is only usable by lld-link
+            if not isinstance(self.linker, ClangClDynamicLinker):
+                raise mesonlib.MesonException(f"LLVM's ThinLTO only works with lld-link, not {self.linker.id}")
+            args.append(f'-flto={mode}')
+        else:
+            assert mode == 'default', 'someone forgot to wire something up'
+            args.extend(super().get_lto_compile_args(threads=threads))
+        return args
+
+    def get_lto_link_args(self, *, threads: int = 0, mode: str = 'default',
+                          thinlto_cache_dir: T.Optional[str] = None) -> T.List[str]:
+        args = []
+        if mode == 'thin' and thinlto_cache_dir is not None:
+            args.extend(self.linker.get_thinlto_cache_args(thinlto_cache_dir))
+        # lld-link /threads:N has the same behaviour as -flto-jobs=N in lld
+        if threads > 0:
+            # clang-cl was released after clang already had LTO support, so it
+            # is safe to assume that all versions of clang-cl support LTO
+            args.append(f'/threads:{threads}')
+        return args