Add support for LLVM's thinLTO

This uses a separate option, b_lto_mode. It works in conjunction with
b_lto_threads.

Fixes #7493

docs/markdown/Builtin-options.md

@@ -128,6 +128,7 @@ available on all platforms or with all compilers:
 | b_lundef      | true           | true, false                                                      | Don't allow undefined symbols when linking                                    |
 | b_lto         | false          | true, false                                                      | Use link time optimization                                                    |
 | b_lto_threads | 0              | Any integer*                                                     | Use multiple threads for lto. *(Added in 0.57.0)*                             |
+| b_lto_mode    | default        | default, thin                                                    | Select between lto modes, thin and default. *(Added in 0.57.0)*               |
 | b_ndebug      | false          | true, false, if-release                                          | Disable asserts                                                               |
 | b_pch         | true           | true, false                                                      | Use precompiled headers                                                       |
 | b_pgo         | off            | off, generate, use                                               | Use profile guided optimization                                               |
@@ -141,6 +142,8 @@ The value of `b_sanitize` can be one of: `none`, `address`, `thread`,
 
 * < 0 means disable, == 0 means automatic selection, > 0 sets a specific number to use
 
+LLVM supports `thin` lto, for more discussion see [LLVM's documentation](https://clang.llvm.org/docs/ThinLTO.html)
+
 <a name="b_vscrt-from_buildtype"></a>
 The default value of `b_vscrt` is `from_buildtype`. The following table is
 used internally to pick the CRT compiler arguments for `from_buildtype` or

docs/markdown/snippets/lto_mode.md

@@ -0,0 +1,5 @@
+## Support added for LLVM's thinLTO
+
+A new `b_lto_mode` option has been added, which may be set to `default` or
+`thin`. Thin only works for clang, and only with gnu gold, lld variants, or
+ld64.

mesonbuild/compilers/compilers.py

@@ -270,6 +270,9 @@ base_options: 'KeyedOptionDictType' = {
     OptionKey('b_lto'): coredata.UserBooleanOption('Use link time optimization', False),
     OptionKey('b_lto'): coredata.UserBooleanOption('Use link time optimization', False),
     OptionKey('b_lto_threads'): coredata.UserIntegerOption('Use multiple threads for Link Time Optimization', (None, None,0)),
+    OptionKey('b_lto_mode'): coredata.UserComboOption('Select between different LTO modes.',
+                                                      ['default', 'thin'],
+                                                      'default'),
     OptionKey('b_sanitize'): coredata.UserComboOption('Code sanitizer to use',
                                                       ['none', 'address', 'thread', 'undefined', 'memory', 'address,undefined'],
                                                       'none'),
@@ -320,7 +323,8 @@ def get_base_compile_args(options: 'KeyedOptionDictType', compiler: 'Compiler')
     try:
         if options[OptionKey('b_lto')].value:
             args.extend(compiler.get_lto_compile_args(
-                threads=get_option_value(options, OptionKey('b_lto_threads'), 0)))
+                threads=get_option_value(options, OptionKey('b_lto_threads'), 0),
+                mode=get_option_value(options, OptionKey('b_lto_mode'), 'default')))
     except KeyError:
         pass
     try:
@@ -942,7 +946,7 @@ class Compiler(metaclass=abc.ABCMeta):
             ret.append(arg)
         return ret
 
-    def get_lto_compile_args(self, *, threads: int = 0) -> T.List[str]:
+    def get_lto_compile_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
         return []
 
     def get_lto_link_args(self) -> T.List[str]:

mesonbuild/compilers/mixins/clang.py

@@ -19,7 +19,7 @@ import shutil
 import typing as T
 
 from ... import mesonlib
-from ...linkers import AppleDynamicLinker
+from ...linkers import AppleDynamicLinker, ClangClDynamicLinker, LLVMDynamicLinker, GnuGoldDynamicLinker
 from ...mesonlib import OptionKey
 from ..compilers import CompileCheckMode
 from .gnu import GnuLikeCompiler
@@ -49,7 +49,8 @@ class ClangCompiler(GnuLikeCompiler):
         super().__init__()
         self.id = 'clang'
         self.defines = defines or {}
-        self.base_options.update({OptionKey('b_colorout'), OptionKey('b_lto_threads')})
+        self.base_options.update(
+            {OptionKey('b_colorout'), OptionKey('b_lto_threads'), OptionKey('b_lto_mode')})
 
         # TODO: this really should be part of the linker base_options, but
         # linkers don't have base_options.
@@ -137,8 +138,16 @@ class ClangCompiler(GnuLikeCompiler):
     def get_coverage_link_args(self) -> T.List[str]:
         return ['--coverage']
 
-    def get_lto_compile_args(self, *, threads: int = 0) -> T.List[str]:
-        args = super().get_lto_compile_args(threads=threads)
+    def get_lto_compile_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
+        args: T.List[str] = []
+        if mode == 'thin':
+            # Thin LTO requires the use of gold, lld, ld64, or lld-link
+            if not isinstance(self.linker, (AppleDynamicLinker, ClangClDynamicLinker, LLVMDynamicLinker, GnuGoldDynamicLinker)):
+                raise mesonlib.MesonException(f"LLVM's thinLTO only works with gnu gold, lld, lld-link, and ld64, 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))
         # In clang -flto=0 means auto
         if threads >= 0:
             args.append(f'-flto-jobs={threads}')

mesonbuild/compilers/mixins/gnu.py

@@ -282,7 +282,7 @@ class GnuLikeCompiler(Compiler, metaclass=abc.ABCMeta):
                 return self._split_fetch_real_dirs(line.split('=', 1)[1])
         return []
 
-    def get_lto_compile_args(self, *, threads: int = 0) -> T.List[str]:
+    def get_lto_compile_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
         # This provides a base for many compilers, GCC and Clang override this
         # for their specific arguments
         return ['-flto']
@@ -387,7 +387,7 @@ class GnuCompiler(GnuLikeCompiler):
     def get_prelink_args(self, prelink_name: str, obj_list: T.List[str]) -> T.List[str]:
         return ['-r', '-o', prelink_name] + obj_list
 
-    def get_lto_compile_args(self, *, threads: int = 0) -> T.List[str]:
+    def get_lto_compile_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
         if threads == 0:
             if mesonlib.version_compare(self.version, '>= 10.0'):
                 return ['-flto=auto']

run_unittests.py

@@ -2864,6 +2864,31 @@ class AllPlatformTests(BasePlatformTests):
                 for e in expected:
                     self.assertIn(e, s['parameters'])
 
+    @skip_if_not_base_option('b_lto_mode')
+    @skip_if_not_base_option('b_lto_threads')
+    def test_lto_mode(self):
+        testdir = os.path.join(self.common_test_dir, '6 linkshared')
+
+        env = get_fake_env(testdir, self.builddir, self.prefix)
+        cc = env.detect_c_compiler(MachineChoice.HOST)
+        if cc.get_id() != 'clang':
+            raise unittest.SkipTest('Only clang currently supports thinLTO')
+        if cc.linker.id not in {'ld.lld', 'ld.gold', 'ld64', 'lld-link'}:
+            raise unittest.SkipTest('thinLTO requires ld.lld, ld.gold, ld64, or lld-link')
+        elif is_windows():
+            raise unittest.SkipTest('LTO not (yet) supported by windows clang')
+
+        self.init(testdir, extra_args=['-Db_lto=true', '-Db_lto_mode=thin', '-Db_lto_threads=8'])
+        self.build()
+        self.run_tests()
+
+        expected = set(cc.get_lto_compile_args(threads=8, mode='thin'))
+        targets = self.introspect('--targets')
+        # This assumes all of the targets support lto
+        for t in targets:
+            for s in t['target_sources']:
+                assert expected.issubset(set(s['parameters'])), f'Incorrect values for {t["name"]}'
+
     def test_dist_git(self):
         if not shutil.which('git'):
             raise unittest.SkipTest('Git not found')