Add support for LLVM's thinLTO

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

Fixes #7493

3 files changed, 21 insertions, 8 deletions

diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 07569a7..08db6d7 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/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]:
diff --git a/mesonbuild/compilers/mixins/clang.py b/mesonbuild/compilers/mixins/clang.py
index 9c17a55..1778c31 100644
--- a/mesonbuild/compilers/mixins/clang.py
+++ b/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}')
diff --git a/mesonbuild/compilers/mixins/gnu.py b/mesonbuild/compilers/mixins/gnu.py
index 5afbb83..464c664 100644
--- a/mesonbuild/compilers/mixins/gnu.py
+++ b/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']