3 files changed, 42 insertions, 5 deletions
diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 0f83f4c..07569a7 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/mesonbuild/compilers/compilers.py
@@ -268,6 +268,8 @@ clike_debug_args = {False: [],
 base_options: 'KeyedOptionDictType' = {
     OptionKey('b_pch'): coredata.UserBooleanOption('Use precompiled headers', True),
     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_sanitize'): coredata.UserComboOption('Code sanitizer to use',
                                                       ['none', 'address', 'thread', 'undefined', 'memory', 'address,undefined'],
                                                       'none'),
@@ -300,11 +302,25 @@ def option_enabled(boptions: T.Set[OptionKey], options: 'KeyedOptionDictType',
     except KeyError:
         return False
 
+
+def get_option_value(options: 'KeyedOptionDictType', opt: OptionKey, fallback: '_T') -> '_T':
+    """Get the value of an option, or the fallback value."""
+    try:
+        v: '_T' = options[opt].value
+    except KeyError:
+        return fallback
+
+    assert isinstance(v, type(fallback)), f'Should have {type(fallback)!r} but was {type(v)!r}'
+    # Mypy doesn't understand that the above assert ensures that v is type _T
+    return v
+
+
 def get_base_compile_args(options: 'KeyedOptionDictType', compiler: 'Compiler') -> T.List[str]:
     args = []  # type T.List[str]
     try:
         if options[OptionKey('b_lto')].value:
-            args.extend(compiler.get_lto_compile_args())
+            args.extend(compiler.get_lto_compile_args(
+                threads=get_option_value(options, OptionKey('b_lto_threads'), 0)))
     except KeyError:
         pass
     try:
@@ -926,7 +942,7 @@ class Compiler(metaclass=abc.ABCMeta):
             ret.append(arg)
         return ret
 
-    def get_lto_compile_args(self) -> T.List[str]:
+    def get_lto_compile_args(self, *, threads: int = 0) -> 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 fcb2225..9c17a55 100644
--- a/mesonbuild/compilers/mixins/clang.py
+++ b/mesonbuild/compilers/mixins/clang.py
@@ -49,7 +49,8 @@ class ClangCompiler(GnuLikeCompiler):
         super().__init__()
         self.id = 'clang'
         self.defines = defines or {}
-        self.base_options.add(OptionKey('b_colorout'))
+        self.base_options.update({OptionKey('b_colorout'), OptionKey('b_lto_threads')})
+
         # TODO: this really should be part of the linker base_options, but
         # linkers don't have base_options.
         if isinstance(self.linker, AppleDynamicLinker):
@@ -135,3 +136,10 @@ 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)
+        # In clang -flto=0 means auto
+        if threads >= 0:
+            args.append(f'-flto-jobs={threads}')
+        return args
diff --git a/mesonbuild/compilers/mixins/gnu.py b/mesonbuild/compilers/mixins/gnu.py
index 95bcd7c..5afbb83 100644
--- a/mesonbuild/compilers/mixins/gnu.py
+++ b/mesonbuild/compilers/mixins/gnu.py
@@ -17,6 +17,7 @@
 import abc
 import functools
 import os
+import multiprocessing
 import pathlib
 import re
 import subprocess
@@ -281,7 +282,9 @@ class GnuLikeCompiler(Compiler, metaclass=abc.ABCMeta):
                 return self._split_fetch_real_dirs(line.split('=', 1)[1])
         return []
 
-    def get_lto_compile_args(self) -> T.List[str]:
+    def get_lto_compile_args(self, *, threads: int = 0) -> T.List[str]:
+        # This provides a base for many compilers, GCC and Clang override this
+        # for their specific arguments
         return ['-flto']
 
     def sanitizer_compile_args(self, value: str) -> T.List[str]:
@@ -330,7 +333,7 @@ class GnuCompiler(GnuLikeCompiler):
         super().__init__()
         self.id = 'gcc'
         self.defines = defines or {}
-        self.base_options.add(OptionKey('b_colorout'))
+        self.base_options.update({OptionKey('b_colorout'), OptionKey('b_lto_threads')})
 
     def get_colorout_args(self, colortype: str) -> T.List[str]:
         if mesonlib.version_compare(self.version, '>=4.9.0'):
@@ -383,3 +386,13 @@ 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]:
+        if threads == 0:
+            if mesonlib.version_compare(self.version, '>= 10.0'):
+                return ['-flto=auto']
+            # This matches clang's behavior of using the number of cpus
+            return [f'-flto={multiprocessing.cpu_count()}']
+        elif threads > 0:
+            return [f'-flto={threads}']
+        return super().get_lto_compile_args(threads=threads)