From: Alessio Podda <alessio@isc.org>
Date: Mon, 29 Sep 2025 06:20:00 +0000 (+0200)
Subject: Add support for more linkers with LTO
X-Git-Tag: v9.21.16~24^2~2
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fbecbfd5e27d3718e229e13723cf6c1f1cc23e6e;p=thirdparty%2Fbind9.git

Add support for more linkers with LTO

Link-time optimization requires close coordination between the compiler
and the linker, so not all combinations of compiler and linker support
it.

Previously, when compiling with Clang, we checked only for lld. With
this commit, we expand the list of supported linkers we check for.
---

diff --git a/meson.build b/meson.build
index 4315d697c64..66b76da732c 100644
--- a/meson.build
+++ b/meson.build
@@ -923,13 +923,33 @@ assert(
 
 # LTO
 
+# Taken from Meson's compilers/mixins/clang.py:get_lto_compile_args
+# (technically mold requires version 1.1)
+supported_clang_lto_linkers = [
+    'ld64',
+    'lld-link',
+    'ld.lld',
+    'ld.gold',
+    'ld.mold',
+]
+
+# On Mac OS, the has_argument check returns true, but compilation fails, so we
+# simply disable LTO.
+has_fat_lto = cc.has_argument('-ffat-lto-objects') and host_machine.system() != 'darwin'
+if not has_fat_lto
+    warning(
+        'Your platform does not support fat lto objects but -Dnamed-lto was not set to off. Building without LTO anyway.',
+    )
+    named_lto_opt = 'off'
+endif
+
 static_lto_c_args = []
 static_lto_link_args = []
 
 if named_lto_opt == 'full'
     static_lto_c_args = ['-ffat-lto-objects', '-flto']
     static_lto_link_args = ['-flto']
-elif named_lto_opt == 'thin' and cc.get_id() == 'clang' and cc.get_linker_id() == 'ld.lld'
+elif named_lto_opt == 'thin' and cc.get_id() == 'clang' and cc.get_linker_id() in supported_clang_lto_linkers
     # Per LLVM docs [1], -ffat-lto-objects is supported only with lld and gold,
     # and gold is deprecated/unmantained.
     # [1]: https://llvm.org/docs/FatLTO.html
@@ -940,7 +960,17 @@ elif named_lto_opt == 'thin' and cc.get_id() == 'gcc'
     static_lto_c_args = ['-ffat-lto-objects', '-flto=auto']
     static_lto_link_args = ['-flto=auto']
 elif named_lto_opt == 'thin'
-    error('LTO requires clang with ld.lld, or gcc with any linker')
+    if cc.get_id() == 'clang'
+        error(
+            'Clang ThinLTO only works with gold, lld, lld-link, ld64 or mold, not '
+            + cc.get_linker_id()
+            + '. To build, use a supported linker or disable LTO with -Dnamed-lto=off.',
+        )
+    else
+        error(
+            'Unsupported configuration for LTO. To build, use a supported linker or disable LTO with -Dnamed-lto=off.',
+        )
+    endif
 endif
 
 add_project_arguments(static_lto_c_args, language: 'c')