fix: dev: 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.

Closes #5536

Merge branch '5536-more-supported-linker-ids' into 'main'

See merge request isc-projects/bind9!11022

.gitlab-ci.yml

@@ -381,6 +381,7 @@ stages:
       -Ddeveloper=enabled
       -Dleak-detection=enabled
       -Doptimization=1
+      -Dnamed-lto=thin
       $EXTRA_CONFIGURE
       build
 
@@ -1511,7 +1512,7 @@ clang:tsan:
     CC: "${CLANG}"
     CFLAGS: "${CFLAGS_COMMON}"
     LDFLAGS: "-Wl,--disable-new-dtags"
-    EXTRA_CONFIGURE: "${TSAN_CONFIGURE_FLAGS_COMMON} -Db_lundef=false -Dnamed-lto=off --native-file ci/clang-trixie.ini"
+    EXTRA_CONFIGURE: "${TSAN_CONFIGURE_FLAGS_COMMON} -Db_lundef=false -Dnamed-lto=disabled --native-file ci/clang-trixie.ini"
   <<: *build_job
 
 system:clang:tsan:
@@ -1985,7 +1986,7 @@ respdiff:tsan:
     CC: "${CLANG}"
     CFLAGS: "${CFLAGS_COMMON}"
     LDFLAGS: "-Wl,--disable-new-dtags"
-    EXTRA_CONFIGURE: "${TSAN_CONFIGURE_FLAGS_COMMON} -Dnamed-lto=off -Db_lundef=false"
+    EXTRA_CONFIGURE: "${TSAN_CONFIGURE_FLAGS_COMMON} -Dnamed-lto=disabled -Db_lundef=false"
     MAX_DISAGREEMENTS_PERCENTAGE: "0.3"
     TSAN_OPTIONS: "${TSAN_OPTIONS_DEBIAN}"
   script:

doc/arm/build.inc.rst

@@ -89,9 +89,11 @@ required when in use.
 
 To further improve performance, compilation with link-time optimization is
 recommended. This is enabled by default via the ``-Dnamed-lto`` option
-(default: ``thin``). Link-time optimization can be disabled if needed by
-using ``-Dnamed-lto=off``. The optimization level can be controlled with
-``-Dnamed-lto=thin`` or ``-Dnamed-lto=full``.
+(default: ``auto``). Link-time optimization can be disabled if needed by
+using ``-Dnamed-lto=disabled``. The optimization level can be controlled with
+``-Dnamed-lto=thin`` or ``-Dnamed-lto=full``. The ``auto`` option enables
+thin LTO when supported by the compiler and linker combination, and disables
+LTO otherwise.
 
 Link-time optimization requires close coordination between the compiler and
 the linker. Due to ``clang`` limitations, compiling ``named`` with ``clang``

meson.build

@@ -923,28 +923,79 @@ 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',
+]
+
+is_darwin = host_machine.system() == 'darwin'
+is_auto_lto = named_lto_opt == 'auto'
+is_supported_linker = cc.get_linker_id() in supported_clang_lto_linkers
+has_fat_lto = cc.has_argument('-ffat-lto-objects') and not is_darwin
+has_fuzzing_backend = fuzzing_backend_opt != 'none'
+
+# First, resolution of the 'auto' case for LTO.
+if is_auto_lto
+    if has_fuzzing_backend
+        # Fuzzers might need sanitizer converage sections, which LTO discards.
+        # Disable LTO when a fuzzing backend is configured.
+        # NB: enabling -Dfuzzing is fine since it justs builds the binaries.
+        named_lto_opt = 'disabled'
+    elif is_darwin
+        # On Mac OS, the has_argument('-ffat-lto-objects') check returns true,
+        # but compilation fails, so we simply disable LTO.
+        named_lto_opt = 'disabled'
+    elif not has_fat_lto
+        # Some very old compilers don't support fat lto objects.
+        named_lto_opt = 'disabled'
+    elif cc.get_id() == 'clang' and not is_supported_linker
+        # Per LLVM docs [1], -ffat-lto-objects is supported only with lld and gold,
+        # and gold is deprecated/unmantained. Manual testing shows that mold
+        # works too though.
+        named_lto_opt = 'disabled'
+    elif get_option('auto_features').disabled()
+        # Fake being a boolean option for the purpose of -Dauto-features
+        named_lto_opt = 'disabled'
+    else
+        named_lto_opt = 'thin'
+    endif
+endif
+
 static_lto_c_args = []
 static_lto_link_args = []
 
-if named_lto_opt == 'full'
+if developer_mode and is_auto_lto
+    # Build a static `named` but do not enable -flto to improve compilation speed.
+elif 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'
-    # 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
-
+elif named_lto_opt == 'thin' and cc.get_id() == 'clang' and is_supported_linker
     static_lto_c_args = ['-ffat-lto-objects', '-flto=thin']
     static_lto_link_args = ['-flto=thin']
 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=disabled.',
+        )
+    else
+        error(
+            'Unsupported configuration for LTO. To build, use a supported linker or disable LTO with -Dnamed-lto=disabled.',
+        )
+    endif
 endif
 
 add_project_arguments(static_lto_c_args, language: 'c')
-if named_lto_opt != 'off' and cc.get_id() == 'clang' and sanitizer.contains('address')
+if named_lto_opt != 'disabled' and cc.get_id() == 'clang' and sanitizer.contains('address')
     # Needed to suppress the
     #    warning: Redundant instrumentation detected, with module flag:
     #    nosanitize_address [-Werror,-Wbackend-plugin]
@@ -957,7 +1008,7 @@ if named_lto_opt != 'off' and cc.get_id() == 'clang' and sanitizer.contains('add
     add_project_arguments('-Wno-backend-plugin', language: 'c')
 endif
 
-if meson_lto and named_lto_opt != 'off'
+if meson_lto and named_lto_opt != 'disabled'
     # Meson's builtin LTO settings do not set -ffat-lto-objects, which can cause
     # build issues.
     # Since we don't want two, possibly conflicting, sets of LTO flags, we
@@ -966,7 +1017,7 @@ if meson_lto and named_lto_opt != 'off'
     error(
         '''
             Meson builtin -Db_lto and BIND's -Dnamed-lto options are incompatible.
-            Either disable named-lto with -Dnamed-lto=off, or avoid setting
+            Either disable named-lto with -Dnamed-lto=disabled, or avoid setting
             -Db_lto.
 
             Note that using -Db_lto is not a recommended configuration, might
@@ -1542,7 +1593,7 @@ executable(
 
 named_deps = []
 
-if named_lto_opt == 'off'
+if named_lto_opt == 'disabled'
     named_deps = [
         libdns_dep,
         libisc_dep,

meson.options

@@ -199,7 +199,7 @@ option(
 option(
     'named-lto',
     type: 'combo',
-    choices: ['off', 'thin', 'full'],
-    value: 'thin',
+    choices: ['disabled', 'auto', 'thin', 'full'],
+    value: 'auto',
     description: 'Enable Link Time Optimization for named.',
 )