buildsystem: support choosing default linker

Default linker can be set with DEFAULT_LINKER in options.

Packages can influence linker selection both by positive and/or
negative PKG_BUILD_FLAGS, eg +bfd or -gold.

Positive build flags take priority over the default linker so eg
DEFAULT_LINKER="gold" and PKG_BUILD_FLAGS="+bfd" will select bfd.

Negative flags mean a specific linker should not be used, eg -gold
prevents using gold.

If the default linker is disabled via a build flag then any other
available linker will be used.

Optional linkers like gold have to be enabled with eg GOLD_SUPPORT="yes"
in options. If an optional linker is not enabled it won't be a candidate
for linker selection. So eg "+mold" will have no effect if MOLD_SUPPORT
isn't set to "yes".

Signed-off-by: Matthias Reichl <hias@horus.com>

config/functions

@@ -254,8 +254,50 @@ check_toolchain_config() {
   done
 }
 
+# args: linker, default availability yes/no
+linker_allowed() {
+  if flag_enabled "$1" "$2"; then
+    # bfd is always available, others need to be enabled with <LINKER>_SUPPORT="yes"
+    local linker_support="${1^^}_SUPPORT"
+    if [ "$1" = "bfd" ] || [ "${!linker_support}" = "yes" ]; then
+      return 0
+    fi
+  fi
+  return 1
+}
+
+# return target linker to use for a package
+get_target_linker() {
+  # all known linkers, in descending order of priority
+  # those are candidates for explicit opt-in via PKG_BUILD_FLAGS
+  local all_linkers="gold bfd"
+
+  # linkers to choose from unless disabled via PKG_BUILD_FLAGS
+  local linker_candidates="${DEFAULT_LINKER:-bfd} ${all_linkers}"
+
+  local linker
+
+  # check if package prefers a specific linker
+  for linker in ${all_linkers}; do
+    if linker_allowed "${linker}" "no"; then
+      echo "${linker}"
+      return
+    fi
+  done
+
+  # select linker which isn't disabled by PKG_BUILD_FLAGS
+  for linker in ${linker_candidates}; do
+    if linker_allowed "${linker}" "yes"; then
+      echo "${linker}"
+      return
+    fi
+  done
+
+  # none of our linkers matched, use the compiler's default linker
+  echo "compiler_default"
+}
+
 setup_toolchain() {
-  local have_gold="no"
   if [ "$LTO_SUPPORT" = "yes" ]; then
     if flag_enabled "lto-parallel" "no"; then
       TARGET_CFLAGS+=" $FLAGS_OPTIM_LTO_PARALLEL $FLAGS_OPTIM_LTO_NO_FAT"
@@ -278,15 +320,14 @@ setup_toolchain() {
     TARGET_LDFLAGS+=" $FLAGS_OPTIM_LTO_OFF"
   fi
 
-  # gold flag
-  if flag_enabled "gold" "$GOLD_SUPPORT" "only-disable"; then
-    TARGET_LDFLAGS+=" $LDFLAGS_OPTIM_GOLD"
-    have_gold="yes"
-  fi
+  local linker="$(get_target_linker)"
+  local linker_opts="LDFLAGS_OPTIM_LINKER_${linker^^}"
+
+  TARGET_LDFLAGS+=" ${!linker_opts}"
 
   # compiler optimization, descending priority: speed, size, default
   if [ "${BUILD_WITH_DEBUG}" = "yes" ]; then
-    if [ "${SPLIT_DEBUG_INFO}" = "yes" -a "${have_gold}" = "yes" ]; then
+    if [ "${SPLIT_DEBUG_INFO}" = "yes" -a "${linker}" = "gold" ]; then
       TARGET_CFLAGS+=" $CFLAGS_OPTIM_DEBUG_SPLIT"
       TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_DEBUG_SPLIT"
       TARGET_LDFLAGS+=" $LDFLAGS_OPTIM_DEBUG_SPLIT"

config/optimize

@@ -27,8 +27,10 @@ FLAGS_OPTIM_LTO_FAT="-ffat-lto-objects"
 FLAGS_OPTIM_LTO_OFF="-fno-lto"
 LDFLAGS_OPTIM_LTO_COMMON="-fuse-linker-plugin"
 
-# gold flags
-LDFLAGS_OPTIM_GOLD="-fuse-ld=gold"
+# linker specific flags
+LDFLAGS_OPTIM_LINKER_COMPILER_DEFAULT=""
+LDFLAGS_OPTIM_LINKER_BFD="-fuse-ld=bfd"
+LDFLAGS_OPTIM_LINKER_GOLD="-fuse-ld=gold"
 
 # default compiler optimization
 CFLAGS_OPTIM_DEFAULT="-O2 -fomit-frame-pointer -DNDEBUG"

config/show_config

@@ -31,6 +31,7 @@ show_config() {
   config_message+="\n - CPU features:\t\t\t ${TARGET_FEATURES}"
   config_message+="\n - LTO (Link Time Optimization) support: ${LTO_SUPPORT}"
   config_message+="\n - GOLD (Google Linker) Support:\t ${GOLD_SUPPORT}"
+  config_message+="\n - Default Linker:\t\t\t ${DEFAULT_LINKER}"
   config_message+="\n - LLVM support:\t\t\t ${LLVM_SUPPORT}"
   config_message+="\n - DEBUG:\t\t\t\t ${DEBUG:-no}"
   config_message+="\n - CFLAGS:\t\t\t\t ${TARGET_CFLAGS}"

distributions/LibreELEC/options

@@ -32,6 +32,9 @@
 # GOLD (Google Linker) support
   GOLD_SUPPORT="yes"
 
+# default linker (bfd / gold)
+  DEFAULT_LINKER="gold"
+
 # HARDENING (security relevant linker and compiler flags) support
   HARDENING_SUPPORT="no"
 

packages/readme.md

@@ -126,7 +126,8 @@ Set the variable `PKG_BUILD_FLAGS` in the `package.mk` to enable/disable the sin
 | lto-parallel | disabled | target, init  | same as `lto` but enables parallel optimization at link stage. Only enable this if the package build doesn't run multiple linkers in parallel otherwise this can result in lots of parallel processes! |
 | lto-fat  | disabled | target, init      | same as `lto` but compile fat LTO objects (bytecode plus optimized assembly). This increases compile time but can be useful to create static libraries suitable both for LTO and non-LTO linking |
 | lto-off  | disabled | target, init      | explicitly disable LTO in the compiler and linker |
-| gold     | enabled by `GOLD_SUPPORT` | target, init | do not use GOLD-Llinker (can only disable) |
+| bfd      | - | target, init | `+bfd` prefers bfd linker over default linker, `-bfd` disables using bfd |
+| gold     | - | target, init | `+gold` prefers gold linker over default linker, `-gold` disables using gold |
 | parallel | enabled  | all | `make` or `ninja` builds with multiple threads/processes (or not) |
 | strip    | enabled  | target | strips executables (or not) |
 | sysroot  | enabled  | target | installs the package to the sysroot folder (or not) |