build: remove --enable-lto

This has outlived its usefulness, doesn't gel well with
newer compilers & `-flto` related options, i.e thin vs full, or `=auto`,
and having `-flto` as the only option means that sometimes this just
needs to be worked around, i.e in oss-fuzz:
https://github.com/google/oss-fuzz/blob/master/projects/bitcoin-core/build.sh.

While it was convenient when `-flto` was newer, support for `-flto` is now
in all compilers we use, and there's also no-longer any real need
for us to treat `-flto` different to any other optimization option.

Remove it, to remove build complexity, and so there's no need
to port a similar option to CMake.

Note that the LTO option remains in depends, because we still a way to
build packages that have LTO specific patches/options.

If we decide to merge this, I'll follow up downstream in oss-fuzz first,
to make sure we don't break the build.

1 files changed, 2 insertions, 15 deletions

diff --git a/configure.ac b/configure.ac
index 7b286df603..19a9134bd2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -321,11 +321,6 @@ AC_ARG_ENABLE([external-signer],
     [use_external_signer=$enableval],
     [use_external_signer=auto])
 
-AC_ARG_ENABLE([lto],
-    [AS_HELP_STRING([--enable-lto],[build using LTO (default is no)])],
-    [enable_lto=$enableval],
-    [enable_lto=no])
-
 AC_LANG_PUSH([C++])
 
 dnl Check for a flag to turn compiler warnings into errors. This is helpful for checks which may
@@ -377,11 +372,6 @@ if test "$enable_debug" = "yes"; then
   AX_CHECK_COMPILE_FLAG([-ftrapv], [DEBUG_CXXFLAGS="$DEBUG_CXXFLAGS -ftrapv"], [], [$CXXFLAG_WERROR])
 fi
 
-if test "$enable_lto" = "yes"; then
-  AX_CHECK_COMPILE_FLAG([-flto], [LTO_CXXFLAGS="$LTO_CXXFLAGS -flto"], [AC_MSG_ERROR([compile failed with -flto])], [$CXXFLAG_WERROR])
-  AX_CHECK_LINK_FLAG([-flto], [LTO_LDFLAGS="$LTO_LDFLAGS -flto"], [AC_MSG_ERROR([link failed with -flto])], [$CXXFLAG_WERROR])
-fi
-
 if test "$use_sanitizers" != ""; then
   dnl First check if the compiler accepts flags. If an incompatible pair like
   dnl -fsanitize=address,thread is used here, this check will fail. This will also
@@ -1893,8 +1883,6 @@ AC_SUBST(GPROF_LDFLAGS)
 AC_SUBST(HARDENED_CXXFLAGS)
 AC_SUBST(HARDENED_CPPFLAGS)
 AC_SUBST(HARDENED_LDFLAGS)
-AC_SUBST(LTO_CXXFLAGS)
-AC_SUBST(LTO_LDFLAGS)
 AC_SUBST(PIC_FLAGS)
 AC_SUBST(PIE_FLAGS)
 AC_SUBST(SANITIZER_CXXFLAGS)
@@ -2002,7 +1990,6 @@ echo "  sanitizers      = $use_sanitizers"
 echo "  debug enabled   = $enable_debug"
 echo "  gprof enabled   = $enable_gprof"
 echo "  werror          = $enable_werror"
-echo "  LTO             = $enable_lto"
 echo
 echo "  target os       = $host_os"
 echo "  build os        = $build_os"
@@ -2011,8 +1998,8 @@ echo "  CC              = $CC"
 echo "  CFLAGS          = $PTHREAD_CFLAGS $CFLAGS"
 echo "  CPPFLAGS        = $DEBUG_CPPFLAGS $HARDENED_CPPFLAGS $CORE_CPPFLAGS $CPPFLAGS"
 echo "  CXX             = $CXX"
-echo "  CXXFLAGS        = $LTO_CXXFLAGS $DEBUG_CXXFLAGS $HARDENED_CXXFLAGS $WARN_CXXFLAGS $NOWARN_CXXFLAGS $ERROR_CXXFLAGS $GPROF_CXXFLAGS $CORE_CXXFLAGS $CXXFLAGS"
-echo "  LDFLAGS         = $LTO_LDFLAGS $PTHREAD_LIBS $HARDENED_LDFLAGS $GPROF_LDFLAGS $CORE_LDFLAGS $LDFLAGS"
+echo "  CXXFLAGS        = $DEBUG_CXXFLAGS $HARDENED_CXXFLAGS $WARN_CXXFLAGS $NOWARN_CXXFLAGS $ERROR_CXXFLAGS $GPROF_CXXFLAGS $CORE_CXXFLAGS $CXXFLAGS"
+echo "  LDFLAGS         = $PTHREAD_LIBS $HARDENED_LDFLAGS $GPROF_LDFLAGS $CORE_LDFLAGS $LDFLAGS"
 echo "  AR              = $AR"
 echo "  ARFLAGS         = $ARFLAGS"
 echo