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.

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 "  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 "  LDFLAGS         = $PTHREAD_LIBS $HARDENED_LDFLAGS $GPROF_LDFLAGS $CORE_LDFLAGS $LDFLAGS"
 echo "  AR              = $AR"
 echo "  ARFLAGS         = $ARFLAGS"
 echo

depends/README.md

@@ -123,7 +123,7 @@ The following can be set when running make: `make FOO=bar`
 - `LOG`: Use file-based logging for individual packages. During a package build its log file
   resides in the `depends` directory, and the log file is printed out automatically in case
   of build error. After successful build log files are moved along with package archives
-- `LTO`: Use LTO when building packages.
+- `LTO`: Enable options needed for LTO. Does not add `-flto` related options to *FLAGS.
 - `NO_HARDEN=1`: Don't use hardening options when building packages
 
 If some packages are not built, for example `make NO_WALLET=1`, the appropriate

depends/config.site.in

@@ -78,10 +78,6 @@ if test "@host_os@" = darwin; then
   BREW=no
 fi
 
-if test -z "$enable_lto" && test -n "@lto@"; then
-  enable_lto=yes
-fi
-
 if test -z "$enable_hardening" && test -n "@no_harden@"; then
   enable_hardening=no
 fi

depends/hosts/android.mk

@@ -9,11 +9,6 @@ endif
 android_CFLAGS=-std=$(C_STANDARD)
 android_CXXFLAGS=-std=$(CXX_STANDARD)
 
-ifneq ($(LTO),)
-android_CFLAGS += -flto
-android_LDFLAGS += -flto
-endif
-
 android_AR=$(ANDROID_TOOLCHAIN_BIN)/llvm-ar
 android_RANLIB=$(ANDROID_TOOLCHAIN_BIN)/llvm-ranlib
 

depends/hosts/darwin.mk

@@ -97,12 +97,6 @@ darwin_CFLAGS=-pipe -std=$(C_STANDARD)
 darwin_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 darwin_LDFLAGS=-Wl,-platform_version,macos,$(OSX_MIN_VERSION),$(OSX_SDK_VERSION)
 
-ifneq ($(LTO),)
-darwin_CFLAGS += -flto
-darwin_CXXFLAGS += -flto
-darwin_LDFLAGS += -flto
-endif
-
 darwin_release_CFLAGS=-O2
 darwin_release_CXXFLAGS=$(darwin_release_CFLAGS)
 

depends/hosts/freebsd.mk

@@ -1,12 +1,6 @@
 freebsd_CFLAGS=-pipe -std=$(C_STANDARD)
 freebsd_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
-ifneq ($(LTO),)
-freebsd_CFLAGS += -flto
-freebsd_CXXFLAGS += -flto
-freebsd_LDFLAGS += -flto
-endif
-
 freebsd_release_CFLAGS=-O2
 freebsd_release_CXXFLAGS=$(freebsd_release_CFLAGS)
 

depends/hosts/linux.mk

@@ -2,10 +2,6 @@ linux_CFLAGS=-pipe -std=$(C_STANDARD)
 linux_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
-linux_CFLAGS += -flto
-linux_CXXFLAGS += -flto
-linux_LDFLAGS += -flto
-
 linux_AR = $(host_toolchain)gcc-ar
 linux_NM = $(host_toolchain)gcc-nm
 linux_RANLIB = $(host_toolchain)gcc-ranlib

depends/hosts/mingw32.mk

@@ -6,10 +6,6 @@ mingw32_CFLAGS=-pipe -std=$(C_STANDARD)
 mingw32_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
-mingw32_CFLAGS += -flto
-mingw32_CXXFLAGS += -flto
-mingw32_LDFLAGS += -flto
-
 mingw32_AR = $(host_toolchain)gcc-ar
 mingw32_NM = $(host_toolchain)gcc-nm
 mingw32_RANLIB = $(host_toolchain)gcc-ranlib

depends/hosts/netbsd.mk

@@ -2,10 +2,6 @@ netbsd_CFLAGS=-pipe -std=$(C_STANDARD)
 netbsd_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
-netbsd_CFLAGS += -flto
-netbsd_CXXFLAGS += -flto
-netbsd_LDFLAGS += -flto
-
 netbsd_AR = $(host_toolchain)gcc-ar
 netbsd_NM = $(host_toolchain)gcc-nm
 netbsd_RANLIB = $(host_toolchain)gcc-ranlib

depends/hosts/openbsd.mk

@@ -1,12 +1,6 @@
 openbsd_CFLAGS=-pipe -std=$(C_STANDARD)
 openbsd_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
-ifneq ($(LTO),)
-openbsd_CFLAGS += -flto
-openbsd_CXXFLAGS += -flto
-openbsd_LDFLAGS += -flto
-endif
-
 openbsd_release_CFLAGS=-O2
 openbsd_release_CXXFLAGS=$(openbsd_release_CFLAGS)
 

src/Makefile.am

@@ -8,8 +8,8 @@ print-%: FORCE
 
 DIST_SUBDIRS = secp256k1
 
-AM_LDFLAGS = $(LIBTOOL_LDFLAGS) $(HARDENED_LDFLAGS) $(GPROF_LDFLAGS) $(SANITIZER_LDFLAGS) $(LTO_LDFLAGS) $(CORE_LDFLAGS)
+AM_LDFLAGS = $(LIBTOOL_LDFLAGS) $(HARDENED_LDFLAGS) $(GPROF_LDFLAGS) $(SANITIZER_LDFLAGS) $(CORE_LDFLAGS)
-AM_CXXFLAGS = $(DEBUG_CXXFLAGS) $(HARDENED_CXXFLAGS) $(WARN_CXXFLAGS) $(NOWARN_CXXFLAGS) $(ERROR_CXXFLAGS) $(GPROF_CXXFLAGS) $(SANITIZER_CXXFLAGS) $(LTO_CXXFLAGS) $(CORE_CXXFLAGS)
+AM_CXXFLAGS = $(DEBUG_CXXFLAGS) $(HARDENED_CXXFLAGS) $(WARN_CXXFLAGS) $(NOWARN_CXXFLAGS) $(ERROR_CXXFLAGS) $(GPROF_CXXFLAGS) $(SANITIZER_CXXFLAGS) $(CORE_CXXFLAGS)
 AM_CPPFLAGS = $(DEBUG_CPPFLAGS) $(HARDENED_CPPFLAGS) $(CORE_CPPFLAGS)
 AM_LIBTOOLFLAGS = --preserve-dup-deps
 PTHREAD_FLAGS = $(PTHREAD_CFLAGS) $(PTHREAD_LIBS)