Protobuf is compiled with -g -O2 despite what configure.ac claims

[neongreen]

By default, protobuf is compiled with -O2 -g, even though configure.ac claims it shouldn't be:

protobuf/configure.ac

Lines 39 to 44 in a9f390f

	# autoconf's default CXXFLAGS are usually "-g -O2". These aren't necessarily
	# the best choice for libprotobuf.
	AS_IF([test "x${ac_cv_env_CFLAGS_set}" = "x"],
	[CFLAGS=""])
	AS_IF([test "x${ac_cv_env_CXXFLAGS_set}" = "x"],
	[CXXFLAGS=""])

I'm building latest master as follows:

./autogen.sh
./configure
make

Here's a sample compiler invocation during the build:

/Library/Developer/CommandLineTools/usr/bin/clang++ -DHAVE_CONFIG_H -I. -I..
  -pthread -DHAVE_PTHREAD=1 -DHAVE_ZLIB=1 -Wall -Wno-sign-compare
  -O2 -g -std=c++11 -DNDEBUG -MT google/protobuf/generated_enum_util.lo -MD -MP
  -MF google/protobuf/.deps/generated_enum_util.Tpo 
  -c google/protobuf/generated_enum_util.cc 
  -fno-common -DPIC -o google/protobuf/.libs/generated_enum_util.o

What would be the best way to compile protobuf without -g? This would help a lot with NixOS/nixpkgs#73919.

[veprbl]

Turns out there is some bit of code which has an opposite intention:

protobuf/configure.ac

Lines 85 to 101 in 3be9322

	# test_util.cc takes forever to compile with GCC and optimization turned on.
	AC_MSG_CHECKING([C++ compiler flags...])
	AS_IF([test "x${ac_cv_env_CXXFLAGS_set}" = "x"],[
	AS_IF([test "$GCC" = "yes"],[
	PROTOBUF_OPT_FLAG="-O2"
	CXXFLAGS="${CXXFLAGS} -g"
	])
	# Protocol Buffers contains several checks that are intended to be used only
	# for debugging and which might hurt performance. Most users are probably
	# end users who don't want these checks, so add -DNDEBUG by default.
	CXXFLAGS="$CXXFLAGS -std=c++11 -DNDEBUG"
	AC_MSG_RESULT([use default: $PROTOBUF_OPT_FLAG $CXXFLAGS])
	],[
	AC_MSG_RESULT([use user-supplied: $CXXFLAGS])
	])

I'm not aware of what is considered the best practice for handling debug/release builds in autotools, but it seems like, in the case of protobuf, downstream needs to explicitly set CXXFLAGS to achieve a full release build without "-g" option everywhere and with "-O2" on clang.

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