What is the right way to pass optimization flags when building (cross) GCC?

[Sergio <year1943 at yandex dot ru>]

Hello.

What is the right way to pass optimization flags when building GCC and when building cross-GCC?

At first I tried to build native gcc, I ran configure this way (I tried 4.8.2 release and 4.9.0 from svn):

$ flags='-march=core2 -mtune=core2 -O2 -pipe'
$ ../gcc/configure CFLAGS="$flags" CXXFLAGS="$flags"  --with-arch=core2 --with-tune=core2  \
      --enable-languages='c,c++' --prefix=/opt/gcc-4.8 --disable-nls

compilation was successful, but I saw a lot of lines in output, telling that lots of objects were compiled without my options but with some kind of default ones, like "-g" or "-g -O2", sometimes options were combined, and "-g" and "-march=core2" were used together. 
It is unclear for me - is it by design, or I did something improperly? 


At second, I tried to build cross-compiler (gcc 4.8.2 + eglibc 2.15 (this version is used on target device)).
I successfully built cross-gcc without optimization, then I tried again with passing flags (last 3rd build of gcc):

$ build_flags='-march=core2 -mtune=core2 -O2 -pipe'
$ ../gcc/configure CFLAGS="$build_flags" CXXFLAGS="$build_flags"  \
    --with-arch=armv7-a --with-tune=marvell-pj4 --with-mode=thumb --with-float=hard --with-fpu=vfpv3  \
    --enable-languages=c --target=$target --prefix=$prefix --with-sysroot=$sysroot  \
    --disable-libssp --disable-libgomp --disable-libmudflap --disable-nls
$ make 

This failed with error:

checking for suffix of object files... configure: error: in `/path/to/xgcc/obj/gcc3/arm-linux-gnueabi/libgcc':
configure: error: cannot compute suffix of object files: cannot compile

File arm-linux-gnueabi/libgcc/config.log contains:

xgcc: error: unrecognized argument in option '-march=core2'
xgcc: note: valid arguments to '-march=' are: armv2 armv2a ....... armv8-a+crc iwmmxt iwmmxt2 native
xgcc: error: unrecognized argument in option '-mtune=core2'
xgcc: note: valid arguments to '-mtune=' are: arm1020e .....

I guess make failed because at some moment build_flags (for amd64 architecture) were passed to cross-compiler, while it was expecting arm-architecture-compatible optimization options' values.

I looked into Makefile and discovered variables ?FLAGS_FOR_BUILD and ?FLAGS_FOR_TARGET.

Next I ran configure with vars:

CFLAGS_FOR_BUILD="$build_flags" CXXFLAGS_FOR_BUILD="$build_flags"

it compiled successfully, but again I saw lots of objects were compiled without my options.
I roughly counted entries passing make's output through grep and got:
mtune=core2	   127
-g -O2          4371

It seems only little number of objects were compiled with flags I passed to configure.
Next I tried to experiment and ran configure with such vars defined (trying to override "global flags" with "target flags" where needed):

target_flags="-march=armv7-a -mtune=marvell-pj4 -O2 -pipe -mthumb -mthumb-interwork -mfloat-abi=hard -mfpu=vfpv3"
CFLAGS="$build_flags" CXXFLAGS="$build_flags" CFLAGS_FOR_TARGET="$target_flags" CXXFLAGS_FOR_TARGET="$target_flags"

It was built with success, and output contained:
mtune=core2     799
-g -O2              0
march=armv7-a    3701

But this is only if --enable-languages=c
With --enable-languages=c,c++ compilation again failed with:

make[3]: Entering directory `/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3'
Making all in include
make[4]: Entering directory `/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/include'
make[4]: Nothing to be done for `all'.
make[4]: Leaving directory `/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/include'
Making all in libsupc++
make[4]: Entering directory `/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/libsupc++'
/bin/bash ../libtool --tag CXX --tag disable-shared   --mode=compile /mnt/home/sergio/wrk/xgcc/obj/gcc3/./gcc/xgcc -shared-libgcc -B/mnt/home/sergio/wrk/xgcc/obj/gcc3/./gcc -nostdinc++ -L/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/src -L/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/src/.libs -L/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/libsupc++/.libs -B/home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/bin/ -B/home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/lib/ -isystem /home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/include -isystem /home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/sys-include    -I/mnt/home/sergio/wrk/xgcc/src/gcc-svn-208890/libstdc++-v3/../libgcc -I/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/include/arm-linux-gnueabi -I/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/include -I/mnt/home/sergio/wrk/xgcc/src/gcc-svn-208890/libstdc++-v3/libsupc++  -prefer-pic -D_GLIBCXX_SHARED -fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual -Wabi  -fdiagnostics-show-location=once    -ffunction-sections -fdata-sections  -frandom-seed=array_type_info.lo -march=core2 -mtune=core2 -O2 -pipe -D_GNU_SOURCE  -c -o array_type_info.lo /mnt/home/sergio/wrk/xgcc/src/gcc-svn-208890/libstdc++-v3/libsupc++/array_type_info.cc
libtool: compile:  /mnt/home/sergio/wrk/xgcc/obj/gcc3/./gcc/xgcc -shared-libgcc -B/mnt/home/sergio/wrk/xgcc/obj/gcc3/./gcc -nostdinc++ -L/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/src -L/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/src/.libs -L/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/libsupc++/.libs -B/home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/bin/ -B/home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/lib/ -isystem /home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/include -isystem /home/sergio/opt/xgcc-armada370/arm-linux-gnueabi/sys-include -I/mnt/home/sergio/wrk/xgcc/src/gcc-svn-208890/libstdc++-v3/../libgcc -I/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/include/arm-linux-gnueabi -I/mnt/home/sergio/wrk/xgcc/obj/gcc3/arm-linux-gnueabi/libstdc++-v3/include -I/mnt/home/sergio/wrk/xgcc/src/gcc-svn-208890/libstdc++-v3/libsupc++ -D_GLIBCXX_SHARED -fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual -Wabi -fdiagnostics-show-location=once -ffunction-sections -fdata-sections -frandom-seed=array_type_info.lo -march=core2 -mtune=core2 -O2 -pipe -D_GNU_SOURCE -c /mnt/home/sergio/wrk/xgcc/src/gcc-svn-208890/libstdc++-v3/libsupc++/array_type_info.cc  -D_GLIBCXX_SHARED
xgcc: error: unrecognized argument in option '-march=core2'
xgcc: note: valid arguments to '-march=' are: armv2 armv2a armv3 armv3m armv4 armv4t armv5 armv5e armv5t armv5te armv6 armv6-m armv6j armv6k armv6s-m armv6t2 armv6z armv6zk armv7 armv7-a armv7-m armv7-r armv7e-m armv7ve armv8-a armv8-a+crc iwmmxt iwmmxt2 native
xgcc: error: unrecognized argument in option '-mtune=core2'
xgcc: note: valid arguments to '-mtune=' are: arm1020e arm1020t ...........

Once again CFLAGS/CXXFLAGS were occasionally passed to arm-targeted crosscompiler...



Another issue is that the built crosscompiler links helloworld to ld-linux-armhf.so.3 , while the built library had installed only ld-linux.so.3
Am I right thinking this is due to aged version of library, ld-linux naming scheme for hard-fp was changed after eglibc 2.15 release? and I have to rename installed library to new name? 
Or this may be due to some mess with build options, and library could actually be built not for float-point=hard? 
Is there any method to discover ABI-specs of built executables and .so? objdump or readelf or somewhat else? what to seek for in their output?

Thanks.

/sergio