Question

I have built GCC 4.7 on my x86 32-bit linux system. When I try to cross-compile with the -m64 flag I get the following:

sorry, unimplemented: 64-bit mode not compiled in

while the compiler provided by default by my Linux distribution can cross-compile with -m64.

What do I have to pass to ./configure to enable the 64bit mode in GCC? These are the options I used to build GCC 4.7:

$ /usr/local/bin/g++ -v Using built-in specs.
COLLECT_GCC=/usr/local/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/i686-pc-linux-gnu/4.7.0/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: ./configure --enable-multiarch --with-cloog=/usr/local/ --with-mpfr=/usr/local/ --with-ppl=/usr/local/ --verbose --enable-languages=c,c++
Thread model: posix gcc version 4.7.0 20120113 (experimental) (GCC)

EDIT:

–enable-multilib and –enable-targets=i686-pc-linux-gnu,x86_64-pc-linux-gnu

do not change the situation. The compiler still complains about 64 bit mode not compiled in:

$ g++ -v Using built-in specs. COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/i686-pc-linux-gnu/4.7.0/lto-wrapper
Target: i686-pc-linux-gnu Configured with: ./configure
–enable-multiarch –with-cloog=/usr/local/ –with-mpfr=/usr/local/ –with-ppl=/usr/local/ –verbose –enable-languages=c,c++ –enable-multilib –enable-targets=i686-pc-linux-gnu,x86_64-pc-linux-gnu Thread model: posix gcc version 4.7.0 20120113 (experimental) (GCC)

$ g++ -m64 c.cpp c.cpp:1:0: sorry, unimplemented: 64-bit mode not
compiled in

Answer 1

You will need both binutils and gcc configured with:

--enable-multilib

and probably:

--enable-targets=i686-pc-linux-gnu,x86_64-pc-linux-gnu

to support multilib (the -m64 and/or -m32 options). You’ll also need two versions of stuff like glibc to be able to link and run the resulting binaries.