G++ sees float as illegal instruction - c++

Question

In my file "six.cpp", I try to print a float, but the compiler keeps throwing an error. Here is the file "six.cpp":
#include <iostream>
#include <iomanip>
using namespace std;
int main(){
double f = 5.3;
cout << f << endl;
return 0;
}
Then when I try to compile using g++ six.cpp I get the following output using the -v tag.
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/lto-wrapper.exe
Target: x86_64-pc-cygwin
Configured with: /cygdrive/i/szsz/tmpp/gcc/gcc-4.9.3-1.x86_64/src/gcc-4.9.3/configure --srcdir=/cygdrive/i/szsz/tmpp/gcc/gcc-4.9.3-1.x86_64/src/gcc-4.9.3 --prefix=/usr --exec-prefix=/usr --localstatedir=/var --sysconfdir=/etc --docdir=/usr/share/doc/gcc --htmldir=/usr/share/doc/gcc/html -C --build=x86_64-pc-cygwin --host=x86_64-pc-cygwin --target=x86_64-pc-cygwin --without-libiconv-prefix --without-libintl-prefix --libexecdir=/usr/lib --enable-shared --enable-shared-libgcc --enable-static --enable-version-specific-runtime-libs --enable-bootstrap --enable-__cxa_atexit --with-dwarf2 --with-tune=generic --enable-languages=ada,c,c++,fortran,lto,objc,obj-c++ --enable-graphite --enable-threads=posix --enable-libatomic --enable-libgomp --disable-libitm --enable-libquadmath --enable-libquadmath-support --enable-libssp --enable-libada --enable-libgcj-sublibs --disable-java-awt --disable-symvers --with-ecj-jar=/usr/share/java/ecj.jar --with-gnu-ld --with-gnu-as --with-cloog-include=/usr/include/cloog-isl --without-libiconv-prefix --without-libintl-prefix --with-system-zlib --enable-linker-build-id
Thread model: posix
gcc version 4.9.3 (GCC)
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/cc1plus.exe -quiet -v -Dunix -idirafter /usr/lib/gcc/x86_64-pc-cygwin/4.9.3/../../../../lib/../include/w32api -idirafter /usr/lib/gcc/x86_64-pc-cygwin/4.9.3/../../../../x86_64-pc-cygwin/lib/../lib/../../include/w32api six.cpp -quiet -dumpbase six.cpp -mtune=generic -march=x86-64 -auxbase six -version -o /tmp/ccfHbtlL.s
GNU C++ (GCC) version 4.9.3 (x86_64-pc-cygwin)
compiled by GNU C version 4.9.3, GMP version 6.0.0, MPFR version 3.1.2-p11, MPC version 1.0.3
warning: GMP header version 6.0.0 differs from library version 6.1.0.
warning: MPFR header version 3.1.2-p11 differs from library version 3.1.3.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/../../../../x86_64-pc-cygwin/include"
ignoring duplicate directory "/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/../../../../x86_64-pc-cygwin/lib/../lib/../../include/w32api"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/x86_64-pc-cygwin
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/backward
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include-fixed
/usr/include
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/../../../../lib/../include/w32api
End of search list.
GNU C++ (GCC) version 4.9.3 (x86_64-pc-cygwin)
compiled by GNU C version 4.9.3, GMP version 6.0.0, MPFR version 3.1.2-p11, MPC version 1.0.3
warning: GMP header version 6.0.0 differs from library version 6.1.0.
warning: MPFR header version 3.1.2-p11 differs from library version 3.1.3.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: a7b6fac296390f5db29d753ab65194e7
six.cpp:6:3: internal compiler error: Illegal instruction
double f = 5.3;
^
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.
I just started programming in C++, and I'm very confused to as to why this is happening. I've also done a few google searches and have not found anything about this.

Answer 1

You've got an internal compiler error: Illegal instruction, which means the gcc compiler itself has failed. From the warnings it looks like you've mis-installed the gcc tool chain. Try removing it and reinstalling or upgrading.

Answer 2

The portion of source code where it failed is not especially interesting here, the important bit is "internal compiler error: Illegal instruction".
This means that your copy of g++ is built for a newer subarchitecture than the CPU you are running it on. For example, SSE4 instructions were introduced in 2006 and older CPU designs don't support them. Because SSE is used for floating-point calculations, it makes sense that finding a floating-point literal triggered the error, but the root cause is the instruction set mismatch.
If you check /proc/cpuinfo, you can find out what instruction set extensions are supported on your CPU, which will help you pick the right g++ build options (whether you build yourself or download a package built with those options).
Relevant options are the ones listed here, particularly -march=, -mfpmath=, and -m(no)fused-madd.