Compiling with -mpopcnt causes Illegal instruction error - c++

Question

I compile the following C++ code
// main.cpp
#include <cstdio>
int main() {
unsigned char tab[4] = {0};
printf("%d\n", __builtin_popcount(*((int *)tab)));
}
using command line:
g++ -o prog main.cpp -mpopcnt
When I run the program I get error:
Illegal instruction
Compiling without -mpopcnt does not give an error (it just prints 0).
Question: what is causing this error?
I am compiling and running the program on the same machine.
Valgrind detects no problem. Running
valgrind --leak-check=full ./prog
gives
==12917== Memcheck, a memory error detector
==12917== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==12917== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==12917== Command: ./prog
==12917==
0
==12917==
==12917== HEAP SUMMARY:
==12917== in use at exit: 0 bytes in 0 blocks
==12917== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==12917==
==12917== All heap blocks were freed -- no leaks are possible
==12917==
==12917== For counts of detected and suppressed errors, rerun with: -v
==12917== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
Below I give some specifications of my system.
I'm using Ubuntu 12.04. Running
uname -a
gives me
Linux wtu-82 3.2.0-65-generic #99-Ubuntu SMP Fri Jul 4 21:03:29 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
Running
g++ -v
gives
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.4-1ubuntu1~12.04' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.4 (Ubuntu/Linaro 4.6.4-1ubuntu1~12.04)
The output of
cat /proc/cpuinfo
is
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 23
model name : Intel(R) Core(TM)2 Duo CPU E8500 # 3.16GHz
stepping : 10
microcode : 0xa0c
cpu MHz : 2000.000
cache size : 6144 KB
physical id : 0
siblings : 2
core id : 0
cpu cores : 2
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm constant_tsc arch_perfmon pebs bts rep_good nopl aperfmperf pni dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm sse4_1 xsave lahf_lm dtherm tpr_shadow vnmi flexpriority
bogomips : 6317.48
clflush size : 64
cache_alignment : 64
address sizes : 36 bits physical, 48 bits virtual
power management:
processor : 1
vendor_id : GenuineIntel
cpu family : 6
model : 23
model name : Intel(R) Core(TM)2 Duo CPU E8500 # 3.16GHz
stepping : 10
microcode : 0xa0c
cpu MHz : 2000.000
cache size : 6144 KB
physical id : 0
siblings : 2
core id : 1
cpu cores : 2
apicid : 1
initial apicid : 1
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm constant_tsc arch_perfmon pebs bts rep_good nopl aperfmperf pni dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm sse4_1 xsave lahf_lm dtherm tpr_shadow vnmi flexpriority
bogomips : 6317.38
clflush size : 64
cache_alignment : 64
address sizes : 36 bits physical, 48 bits virtual
power management:

Answer 1

POPCNT was introduced in SSE 4.2. Your processor is SSE 4.1. So, the instruction is simply missing. You get an illegal instruction error when you force the compiler, with -mpopcnt, to generate code using an instruction your processor doesn't know about.