valgrind error with __builtin_ctz - c++

Question

I'm trying to profile my code but run into problems.
If I run the following code:
#include <iostream>
int main() {
size_t val = 8;
std::cout << sizeof(val) << std::endl;
std::cout << __builtin_ctz(val) << std::endl;
}
It returns as expected
8
3
If I run valgrind on it it returns:
==28602== Memcheck, a memory error detector
==28602== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==28602== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==28602== Command: ./test
==28602==
8
vex amd64->IR: unhandled instruction bytes: 0xF3 0xF 0xBC 0xC0 0x89 0xC6 0xBF 0x60
==28602== valgrind: Unrecognised instruction at address 0x400890.
==28602== at 0x400890: main (in /home/magu_/sod/test/test)
==28602== Your program just tried to execute an instruction that Valgrind
==28602== did not recognise. There are two possible reasons for this.
==28602== 1. Your program has a bug and erroneously jumped to a non-code
==28602== location. If you are running Memcheck and you just saw a
==28602== warning about a bad jump, it's probably your program's fault.
==28602== 2. The instruction is legitimate but Valgrind doesn't handle it,
==28602== i.e. it's Valgrind's fault. If you think this is the case or
==28602== you are not sure, please let us know and we'll try to fix it.
==28602== Either way, Valgrind will now raise a SIGILL signal which will
==28602== probably kill your program.
==28602==
==28602== Process terminating with default action of signal 4 (SIGILL)
==28602== Illegal opcode at address 0x400890
==28602== at 0x400890: main (in /home/magu_/sod/test/test)
==28602==
==28602== HEAP SUMMARY:
==28602== in use at exit: 0 bytes in 0 blocks
==28602== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==28602==
==28602== All heap blocks were freed -- no leaks are possible
==28602==
==28602== For counts of detected and suppressed errors, rerun with: -v
==28602== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
Illegal instruction (core dumped)
Is this an bug of valgrind or should I not use __builtin_ctz with my computer? __builtin_popcount does not raise any errors.
My system:
g++ (Ubuntu 4.8.1-2ubuntu1~12.04) 4.8.1
CPU : Intel Core Duo T7500

Answer 1

You need to upgrade valgrind to at least 4.8.1 or use an gcc older than v4.8.
The opcode you ran into -- F3 0F BC -- is the TZCNT opcode, introduced in BMI1, which your CPU doesn't implement. However, it is also REP;BSF (F3 is REP) and older CPUs, including yours, ignore the REP for this opcode, and the similar LZCNT == REP;BSR pair. There is very little difference between TZCNT and BSF (they differ in how they handle 0).
Older gcc versions used BSF for older CPUs and TZCNT for newer ones, but since the opcode is relatively rare, in newer gcc versions the logic was simplified and TZCNT is always used, since both older and newer CPUs understand it.
Unfortunately, valgrind did not correctly fallback from TZCNT to BSF until v4.8.1. See bug 295808.

Answer 2

On Debian/Sid/x86-64 (Intel i4750HQ processor) with gcc version 4.9.1 (Debian 4.9.1-4) and valgrind-3.9.0 your test is working ok (and valgrind runs successfully without reporting any errors).
So I suggest you to upgrade your GCC compiler and most importantly valgrind. Start first by compiling valgrind from its valgrind-3.9.0 source code tarball (and use aptitude build-dep valgrind before).
BTW, your distribution version is quite old. Did you consider upgrading to Ubuntu 14.0 LTS?
If you don't have root access, consider passing some explicit --prefix (e.g. $HOME/pub/ ) to valgrind-3.9.0/configure