GNU c和c ++中的SIMD函数问题

Question

Environment Details:

Machine: Core i5 M540 processor running Centos 64 bits in a virtual machine in VMware player. GCC: 4.8.2 built from source tar.

Issue: I am trying to learn more about SIMD functions in C/C++ and for that I created the following helloworld program.

#include <iostream>
#include <pmmintrin.h>

int main(void){
    __m128i a, b, c;
    a = _mm_set_epi32(1, 1, 1, 1);
    b = _mm_set_epi32(2, 3, 4, 5);
    c = _mm_add_epi32(a,b);
    std::cout << "Value of first int: " << c[0];
}

When I look at the assembly output for it using the following command I do not see the SIMD instructions.

g++ -S -I/usr/local/include/c++/4.8.2 -msse3 -O3 hello.cpp

Sample of the assembly generated:

movl    $.LC2, %esi
movl    $_ZSt4cout, %edi
call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
movabsq $21474836486, %rsi
movq    %rax, %rdi
call    _ZNSo9_M_insertIxEERSoT_
xorl    %eax, %eax

Please advise the correct way of writing or compiling the SIMD code.

Thanks you!!

Answer 1

It looks like your compiler is optimizing away the calls to _mm_foo_epi32 , since all the values are known. Try taking all the relevant inputs from the user and see what happens.

Alternately, compile with -O0 instead of -O3 and see what happens.