GCC -msse2 does not generate SIMD code

Question

I am trying to figure out why g++ does not generate a SIMD code.

Info GCC / OS / CPU:

$ gcc -v
gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)

$ cat /proc/cpuinfo
...
model name  : Intel(R) Core(TM)2 Duo CPU     P8600  @ 2.40GHz
... 

and here is my C++ code:

#include <iostream>
#include <cstdlib>

//function that fills an array with random numbers
template<class T>
void fillArray(T *array, int n){
    srand(1);
    for (int i = 0; i < n; i++) {
        array[i] = (float) (rand() % 10);
    }
}
// function that computes the dotprod of two vectors (loop unrolled)
float dotCPP(float *src1, float *src2, int n){
    float dest = 0;
    for (int i = 0; i < n; i+=2) {
        dest += (src1[i] * src2[i]) + (src1[i+1] * src2[i+1]);                
    }
    return dest;
}

int main(int argc, char *argv[])
{

    const int n = 1200000;           
    float *a = new float[n];   //allocate data on the heap
    float something_else;      //store result
    fillArray<float>(a,n);     //function that fills the array with random numbers
    something_else = dotCPP(a, a, n);  //call function and store return value

    return 0;
}  

I compile the code with:

makefile:
CXX = g++
CXXFLGS = -g -Wall -std=c++11 -msse2 -O3
SRC = main.o dot.o 
EXEC = dot

$(EXEC): $(SRC)
    $(CXX) $(CXXFLGS) $(SRC) -o $(EXEC)
main.o: dot.cpp
    $(CXX) $(CXXFLGS) -c dot.cpp -o main.o

and use gdb to inspect the generated code:

$gdb dot
... 
(gdb) b dotCPP
(gdb) r
...
(gdb) disass
Dump of assembler code for function dotCPP(float*, float*, int):
=> 0x08048950 <+0>:     push   %ebx
   0x08048951 <+1>:     mov    0x10(%esp),%ebx
   0x08048955 <+5>:     mov    0x8(%esp),%edx
   0x08048959 <+9>:     mov    0xc(%esp),%ecx
   0x0804895d <+13>:    test   %ebx,%ebx
   0x0804895f <+15>:    jle    0x8048983 <dotCPP(float*, float*, int)+51>
   0x08048961 <+17>:    xor    %eax,%eax
   0x08048963 <+19>:    fldz   
   0x08048965 <+21>:    lea    0x0(%esi),%esi
   0x08048968 <+24>:    flds   (%edx,%eax,4)
   0x0804896b <+27>:    fmuls  (%ecx,%eax,4)
   0x0804896e <+30>:    flds   0x4(%edx,%eax,4)
   0x08048972 <+34>:    fmuls  0x4(%ecx,%eax,4)
   0x08048976 <+38>:    add    $0x2,%eax
   0x08048979 <+41>:    cmp    %eax,%ebx
   0x0804897b <+43>:    faddp  %st,%st(1)
   0x0804897d <+45>:    faddp  %st,%st(1)
   0x0804897f <+47>:    jg     0x8048968 <dotCPP(float*, float*, int)+24>
   0x08048981 <+49>:    pop    %ebx
   0x08048982 <+50>:    ret    
   0x08048983 <+51>:    fldz   
   0x08048985 <+53>:    pop    %ebx
   0x08048986 <+54>:    ret    
End of assembler dump.

Now am I missing something or should gcc make use of the xmm registers?

I would really appreciate any suggestions that would help me understand why gcc does not generate code that uses the xmm registers.

Please let me know if you need further information on anything.

Answer 1

-march=core2 means that gcc can assume (along with 64 bit ISA) up to SSSE3 (e.g., MMX, SSE, SSE2, SSE3) is available.

-mfpmath=sse can then force the use of SSE for floating-point arithmetic (the default in 64-bit mode), rather than 387 (the default in 32-bit -m32 mode).

See: "Intel 386 and AMD x86-64 Options" section in the man page.

Unfortunately, you still have the limitations of 32-bit mode and a 32-bit ABI. e.g., only registers XMM0 .. XMM7 are available; XMM8 .. XMM15 are only available in 64 bit mode.