[Bug target/18767] New: No vectorization for simple loop

[bangerth at dealii dot org]

As observed in PR 17619: 
 
Take this code: 
--------------------------------  
struct X { float array[4]; };  
  
X a,b;  
  
float foobar () {  
  float s = 0;  
  for (unsigned int d=0; d<4; ++d)  
    s += a.array[d] * b.array[d];  
  return s;  
}  
--------------------------  
It compiles to 
-------------------------- 
        flds b+12 
        fmuls   a+12 
        movss   b, %xmm1 
        mulss   a, %xmm1 
        addss   .LC0, %xmm1 
        movss   b+4, %xmm0 
        mulss   a+4, %xmm0 
        addss   %xmm0, %xmm1 
        movss   b+8, %xmm0 
        mulss   a+8, %xmm0 
        addss   %xmm0, %xmm1 
        movss   %xmm1, -4(%ebp) 
        flds -4(%ebp) 
        faddp   %st, %st(1) 
-------------------------- 
 
However, what should really happen is that the compiler should vectorize 
the loop. As Uros points out in PR 17619, this isn't happening, although 
this modified function here 
-------------------------- 
struct X 
{ 
  float array[4]; 
}; 
 
float foobar() 
{ 
  X a, b, c; 
 
  float s = 0; 
  for (unsigned int d = 0; d < 4; ++d) 
    c.array[d] = a.array[d] * b.array[d]; 
 
  for (unsigned int d = 0; d < 4; ++d) 
    s += c.array[d]; 
 
  return s; 
} 
-------------------------- 
generates the optimal code: 
-------------------------- 
        movaps  32(%esp), %xmm0 
        mulps   16(%esp), %xmm0 
        movaps  %xmm0, (%esp) 
        flds    4(%esp) 
        fadds   (%esp) 
        fadds   8(%esp) 
        fadds   12(%esp) 
-------------------------- 
The compiler should be able to make this transformation by itself. 
 
Thanks 
 W. 
--------------------------

-- 
           Summary: No vectorization for simple loop
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bangerth at dealii dot org
                CC: gcc-bugs at gcc dot gnu dot org,uros at gcc dot gnu dot
                    org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18767