"You can't beat a good compiler... right?"

[pocopito]

Hi all

I just found this blog entry about how to improve a piece of C code by studying the assembly output:

http://lbrandy.com/blog/2010/06/you-can ... d-compile/

What called my attention was the simpleness of the improved code and the "improving factor" (almost 5 times faster). The original code looks like:

Code: Select all

#define LENGTH 5000

// a bad impersonation of a gaussian filter
const float filter[] = {0.01f, 0.2f, 0.58f, 0.2f, 0.01f};

void naive(float* in, float* out)
{
  int i,j;
  for (i=0;i<LENGTH-4;i++)
  {
    out[i]=0.0f;
    for (j = 0;j<5;j++)
      out[i] += filter[j] * in[i+j];
  }
}

My knowledge on the topic is almost null, so probably it's not that interesting, but just in case I post it in the forum.

Best

E Diaz

[mar]

Hi,
this is about aliasing. The compiler doesn't know wheter in and out overlap. You can explicitly tell with restrict (a hint), but you have no guarantee that your compiler actually supports it.
Any (sane) programmer would use a temporary variable instead, which will be stored in a register:

Code: Select all

void naive(float* in, float* out)
{
  int i,j;
  for (i=0;i<LENGTH-4;i++)
  {
    float temp=0;
    for (j = 0;j<5;j++)
      temp += filter[j] * in[i+j];
    out[i] = temp;
  }
}

[lucasart]

Hi,
this is about aliasing. The compiler doesn't know wheter in and out overlap. You can explicitly tell with restrict (a hint), but you have no guarantee that your compiler actually supports it.
Any (sane) programmer would use a temporary variable instead, which will be stored in a register:

Code: Select all

void naive(float* in, float* out)
{
  int i,j;
  for (i=0;i<LENGTH-4;i++)
  {
    float temp=0;
    for (j = 0;j<5;j++)
      temp += filter[j] * in[i+j];
    out[i] = temp;
  }
}

Thanks, that's interesting to know. I didn't read the article, but after looking at the initial code sample, I didn't see what was wrong with it. I naively assumed that out would be seen by the compiler as a loop invariant, but as you say, that's only OK if in and out do not overlap.

Something to think about when writing performance critical code.