[tree-ssa] performance with loops

[Steven Bosscher <s dot bosscher at student dot tudelft dot nl>]

Diego,

A C++ fluid dynamics code I am working with performs really bad when
compiled with tree-ssa compared to mainline.  It takes about 3 times as
long for tree-ssa (that's the difference between running a simulation
overnight or having to wait a whole day and not being able to use your
computer...).

I have tried to narrow down the code to a small test case, and I have
found an example that shows ~66% slowdown; the code I used for these
timings is attached.

Timings for tree-ssa:
real    0m6.852s	0m6.846s	0m6.882s
user    0m6.680s	0m6.690s	0m0.160s
sys     0m0.170s	0m6.690s	0m0.190s

Timings for mainline:
real    0m4.090s	0m4.086s	0m4.087s
user    0m3.910s	0m3.880s	0m3.870s
sys     0m0.180s	0m0.200s	0m0.220s

Ratio:	1,67		1.68		1.68

(The machine is an Athlon XP2000, 256MB ram)

Maybe something like this can also explain the slowdown in 183.equake
(which is the only SPECfp2000 benchmark that slows down with
tree-ssa)???

Gr.
Steven


---------------------------------------
#define L 1000
#define W 200
#define H 200

float ***data1, ***data2, ***data3;

void __attribute__((noinline))
foo (void)
{
  int i, j, k;

  for (i = 0; i < L; i++)
    for (j = 0; j < W; j++)
      for (k = 0; k < H; k++)
	{
	  data1[i][j][k] = data2[i][j][k] * data3[i][j][k];
	}
}

int
main (void)
{
  float ***x;
  int i, j;

  x = (float ***) malloc (L*sizeof(float**));
  for (i = 0; i < L; i++)
    {
      x[i] = (float **) malloc (W*sizeof(float*));
      for (j = 0; j < W; j++)
	x[i][j] = (float *) malloc (H*sizeof(float));
    }
  data1 = data2 = data3 = x;

  /* Loops 10 times to spread the
     overhead of the malloc.  */
  for (i = 1; i < 10; i++)
    foo ();
  free (data1);
}