[tree-ssa] performance with loops

[Toon Moene]

Steven Bosscher wrote:

> 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);
> }

Hmm, how about initializing the data you use (malloc just allocates the 
space) - without it you could run into NaNs which would distort the 
timing picture completely.

BTW, what's wrong with:

PROGRAM TEST
REAL, ALLOCATABLE :: A(:,:,:), B(:,:,:), C(:,:,:)
READ*,L,M,N
ALLOCATE(A(L,M,N),B(L,N,M),C(L,N,M))
A=1.0;B=2.0;C=A+B
PRINT*,SUM(C)
END PROGRAM TEST

-- 
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
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