[LLVMdev] Re: Dependence Analysis

[Chris Lattner]

On Thu, 21 Jul 2005, Naftali Schwartz wrote:
>> If you're interested in dependence analysis, the next important step is 
> to
>> start analyzing distance and direction vectors.
>
> Well, specifically, I was thinking of a mechanism to turn this:

The indvars pass is *intentionally* restricted to only promoting affine 
expressions to array subscripts, not arbitrary expressions.  To enable 
this, remove the two if's at IndVarSimplify.cpp:647 (unconditionally 
pushing the discovered indvar on the IndVars list).

See the comments in that code for a justification.

-Chris

> int A[100], B[100], C[100], X, Y, Z;
>
>        int *p_a = &A[0];
>        int *p_b = &B[0];
>        int *p_c = &C[0];
>
>        int i, j, k, f;
>        for ( k = 0; k < Z; k++ )
>        {
>                p_a = &A[0];
>                for ( i = 0; i < X; i++ )
>                {
>                        p_b = &B[k*Y];
>                        *p_c = *p_a++ * *p_b++;
>                        for ( f = 0; f < Y-2; f++ )
>                                *p_c += *p_a++ * *p_b++;
>                        *p_c++ += *p_a++ * *p_b++;
>                }
>        }
>
> ...into:
>
> int A[100], B[100], C[100], X, Y, Z;
>
>        int i, j, k, f;
>        for ( k = 0; k < Z; k++ )
>                for ( i = 0; i < X; i++ )
>                {
>                        C[X*k+i] = A[Y*i] * B[Y*k];
>                        for (f = 0; f < Y-2; f++ )
>                                C[X*k+i] += A[Y*i+f+1] * B[Y*k+f+1];
>                        C[X*k+i] += A[Y*i+Y-1] * B[Y*k+Y-1];
>                }
>
> a la Frank and O'Boyle, which -indvars seems not to be able to handle (unless 
> I'm doing something wrong...)
>
> Naftali
>
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-Chris

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