Questions in DSPF_sp_fircirc

He Zhang

Hi there,

I'm trying to understand the function from dsplib 'DSPF_sp_fircirc'. Could anyone tell me why '#pragma MUST_ITERATE(2,,)' is here? Thanks!

Here is some code:

    for (i = 0; i < ny; i += 4) 
    {           
        y0 = 0;
        y1 = 0;
        y2 = 0;
        y3 = 0;
        
        x_0 = x[(i + index) & (mod - 1)];
        x_1 = x[(i + 1 + index) & (mod - 1)];
        x_2 = x[(i + 2 + index) & (mod - 1)];
        
        #pragma MUST_ITERATE(2,,)
        for (j = 0; j < nh; j++)
        {
            x_3 = x[(i + j + 3 + index) & (mod - 1)];
            
            y0 += x_0 * h[j];
            y1 += x_1 * h[j];
            y2 += x_2 * h[j];
            y3 += x_3 * h[j];
            
            x_0 = x_1;
            x_1 = x_2;
            x_2 = x_3;
        }
        
        y[i] = y0;
        y[i + 1] = y1;
        y[i + 2] = y2;
        y[i + 3] = y3;
    }

over 9 years ago

0 Sivaraj Kuppuraj over 9 years ago

TI__Mastermind 35645 points

Hi,

Thanks for your post.

To understand more on DSPF_sp_fircirc kernel which performs FIR filtering on circular input array x with coefficient array h, please see the C67x DSPLIB reference guide below:

http://www.ti.com/lit/ug/spru657c/spru657c.pdf (page no 75)

http://processors.wiki.ti.com/index.php/C674x_DSPLIB#DSPF_sp_fircirc_.28FIR_Filter_with_Circular_Input.29

Please be aware that the #pragma MUST_ITERATE informs the compiler about the loop behaviour, but it is the optimizer that makes the final decision about the generated code. To know the usage of "MUST_ITERATE Pragma", please refer section 6.8.15 in the optimizing compiler user guide as below:

http://www.ti.com/lit/ug/spru187o/spru187o.pdf

Additional information about hand-optimizing loops can be obtained in this interesting app note (SPRA666) which, despite talking about another DSP family (C6000), explains the use of these two pragmas (page 19 and 20) and shows considerations about their use.

http://www.ti.com/lit/an/spra666/spra666.pdf

Thanks & regards,

Sivaraj K

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