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Getting the compiler to consistently use LDM ?

Christoph Franck

Hello,

I am trying to optimize the implementation of a FIR filter. One major improvement would be to use LDM for loading all data/coefficients to minimize the amount of cycles used for memory access.

How can I get the compiler do this consistently ?

long fir_coef[] = ...;
long dbuf = ...;
long out_buf;

void test(void)
{
register long c1, c2, c3, c4;
register long d1, d2, d3, d4;
registerlong accu;

c1 = fir_coef[0];
c2 = fir_coef[1];
c3 = fir_coef[2];
c4 = fir_coef[3];
d1 = dbuf[0];
d2 = dbuf[1];
d3 = dbuf[2];
d4 = dbuf[3];

accu = 0;
accu += c1 * d1;
accu += c2 * d2;
accu += c3 * d3;
accu += c4 * d4;

out_buf = accu;
}

seems to use LDM sporadically to load two registers at once (at -O3), but ideally I would like to see only two LDM instructions in the above code.

Can this be done in C, or is it time to get out the assembler ?

Parents
  • 0 in reply to Per Westermark

    I unrolled the innermost loops (the filter acts on several channels of data, and produces several output sample per call), since the ARM architecture does not have zero-overhead-looping functions.

    However, I found working with pointers that are incremented actually slows things down since after every output sample, I need to reset the coefficient pointer to the start of the filter. Instead, I used regular indexing:

    acc = a[0] * b[0];
acc += a[1] * b[1];
...

    since it does not matter to the processor whether it writes back the modified address or just uses a temporary index.

Reply
  • 0 in reply to Per Westermark

    I unrolled the innermost loops (the filter acts on several channels of data, and produces several output sample per call), since the ARM architecture does not have zero-overhead-looping functions.

    However, I found working with pointers that are incremented actually slows things down since after every output sample, I need to reset the coefficient pointer to the start of the filter. Instead, I used regular indexing:

    acc = a[0] * b[0];
acc += a[1] * b[1];
...

    since it does not matter to the processor whether it writes back the modified address or just uses a temporary index.

Children
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