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POUsing Multiply Accumulate Instruction Inline Assembly in C++
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  1. POUsing Multiply Accumulate Instruction Inline Assembly in C++
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    copied!<p>I am implementing a FIR filter on an ARM9 processor and am trying to use the SMLAL instruction. </p> <p>Initially I had the following filter implemented and it worked perfectly, except this method uses too much processing power to be used in our application. </p> <pre><code>uint32_t DDPDataAcq::filterSample_8k(uint32_t sample) { // This routine is based on the fir_double_z routine outline by Grant R Griffin // - www.dspguru.com/sw/opendsp/alglib.htm int i = 0; int64_t accum = 0; const int32_t *p_h = hCoeff_8K; const int32_t *p_z = zOut_8K + filterState_8K; /* Cast the sample to a signed 32 bit int * We need to preserve the signdness of the number, so if the 24 bit * sample is negative we need to move the sign bit up to the MSB and pad the number * with 1's to preserve 2's compliment. */ int32_t s = sample; if (s &amp; 0x800000) s |= ~0xffffff; // store input sample at the beginning of the delay line as well as ntaps more zOut_8K[filterState_8K] = zOut_8K[filterState_8K+NTAPS_8K] = s; for (i =0; i&lt;NTAPS_8K; ++i) { accum += (int64_t)(*p_h++) * (int64_t)(*p_z++); } //convert the 64 bit accumulator back down to 32 bits int32_t a = (int32_t)(accum &gt;&gt; 9); // decrement state, wrapping if below zero if ( --filterState_8K &lt; 0 ) filterState_8K += NTAPS_8K; return a; } </code></pre> <p>I have been attempting to replace the multiply accumulate with inline assembly since GCC is not using a MAC instruction even with optimization turned on. I replaced the for loop with the following: </p> <pre><code>uint32_t accum_low = 0; int32_t accum_high = 0; for (i =0; i&lt;NTAPS_4K; ++i) { __asm__ __volatile__("smlal %0,%1,%2,%3;" :"+r"(accum_low),"+r"(accum_high) :"r"(*p_h++),"r"(*p_z++)); } accum = (int64_t)accum_high &lt;&lt; 32 | (accum_low); </code></pre> <p>The output I now get using the SMLAL instruction is not the filtered data I was expecting. I have been getting random values that seem to have no pattern or connection to the original signal or the data I am expecting. </p> <p>I have a feeling I am doing something wrong with splitting the 64 bit accumulator into the high and low registers for the instruction, or I am putting them back together wrong. Either way I not sure why I am not able to get the correct output by swapping the C code with the inline assembly. </p>
 

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