I am implementing a FIR filter on an ARM9 processor and am trying to use the SMLAL instruction.
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.
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 & 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<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 >> 9);
// decrement state, wrapping if below zero
if ( --filterState_8K < 0 )
filterState_8K += NTAPS_8K;
return a;
}
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:
uint32_t accum_low = 0;
int32_t accum_high = 0;
for (i =0; i<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 << 32 | (accum_low);
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.
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.
In computer programming, an inline assembler is a feature of some compilers that allows low-level code written in assembly language to be embedded within a program, among code that otherwise has been compiled from a higher-level language such as C or Ada.
Inline assembly (typically introduced by the asm keyword) gives the ability to embed assembly language source code within a C program. Unlike in C++, inline assembly is treated as an extension in C.
Which compiler version are you using? I tried compiling your C only code using GCC 4.4.3 using the options -O3 -march=armv5te and it generated the smlal instructions.
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