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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 ?
It is not generally a good idea to declare variables with the register qualifier. The RV compiler is a very good optimizing compiler, and you will hamper its decisions when you force register variables.<p>
I believe the compiler will ignore the register qualifier anyway. I put it there to show which variables I'd like to see put in registers.
If you want to go to assembly, you have a few alternatives: a) you can use the inline assembler and code a 'virtual registers' version of the algorithm, since it is a very straightforward computation,
Is there any explanation in the docs about these "virtual registers" ? Any time I try to use inline assembly, I keep running into
main01.c(242): warning: #d1267-D: Implicit physical register R0 should be defined as a variable
warnings, and an
main01.c(242): error #549: variable "R0" is used before its value is set
error when I try to access one of the function arguments which should be passed in R0. I believe this has something to do with "virtual registers", or do I need to do anything to be able to use the "raw" registers in my inline assembly (like all of the examples seem to do) ?
Ok, I think I found it. I should have checked the RealView docs earlier instead of looking at the CARM docs.
Silly me.
Ok, after playing around with the source code for a while, the "pure C" version of the filter runs 4% faster than the version with optimized embedded C.
Back to the drawing board.
It might sometimes be good to do something like:
acc += *a++ * *b++; acc += *a++ * *b++; acc += *a++ * *b++; acc += *a++ * *b++;
when filtering data. A lot depens on what external loops you need, i.e. how much code is part of a filter kernel in comparison to the amount of iterations with sample data.
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.