[music-dsp] C++ performance
rbj at audioimagination.com
Wed Oct 27 11:34:06 EDT 2010
On Oct 27, 2010, at 7:59 AM, Thomas Strathmann wrote:
> Some generic advice first:
> 1. Get to know your compiler and library and write "common sense" code
> that the compiler knows how to deal with in an (near to) optimal
this i agree with.
> 4. To reiterate: Don't think your cleverer than the people who wrote
> the compiler and the runtime and don't try to port idioms from
> other languages in the hope of making your code run faster.
i don't think i agree with this.
here is an example (a simple one-pole LPF with noise shaping):
// transfer function: H(z) = b0/(1 - (a1+1)*z^(-1))
void myProject::LPF(LPFBlock* this_filter, long* input)
register long* output_ptr = &(this_filter->output);
register long b0 = this_filter->b0; // feedforward
register long a1 = this_filter->a1; // feedback
register long long y1 = this_filter->y1; // previous output
Q16.48, roundoff noise state in lower 24 bits
register long output_sample = (long)(y1>>24); // now is
previous output sample, y[n-1]
for (register int i=CHUNK_SIZE; i>0; i--)
y1 += (long long)b0 * (long long)(*input++); // (y[n-1]
+ b0*x[n]) * 2^24
y1 += (long long)a1 * (long long)output_sample; // (y[n-1]
+ b0*x[n] + a1*y[n-1]) * 2^24
output_sample = (long)(y1>>24); // this truncation has
simple roundoff noise shaping
*output_ptr++ = output_sample;
this_filter->y1 = y1; // save state, including
roundoff error state
the use of register variables, the use of simple arithmetic lines with
"+=" in the loop is there to make sure the compiler has no doubt what
i want it to do. the fact that i have "i" count down to zero rather
than up to CHUNK_SIZE is to prevent the compiler from loading
CHUNK_SIZE and subtracting, it only needs to compare to zero.
sometimes you *should* spell it out for the compiler. i only wish the
promotion to (long long) was unnecessary. i wish that in C and C++,
it was automatically understood that the type of the result of
multiplying two N-bit numbers was a type with a 2N-bit number if a 2N-
bit type is available. that is one flaw of C or C++.
r b-j rbj at audioimagination.com
"Imagination is more important than knowledge."
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