Performance: Optimize FFT stage inner loops via loop interchange and variable hoisting#159
Performance: Optimize FFT stage inner loops via loop interchange and variable hoisting#159ysdede wants to merge 1 commit into
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…y lookups and caching typed array accesses.
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Code Review
This pull request optimizes the FFT implementation in src/mel.js by interchanging nested loops to hoist twiddle factor lookups and caching TypedArray accesses into local variables to reduce overhead. These performance improvements are also documented in .jules/bolt.md. Feedback suggests further micro-optimizations, including hoisting the tw.cos and tw.sin property lookups outside the loops and refactoring the inner loop to use p as the iterator to eliminate redundant additions.
| for (let k = 0; k < halfLen; k++) { | ||
| const twIdx = k * step; | ||
| const wCos = tw.cos[twIdx]; | ||
| const wSin = tw.sin[twIdx]; | ||
| for (let i = 0; i < N; i += len) { | ||
| const p = i + k; | ||
| const q = p + halfLen; | ||
| const tRe = re[q] * wCos - im[q] * wSin; | ||
| const tIm = re[q] * wSin + im[q] * wCos; | ||
| re[q] = re[p] - tRe; | ||
| im[q] = im[p] - tIm; | ||
| re[p] += tRe; | ||
| im[p] += tIm; | ||
| const req = re[q]; | ||
| const imq = im[q]; | ||
| const tRe = req * wCos - imq * wSin; | ||
| const tIm = req * wSin + imq * wCos; | ||
| const rep = re[p]; | ||
| const imp = im[p]; | ||
| re[q] = rep - tRe; | ||
| im[q] = imp - tIm; | ||
| re[p] = rep + tRe; | ||
| im[p] = imp + tIm; | ||
| } | ||
| } |
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To further optimize this performance-critical loop, you can hoist the tw.cos and tw.sin array references out of the k loop to avoid repeated property lookups on the tw object. Additionally, using p as the loop variable directly eliminates the redundant i + k addition in the innermost loop, which is a common micro-optimization in FFT implementations.
const twCos = tw.cos;
const twSin = tw.sin;
for (let k = 0; k < halfLen; k++) {
const twIdx = k * step;
const wCos = twCos[twIdx];
const wSin = twSin[twIdx];
for (let p = k; p < N; p += len) {
const q = p + halfLen;
const req = re[q];
const imq = im[q];
const tRe = req * wCos - imq * wSin;
const tIm = req * wSin + imq * wCos;
const rep = re[p];
const imp = im[p];
re[q] = rep - tRe;
im[q] = imp - tIm;
re[p] = rep + tRe;
im[p] = imp + tIm;
}
}1 participant
What changed
In
src/mel.js, within the nested loops for the Fast Fourier Transform (FFT) stages, theiandkloops were swapped (loop interchange) to hoistwCosandwSintwiddle factor computations out of the innermost loop. Additionally, array reads and writes for thereandimFloat32Arrays inside the hot butterfly loop were cached into local variables. A.jules/bolt.mdjournal entry was appended to capture this learning.Why it was needed
V8 Javascript engines struggle to properly optimize deeply nested, tight numerical loops when there are constant indirect lookup costs (
tw.cos[twIdx]) occurring pointlessly inside the primary loop for operations that only vary with an outer variable. Profiling identified thefftstage butterfly loop as heavily bound by TypedArray element access.Impact
Benchmark simulations comparing the nested baseline structure against the swapped loop and local var cache reduced execution times significantly. Over 5000 executions of a dummy size-4096 spectrum, time spent fell from ~1192 ms to ~833 ms, a roughly ~30% improvement in this highly specific routine.
How to verify
npm testPR created automatically by Jules for task 12804144867430842509 started by @ysdede
Summary by Sourcery
Optimize FFT stage computation loops to reduce TypedArray access overhead and improve performance.
Enhancements:
Documentation: