Open-source learnc auto-tunes C constants: 9.1% smaller JPEGs, +4.9 F1 on WebRTC VAD
keep_up_sharma · reddit · 2026-10-13
A developer released learnc (MIT), a tool that tunes named constants in unmodified C code against a score function you write in C, then writes the same file back with new values.
Results, both on held-out data:
- JPEG: tuning all 128 quantization-table entries in stbimagewrite.h (12 Kodak images for tuning, 12 for testing) yields 9.1% BD-rate improvement at equal SSIM, independently verified in libjpeg with scikit-image — vs 2.7% for mozjpeg's hand-optimized tables
- WebRTC VAD: tuning every constant in vadcore.c lifts F1 from 0.810 to 0.859 on LibriParty eval
Known limits: no JPEG gains below 1 bpp, and it's best for tens to hundreds of constants, not large weight sets. A drop-in way to squeeze performance out of legacy magic numbers in any C project.
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