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d514ee9282
Cuts realtime decode on the Sansa e200v1 from 52.1 MHz to 50.8 MHz. clt_mdct_backward was the largest remaining item at 13.5% of decode. Only the three inner loops move to assembly. The setup stays in C, so mdct.c remains readable and the assembly needs no knowledge of mdct_lookup. What the compiled loops lose is registers. Each needs more live values than gcc can hold, so it spills the loop-invariant pointers, strides and limits and reloads them every pass: five stack accesses per iteration in the post-rotation alone. Holding the twiddle as a 16-bit value and accumulating the product pair with smull/smlal is what makes the bookkeeping fit, needing seven live registers where the shifted MULT16_32_Q15 form needs nine. ldm/stm helps only where the addressing allows. The post-rotation walks the buffer from both ends and so reads and writes contiguous pairs. The pre-rotation reads the spectrum through a runtime stride and writes through the bitrev table, so only its 8-byte output pair merges, and the TDAC mirror merges nothing. Over 160 ms of stereo music, traced under qemu: clt_mdct_backward 1,037,962 -> 900,982 -13.2% whole decode 7,695,876 -> 7,558,896 -1.8% loads 650,157 -> 611,667 -5.9% stores 350,605 -> 323,605 -7.7% multiplies 337,493 -> 337,493 unchanged Accuracy improves substantially, because all three loops keep 32 bits of each Q15 product where MULT16_32_Q15_armv4 drops the low bit, and the backward MDCT applies three such rounds per sample. The rounding SNR of the backward transform rises about 9.5 dB, and its worst case error falls from 708 to 186. Decoded output differs from the previous build in 90 of 15,360 samples, each by one LSB. Build with OPUS_ARM_NO_MDCT_ASM to select the C loops instead. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Change-Id: I3c4404b4dbe581d8bcf1f357266a658a068fdb50 |
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| arm_support | ||
| fixedpoint | ||
| libsetjmp | ||
| microtar | ||
| mipsunwinder | ||
| rbcodec | ||
| skin_parser | ||
| tlsf | ||
| unwarminder | ||
| utf8proc | ||