SOLVED: 20% and up to 32% faster fp32 FFT in ASM for ESP32 and ESP32-S3 (AUD-5911)

[f4lc0n-asm]

Hello,

here are the 20% and up to 32% faster ASM drop-in replacements for dsps_fft2r_fc32_ae32_.S and dsps_fft2r_fc32_aes3_.S from ESP-DSP repository.

I achieved this by:

• Replacing slli / add.n pair with addx8 on 3 occasions.
• Carefully reordering instructions for much fewer pipeline stalls and preventing the assembler to automatically insert a NOP in front of a hw loop so that the first instruction in the hw loop entirely fits within a naturally aligned four byte region. Consult Xtensa ISA Summary (LOOPNEZ description) for more info - it's vital for achieving the maximum loop speed!
• Removing some superfluous instructions (forgotten debugging stuff).

The following table shows the number of cycles for various FFT routines:

   N |   ANSI |  ESP32 | ESP32 O|ESP32-S3|ESP32-S3 O
  16 |   1277 |    977 |    817 |    930 |    784
  32 |   2972 |   2318 |   1934 |   2207 |   1853
  64 |   6835 |   5379 |   4483 |   5124 |   4290
 128 |  15514 |  12264 |  10216 |  11689 |   9767
 256 |  34785 |  27565 |  22957 |  26286 |  21932
 512 |  77160 |  61234 |  50994 |  58419 |  48689
1024 | 169583 | 134711 | 112183 | 128568 | 107062
2048 | 369782 | 293948 | 244796 | 280637 | 233531
4096 | 800893 | 636993 | 530497 | 608322 | 505920

Note 1: Passing the 3rd argument float* dsps_fft_w_table_fc32 to FFT functions is actually a good idea. Because when you do parallel computations of say 2 FFTs with different Ns, you need 2 different table pointers for them. The table pointer cannot be thus hardcoded in the FFT functions, as attempted.

Note 2: Optimized ESP32-S3 ASM version is just 11-15% faster than optimized ESP32 ASM version because of some pipeline stalls which cannot be avoided.

Cheers!

f4lc0n

fft2r_fp32_xtensa_v1.2.zip (7-Zip)
Improved: Further speed optimizations of dsps_fft2r_fc32_aes3_.S. Now it is up to 32% faster (N=4096)! The new performance table (see the rightmost column):

   N |   ANSI |  ESP32 | ESP32 O|ESP32-S3|ESP32-S3 O
  16 |   1277 |    977 |    817 |    930 |    735
  32 |   2972 |   2318 |   1934 |   2207 |   1724
  64 |   6835 |   5379 |   4483 |   5124 |   3969
 128 |  15514 |  12264 |  10216 |  11689 |   8998
 256 |  34785 |  27565 |  22957 |  26286 |  20139
 512 |  77160 |  61234 |  50994 |  58419 |  44592
1024 | 169583 | 134711 | 112183 | 128568 |  97845
2048 | 369782 | 293948 | 244796 | 280637 | 213050
4096 | 800893 | 636993 | 530497 | 608322 | 460863