librelane playground

No. 115 / project of 147 on the ladder

Frontend target queue

introduces — FTQ metadata shadow; predicted-target record; frontend steering scaffold

harden statelast run2026-05-06
signoff
  • DRCNOT RUN
  • LVSNOT RUN
  • antennaNOT RUN
artifacts

P115 adds a one-entry frontend target queue shadow beside the existing fetch queue. It carries predicted-target metadata without yet allowing the predictor to redirect fetch.

checkresult
Verilator buildPASS
BusyBox shell workload reaches P115-FILE-OKPASS
FTQ fill/consume accountingPASS
Hardened layoutNOT RUN
countervalue
FTQ fills54,240,789
FTQ consumes54,240,789
FTQ flushes0
valid at end0
metricP114P115
shell window cycles64,501,12765,309,216
S_FETCH cycles7,617,4677,635,432
S_MEM cycles27,708,07527,816,606

This is a scaffold rung, not a speedup claim. The useful result is that the frontend can now carry target metadata through the queue exactly once per queued instruction, which gives P116 a real structure to steer from and flush when wrong.

shell phases label P115 shell workload cycles 219,240,193 cpi 2.53
  1. kernel banner to /init 116,715,436 53.4%
  2. /init to shell banner 1,066,766 0.5%
  3. shell banner to first command 35,520,708 16.3%
  4. echo command 1,649 0%
  5. uname -a 2,576,654 1.2%
  6. ls /bin /usr/share 31,863,870 14.6%
  7. cat sample file 3,038,222 1.4%
  8. touch/write/cat/rm /tmp file 11,583,651 5.3%
  9. 8x ash loop with file I/O 16,244,490 7.4%
  10. final marker 680 0%
state breakdown label P115 FTQ workload cycles 219,240,193 cpi 2.53
  1. fetch 3.5% 7,635,432
  2. execute 39.5% 86,533,538
  3. mem 12.8% 28,096,752
  4. walker 1.2% 2,700,540
  5. writeback 39.5% 86,508,622
  6. mul/div 3.5% 7,763,593