llc -O2 vs. llc -O3 --> same debug-pass=Executions but output.obj differs?


I’m trying to minimize the processing time for llc -O3 by using a three step compilation process of

  1. llc input.bc -stopafter=targetlibinfo -o input.mir
  2. llc -run-pass={…min passes…} input.mir -o opt.mir
  3. llc -startafter=machine-opt-remark-emitter -filetype=obj opt.mir -o final.obj

Examining the passes produced by llc for O1,O2,O3 I compared (with XXX = {1,2,3}):

llc input.bc -filetype=obj -OXXX -o XXXfinal.obj -debug-pass=Executions

This showed the passes were identical for O1,O2,O3 yet the .obj produced was different for O3 versus O1 and O2 (which were the same).

Does anyone happen to know why O3 is different, I’m wondering if this is caused by some other llc flag unrelated to the passes which appear to be the same looking at -debug-pass=Executions.

Thanks for considering this,



Thank you!

For my code at least, adding -tail-dup-placement-aggressive-threshold=4 makes the output match O3.

I understand this is fragile though as there could be other spots in the code doing something special for O3, and perhaps not exposing the equivalent flag.

I really appreciate your help,