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

Mehdi AMINI via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 4 18:57:38 PDT 2020


On Tue, Aug 4, 2020 at 5:19 PM Chad Verbowski via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
> 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.

The opt flag (-O0 -> -O3) are impacting more than the list of passes, it
would also impact some threshold for some optimizations for example (both
for opt and llc).

For example in the backend here as branch on O3:


> Thanks for considering this,
> -Chad
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