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

[Chad Verbowski via llvm-dev]

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,
-Chad

On Tue, Aug 4, 2020 at 6:58 PM Mehdi AMINI <joker.eph at gmail.com> wrote:

> Hi,
>
>
> 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:
> https://github.com/llvm/llvm-project/blob/master/llvm/lib/CodeGen/MachineBlockPlacement.cpp#L3330
>
> --
> Mehdi
>
>
>
>>
>> Thanks for considering this,
>>
>> -Chad
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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