[llvm-dev] Replication -O3 optimizations manually

[Michael Kruse via llvm-dev]

Hi,

it is not possible (or at least difficult) to entirely get the same
behavior. This is because the pass manager builder for the
optimization levels add the passes with additional options. See
PassManagerBuilder.cpp. For instance, the Loop Unroll pass takes the
following options:

LoopUnroll(int OptLevel = 2, Optional<unsigned> Threshold = None,
             Optional<unsigned> Count = None,
             Optional<bool> AllowPartial = None, Optional<bool> Runtime = None,
             Optional<bool> UpperBound = None,
             Optional<bool> AllowPeeling = None)

It is added twice in the typical pipeline: Once to only unroll loops
completely, and another time to allow more general unrolling. What
adding it to the pass pipeline using opt -loop-unroll, it is added
with the default options.
Most passes have command line options to control this behaviour as
well (such as -unroll-runtime), but this would apply to both instances
of the pass. You would have to invoke opt multiple times with
different options for each instance, but this would also split the
module/function/loop pass hierarchy and different preserved analysis
passes (such as alias analysis which have to be re-added again).

Michael



Am Fr., 19. Okt. 2018 um 14:18 Uhr schrieb Han Cui via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> Hi,
>
>
>
> I’m having a very similar question to this thread, about replicating the O3 optimization processing manually using opt.
>
>
>
> This is my setup:
>
>
>
> Process A:
>
> clang -O1 -S -emit-llvm main.c -o main.ll
> opt -O3 main.ll -o main-opt1.ll
> clang -O1 main-opt1.ll -o main-opt1
>
>
>
> Process B:
>
> clang -O1 -S -emit-llvm main.c -o main.ll
>
> 2.  opt [opt flags given by command ‘opt -O3 -disable-output -debug-pass=Arguments main.ll’] main.ll -o main-opt2.ll
>
> clang -O1 main-opt2.ll -o main-opt2
>
>
>
> I tried this with a set of programs and I noticed the execution time of main-opt2 is often much slower than main-opt1. While ‘main.ll’ is the same, ‘main-opt1.ll’ is different to ‘main-opt2.ll’. May I get some suggestions what I should put for opt (to replace the command in the brackets), so that the two process would have identical effects? Any help would be much appreciated.
>
>
>
> Thanks,
>
> Han
>
>
>
>
>
>
>
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