[llvm-dev] Replication -O3 optimizations manually

[Han Cui via llvm-dev]

Hi Philip,


I would like to try different pass sequences with opt and see how they affect program optimization, and I was thinking replicating the O3 sequence would be a start point.


Thanks,

Han

________________________________
From: Philip Pfaffe <philip.pfaffe at gmail.com>
Sent: 21 October 2018 11:36:07
To: Han Cui
Cc: llvm-dev
Subject: Re: [llvm-dev] Replication -O3 optimizations manually

Hi Han,

What is your ultimate goal here?

Cheers,
Philip

On Fri, Oct 19, 2018 at 9:17 PM Han Cui via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi,



I’m having a very similar question to this thread<http://lists.llvm.org/pipermail/llvm-dev/2018-August/125416.html>, about replicating the O3 optimization processing manually using opt.



This is my setup:



Process A:

  1.  clang -O1 -S -emit-llvm main.c -o main.ll
  2.  opt -O3 main.ll -o main-opt1.ll
  3.  clang -O1 main-opt1.ll -o main-opt1



Process B:

  1.  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

  1.  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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