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Re: 50% slowdown with LTO

From: Andi Kleen <andi at firstfloor dot org>
To: Ian Lance Taylor <iant at google dot com>
Cc: Paul_Koning at dell dot com, gcc at gcc dot gnu dot org
Date: Mon, 13 Aug 2012 09:51:45 -0700
Subject: Re: 50% slowdown with LTO
References: <C75A84166056C94F84D238A44AF9F6AD188F79@AUSX10MPC102.AMER.DELL.COM> <CAKOQZ8w86yQyY6PjYJyNDJFaHe9wQHhjGjesKJmJryGeSASrtw@mail.gmail.com>

Ian Lance Taylor <iant@google.com> writes:
>
> Figuring out what has gone wrong is like optimizing any program.  Get
> a profile for your program, e.g., using -pg.  Build the program with
> and without -flto, run it, and look at the resulting profiles.  A 50%
> slowdown should be fairly obvious.  I would guess that GCC has made a
> poor inlining decision, but the profile should show the problem for
> sure.

On modern profiling tools like perf or oprofile you can also
"diff" profiles for this.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only

References:
- 50% slowdown with LTO
  - From: Paul_Koning
- Re: 50% slowdown with LTO
  - From: Ian Lance Taylor

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