Robert Dewar - Re: [RFC] Enabling loop unrolls at -O3? (original) (raw)

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• From: Robert Dewar
• To: Jakub Jelinek
• Cc: Giovanni Bajo , Gabriel Dos Reis ,Steven Bosscher , gcc at gcc dot gnu dot org
• Date: Sun, 06 Nov 2005 09:20:21 -0500
• Subject: Re: [RFC] Enabling loop unrolls at -O3?
• References: <436D712E.3050704@adacore.com> <m3hdaq9d8q.fsf@uniton.integrable-solutions.net> <04f801c5e2ce$2f629f60$09be2997@bagio> <20051106124024.GC16723@sunsite.mff.cuni.cz>

Jakub Jelinek wrote:

Including loop unrolling to -O2 is IMNSHO a bad idea, as loop unrolling increases code size, sometimes a lot. And the distinction between -O2 and -O3 is exactly in the space-for-speed tradeoffs.

That's certainly a valid way of defining the difference (and certainly used to be the case in the old days when the principle extra optimization was inlining)

On many CPUs for many programs, -O3 generates slower code than -O2, because the cache footprint disadvantages override positive effects of the loop unrolling, extra inlining etc.

That's what we have found, though I would have thought it unusual that loop unrolling would run into this cache effect in most cases.

Jakub

• Follow-Ups:
  • Re: [RFC] Enabling loop unrolls at -O3?
    * From: Mattias Engdeg�rd
• References:
  • Re: [RFC] Enabling loop unrolls at -O3?
    * From: Robert Dewar
  • Re: [RFC] Enabling loop unrolls at -O3?
    * From: Gabriel Dos Reis
  • Re: [RFC] Enabling loop unrolls at -O3?
    * From: Giovanni Bajo
  • Re: [RFC] Enabling loop unrolls at -O3?
    * From: Jakub Jelinek