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Re: Live range shrinkage in pre-reload scheduling

From: Vladimir Makarov <vmakarov at redhat dot com>
To: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
Cc: Maxim Kuvyrkov <maxim dot kuvyrkov at linaro dot org>
Date: Wed, 14 May 2014 09:17:49 -0400
Subject: Re: Live range shrinkage in pre-reload scheduling
Authentication-results: sourceware.org; auth=none
References: <5371F395 dot 8050208 at arm dot com>

On 2014-05-13, 6:27 AM, Kyrill Tkachov wrote:

Hi all,

In haifa-sched.c (in rank_for_schedule) I notice that live range
shrinkage is not performed when SCHED_PRESSURE_MODEL is used and the
comment mentions that it results in much worse code.

Could anyone elaborate on this? Was it just empirically noticed on x86_64?

It was empirically noticed on SPEC2000. The practice is a singlecriteria for heuristic optimizations. Sometimes a new heuristicoptimization might look promising but the reality might be quite different.

In this relation I am remembering a story told me by Bob Morgan aboutbin packing RA invention. It was just a quick and simple first RAimplementation for a new compiler. After that DEC compiler team triedmany times to improve the RA implementing more complicated optimizationsbut the first bin packing RA was always better.

Follow-Ups:
- Re: Live range shrinkage in pre-reload scheduling
  - From: Richard Sandiford

References:
- Live range shrinkage in pre-reload scheduling
  - From: Kyrill Tkachov

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