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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:
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 single
criteria for heuristic optimizations. Sometimes a new heuristic
optimization might look promising but the reality might be quite different.
In this relation I am remembering a story told me by Bob Morgan about
bin packing RA invention. It was just a quick and simple first RA
implementation for a new compiler. After that DEC compiler team tried
many times to improve the RA implementing more complicated optimizations
but the first bin packing RA was always better.