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Date:      Sun, 26 Oct 2003 12:46:48 +0100
From:      Alexander Leidinger <Alexander@Leidinger.net>
To:        des@des.no (Dag-Erling =?ISO-8859-1?Q?Sm=F8rgrav?=)
Cc:        cvs-src@freebsd.org
Subject:   Re: cvs commit: src/sys/i386/i386 pmap.c
Message-ID:  <20031026124648.6dcdb7d4.Alexander@Leidinger.net>
In-Reply-To: <xzp3cdgb6am.fsf@dwp.des.no>
References:  <20031026064145.18F0E2A8D5@canning.wemm.org> <xzp3cdgb6am.fsf@dwp.des.no>

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On Sun, 26 Oct 2003 12:13:37 +0100
des@des.no (Dag-Erling Sm=F8rgrav) wrote:

> Is there any way to teach (or trick) gcc to generate a branch which
> the p4 will predict correctly?

First of all:
---snip---
`-fno-guess-branch-probability'
     Do not guess branch probabilities using a randomized model.

     Sometimes gcc will opt to use a randomized model to guess branch
     probabilities, when none are available from either profiling
     feedback (`-fprofile-arcs') or `__builtin_expect'.  This means that
     different runs of the compiler on the same program may produce
     different object code.

     In a hard real-time system, people don't want different runs of the
     compiler to produce code that has different behavior; minimizing
     non-determinism is of paramount import.  This switch allows users
     to reduce non-determinism, possibly at the expense of inferior
     optimization.

     The default is `-fguess-branch-probability' at levels `-O', `-O2',
     `-O3', `-Os'.
---snip---

Except instrumenting the code with "expect" or using "-fprofile-arcs"
there's only the possibility to modify the code of gcc to match our
existing model...

Bye,
Alexander.

--=20
               I believe the technical term is "Oops!"

http://www.Leidinger.net                       Alexander @ Leidinger.net
  GPG fingerprint =3D C518 BC70 E67F 143F BE91  3365 79E2 9C60 B006 3FE7



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