cvs commit: src/sys/i386/i386 pmap.c

[Alexander Leidinger]

On Sun, 26 Oct 2003 12:13:37 +0100
des at des.no (Dag-Erling Smørgrav) 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.

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

http://www.Leidinger.net                       Alexander @ Leidinger.net
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