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Re: Promblem with PREF (prefetching) in memcpy

To: Carsten Langgaard <>
Subject: Re: Promblem with PREF (prefetching) in memcpy
From: Dominic Sweetman <>
Date: Fri, 4 Oct 2002 12:53:22 +0100 (BST)
Cc: Ralf Baechle <>,
In-reply-to: <>
Original-recipient: rfc822;
References: <>
Carsten Langgaard ( writes:

> I think we have a problem with the PREF instructions spread out in the
> memcpy function.

Not really.  The MIPS32 manual (for example):

 "PREF does not cause addressing-related exceptions. If it does happen
  to raise an exception condition, the exception condition is
  ignored. If an addressing-related exception condition is raised and
  ignored, no data movement occurs."
  PREF never generates a memory operation for a location with an
  uncached memory access type."

For a Linux user program, at least, memory pages are "memory-like":
reads are guaranteed to be side-effect free, so any outlying
prefetches are harmless.  It's hard to see any circumstance where an
accessible cacheable location would lead to bad side-effects on read.

Dominic Sweetman, 
MIPS Technologies (UK) - formerly Algorithmics
The Fruit Farm, Ely Road, Chittering, CAMBS CB5 9PH, ENGLAND
phone: +44 1223 706200 / fax: +44 1223 706250 / direct: +44 1223 706205

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