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Re: 2GB userspace limitation in ABI N32

To: Ronny Meeus <ronny.meeus@gmail.com>
Subject: Re: 2GB userspace limitation in ABI N32
From: Ralf Baechle <ralf@linux-mips.org>
Date: Fri, 12 Oct 2012 12:18:17 +0200
Cc: linux-mips@linux-mips.org
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On Wed, Oct 10, 2012 at 05:12:16PM +0200, Ronny Meeus wrote:

> Do you have any clue (rough) on the amount of effort this change would cost?

David Daney's reply should give you more information for an estimate.

> About the limited gain we can discuss: if you have a large application
> that has been created assuming 32bit and it needs to be ported to a
> 64bit architecture, I think the effort can be huge and the risk for
> forgetting things is high. It will be very hard to check whether the
> system behaves well under all conditions.

A 64-bit port could start right away without the delay of waiting for
a usable N32-4GB.  Added benefit - with N64 you can grow beyond the 3GB.

Downside, due to larger pointers thus better cache locality 32-bit code
generally performs better.  And I agree that verification of N32-4GB
probably is easier than for a large application that wasn't written
with the intend of 64-bit support.

In my past as a contractor I've dealth with a few customers who were
trying to avoid going 64-bit at all cost.  They had to pay that cost ...

  Ralf

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