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Re: compressed pointer type implementation in gcc

From: Robert Dewar <dewar at adacore dot com>
To: Yair Lifshitz <luna dot starlight at gmail dot com>
Cc: gcc at gcc dot gnu dot org, Yair Lifshitz <yair dot lifshitz at intel dot com>
Date: Sat, 19 Jul 2008 11:32:31 -0400
Subject: Re: compressed pointer type implementation in gcc
References: <9D3FF9CB8AF1694AB811321FF6378F3603849741@hasmsx415.ger.corp.intel.com> <b90c57030807190825s55fb452ct755617b4e017d0a1@mail.gmail.com> <b90c57030807190829h5e8c097fyf3d0bd436b0f419c@mail.gmail.com>

Yair Lifshitz wrote:

Hi,

I hope I'm not flooding with this topic. I've did some research and
couldn't find anything relevant on this topic.
My team is developing a large scale CAD application that has a large
memory footprint, requiring strong machines to run.

The application uses pointers massively.
During one of our optimization cycles I noticed that since most
objects are aligned on 8-byte boundaries, it's possible to drop the
lower 3 bits of the address and reconstruct the full address later.

Basically, as long as the application is in the 32G range (2^32*2^3),
it's possible to represent aligned pointers using an unsigned int - 4
bytes.


why not just run in 32 bit mode in this case?
What's the point of using 64-bit addresses if you
don't need them?

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References:
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  - From: Yair Lifshitz

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