linux-kernel - Re: [Bug Fix]: Do 32-bit table calculations in pre-processor

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Message-ID: <4A4E5738.6080407@goop.org>
Date:	Fri, 03 Jul 2009 12:08:40 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	lkml@...eThan.org
CC:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: [Bug Fix]: Do 32-bit table calculations in pre-processor

On 07/03/09 11:38, Michael S. Zick wrote:
> I make no claims for it at the moment - too early in the test process.
> Just the general observation that it takes 0.5M to describe 0.5G of ram.
>   
Only if you're using 4k pages.  With large pages, 1 pte can map 2M, so
256 entries can map 512M, so you only need 1/2 a page of pagetable
(assuming PAE; if not a single entry can map 4M).

> Also, 
> the observation that (1<<32) drops the bit off the left end of a 32-bit value.
> You can see the result in the portion of the post you snipped out.  ;)
>   

Those computations aren't done as 32-bit.

$ as << EOF
.data
.byte (1 << 100) >> 100
EOF
$ objdump -D a.out

a.out:     file format elf64-x86-64

Disassembly of section .data:

0000000000000000 <.data>:
   0:    01                       .byte 0x1


    J
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