briandamgaard

Serious bug in widely used AMD publication

Discussion created by briandamgaard on Feb 26, 2013

Publication:

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AMD Athlon Processor x86 Code Optimization Guide

Publication  No.: 22007

Revision: K

Date: February 2002

 

Bug:

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Serious bug in all integer-64 division and remainder assembler routines:

* _ulldiv

* _lldiv

* _ullrem

* _llrem

 

Example:

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An unsigned division example:

(2**64 - 1 ) divided by (2**32 + 3):

 

18446744073709551615 div 4294967299 = 4294967294 (wrong)

The correct result: 4294967293

 

The problem:

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The problem is that the assembler code doesn't implement the "Hacker's Delight" division correctly. A description of the algorithm can be found here:

http://www.hackersdelight.org/hdcodetxt/divDouble.c.txt

 

Importance:

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Please note that:

* The bugs spreads like a virus because many software products and programmers assume the publication it the autoritative implementation of these routines

 

* The use of these routines will not die out just because modern 64-bit hardware has native support for 64-integers. Software always need double-integer support and programmers simply change the four routines to operate on whatever the double-integer bit-size is, e..g, 128 bits nowadays.

 

For this reason it's imperative to make the public aware of the bug. I hope AMD will take action on this.

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