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Re: modulus question

From: James E Wilson <wilson at specifixinc dot com>
To: Andrew Pinski <pinskia at physics dot uc dot edu>
Cc: gcc at gcc dot gnu dot org
Date: Wed, 11 Aug 2004 12:07:55 -0700
Subject: Re: modulus question
References: <OF4ABA5D1F.E14ACD39-ON86256EEB.006F52A4-86256EEB.0070CA81@us.ibm.com> <A731B4A6-EA43-11D8-ADE9-00039351ED8A@physics.uc.edu>

Andrew Pinski wrote:

Fold is changing (u%2)==0 into (u&1)==0 which gets changed to ((u^1) & 1) != 0.

The culprit is fold_single_bit_test in fold-const.c.

This used to be part of do_store_flag, and it does make some sense there. Simplifying a single bit test allows us to eliminate the compare that would otherwise be needed.

However, Jeff Law pulled this code out and started calling it from fold(). Now it gets used for if statement tests, and now we have a potential problem. Since we will need to have a comparison for the branch anyways, some of the simplications performed here may actually make the code worse. -- Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com

Follow-Ups:
- Re: modulus question
  - From: Pat Haugen

References:
- modulus question
  - From: Pat Haugen
- Re: modulus question
  - From: Andrew Pinski

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