Tue, 29 Mar 1994 12:16:07 GMT

Related articles |
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Floating point constant question mahlke@crhc.uiuc.edu (Scott Mahlke) (1994-03-28) |

Re: Floating point constant question joe@sanskrit.ho.att.com (1994-03-29) |

Re: Floating point constant question chase@Think.COM (1994-03-29) |

Re: Floating point constant question bill@amber.csd.harris.com (1994-03-29) |

Re: Floating point constant question maydan@mti.mti.sgi.com (1994-03-29) |

Re: Floating point constant question hbaker@netcom.com (1994-03-30) |

Re: Floating point constant question conte@ece.scarolina.edu (1994-03-30) |

Re: Floating point constant question chase@Think.COM (1994-03-30) |

[2 later articles] |

Newsgroups: | comp.compilers |

From: | joe@sanskrit.ho.att.com (Joe Orost 908- 946-1115) |

Keywords: | arithmetic, optimize |

Organization: | AT&T |

References: | 94-03-157 |

Date: | Tue, 29 Mar 1994 12:16:07 GMT |

Scott Mahlke <mahlke@crhc.uiuc.edu> writes:

*>I have a question regarding the following compiler optimization:*

*>*

*> x = y / 500.0; ===> x = y * 0.002;*

*> ...*

*>Is this or similar optimizations dealing with FP constants ever legal? Do*

*>any compilers out there do optimizations such as this? Thanks.*

It's only a valid replacement if you don't affect the accuracy of the

calculation. For cases where you are dividing by a power of 2, it is

always safe. This is the rule I apply to the compilers I write.

In IEEE single precision, 500.0 is exact (43fa0000), while 0.002 is

actually represented by 0.00200000009499490261077880859375 (3b03126f). On

the other hand, both 512.0 and 0.001953125 are exact.

regards,

joe

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Full Name: Joseph M. Orost

EMail: joe@babel.ho.att.com, attmail!orost

Organization: AT&T Bell Laboratories

SurfaceMail: 943 Holmdel Rd.; Cruz Plaza; Holmdel, NJ 07733

Phone: +1 (908) 946-1115

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