29 Jan 1996 23:22:25 -0500

Related articles |
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[4 earlier articles] |

Re: Compiler optimization of division and remainder prener@watson.ibm.com (1996-01-29) |

Re: Compiler optimization of division and remainder hbaker@netcom.com (1996-01-29) |

Re: Compiler optimization of division and remainder Peter-Lawrence.Montgomery@cwi.nl (1996-01-29) |

Re: Compiler optimization of division and remainder johnmce@world.std.com (1996-01-29) |

Re: Compiler optimization of division and remainder michael.williams@armltd.co.uk (1996-01-29) |

Re: Compiler optimization of division and remainder jgj@ssd.hcsc.com (1996-01-29) |

Re: Compiler optimization of division and remainder dave@occl-cam.demon.co.uk (Dave Lloyd) (1996-01-29) |

Re: Compiler optimization of division and remainder dave@occl-cam.demon.co.uk (Dave Lloyd) (1996-02-02) |

From: | Dave Lloyd <dave@occl-cam.demon.co.uk> |

Newsgroups: | comp.compilers |

Date: | 29 Jan 1996 23:22:25 -0500 |

Organization: | Compilers Central |

References: | 96-01-088 |

Keywords: | arithmetic, optimize |

hbaker@netcom.com (Henry G. Baker) writes:

On another newsgroup, a poster claimed that some compilers could

optimize a program which used both a/b and a%b (both quotient and

remainder with the _same_ arguments) in such a way that the compiler

would perform only a single hardware division operation and arrange to

use both the quotient and remainder from this single operation.

We do this on the x86 where the idiv instruction plonks the quotient

in AX and the remainder in DX. Most other processors either don't have

hardware support or only give you one of the values out of the divide

unit depending on source instruction.

One can play a similar game with other multiple-output instructions:

for example, one gets given both the sin and the cos out of a chordic

algorithm and in many circumstances sin(x) and cos(x) get used

together.

----------------------------------------------------------------------

Dave Lloyd Email: Dave@occl-cam.demon.co.uk

Oxford and Cambridge Compilers Ltd Phone: (44) 1223 572074

55 Brampton Rd, Cambridge CB1 3HJ, UK

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