# Re: Compiler optimization and floating point operations

## christian.bau@cbau.wanadoo.co.uk

Mon, 28 Oct 2013 15:50:07 -0700 (PDT)

*From comp.compilers*

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**From: ** | christian.bau@cbau.wanadoo.co.uk |

**Newsgroups: ** | comp.compilers |

**Date: ** | Mon, 28 Oct 2013 15:50:07 -0700 (PDT) |

**Organization: ** | Compilers Central |

**References: ** | 13-10-026 13-10-029 |

**Keywords: ** | arithmetic, optimize |

**Posted-Date: ** | 28 Oct 2013 18:51:53 EDT |

*> [I think you mean they're not associative. I don't know any situations*

*> where a+b != b+a, but lots where a+(b+c) != (a+b)+c -John]*

Take fused multiply-add, and the expression a*b + c*d.

It can be evaluated either as fma (a, b, c*d) or fma (c, d, a*b).

It would seem natural that the code for a*b + c*d and for c*d + a*b

would be different, with different results.

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