Re: Compiler optimization and floating point operations

christian.bau@cbau.wanadoo.co.ukMon, 28 Oct 2013 15:50:07 -0700 (PDT)

From comp.compilers

Related articles
[2 earlier articles]
Re: Compiler optimization and floating point operations gah@ugcs.caltech.edu (glen herrmannsfeldt) (2013-10-24)
Re: Compiler optimization and floating point operations gneuner2@comcast.net (George Neuner) (2013-10-24)
Re: Compiler optimization and floating point operations jm@bourguet.org (Jean-Marc Bourguet) (2013-10-24)
Re: Compiler optimization and floating point operations sgk@troutmask.apl.washington.edu (Steven G. Kargl) (2013-10-24)
Re: Compiler optimization and floating point operations gneuner2@comcast.net (George Neuner) (2013-10-24)
Re: Compiler optimization and floating point operations gah@ugcs.caltech.edu (glen herrmannsfeldt) (2013-10-24)
Re: Compiler optimization and floating point operations christian.bau@cbau.wanadoo.co.uk (2013-10-28)
Re: Compiler optimization and floating point operations christian.bau@cbau.wanadoo.co.uk (2013-10-28)
Re: Compiler optimization and floating point operations cameron.mcinally@nyu.edu (Cameron McInally) (2013-10-28)
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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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