Re: Optimization techniques

Martin Ward <martin@gkc.org.uk>
Fri, 26 Apr 2019 20:10:10 +0100

          From comp.compilers

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From: Martin Ward <martin@gkc.org.uk>
Newsgroups: comp.compilers
Date: Fri, 26 Apr 2019 20:10:10 +0100
Organization: Compilers Central
References: 19-04-004 19-04-012 19-04-018
Injection-Info: gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970"; logging-data="54988"; mail-complaints-to="abuse@iecc.com"
Keywords: arithmetic
Posted-Date: 26 Apr 2019 15:15:10 EDT

On 23/04/19 08:43, David Brown wrote:
> But mathematical identities such as associativity and commutativity are
> valid because signed integer overflow does not happen - thus "a * (b +
> c)" can be changed to "(a * b) + (a * c)".


If b is large and c has a value close to -b then a * (b + c)
might be OK while (a * b) will overflow and cause undefined
behaviour.


--
Martin


Dr Martin Ward | Email: martin@gkc.org.uk | http://www.gkc.org.uk
G.K.Chesterton site: http://www.gkc.org.uk/gkc | Erdos number: 4


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