Re: Q: division vs multiplication

mikeq@primenet.com (Michael Quinlan)
Sun, 2 Apr 1995 13:50:23 GMT

          From comp.compilers

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Q: division vs multiplication t.hulek@imperial.ac.uk (1995-03-24)
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[9 later articles]
| List of all articles for this month |

Newsgroups: comp.compilers
From: mikeq@primenet.com (Michael Quinlan)
Keywords: arithmetic, optimize
Organization: Primenet
References: 95-04-003
Date: Sun, 2 Apr 1995 13:50:23 GMT

t.hulek@imperial.ac.uk (Mr Tomas Hulek) writes:


[Can one optimize floating division by powers of two into something like a
shift?]


>I would imagine that division by 2.0 could be done very efficiently, just like
>division by 10 in our decimal system. But is it really so?


        On at least one set of machines (the IBM System 370/390 series), division
        of a floating point number by 2 is significantly faster than multiplication
        because the hardware has a special instruction for it.


        Of course, in a C program, division by 2 will only be faster when the
        compiler detects the situation and generates code using the instruction.
        A well-written compiler might detect both situations above and generate
        identical code.


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| Michael Quinlan |
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