Re: Constant divisions, remainders

joe@babel.ho.att.com (Joseph M Orost) writes:


>The problem with this thinking is that you will get inconsistent results
>if you divide by a constant power-of-2 versus dividing by a variable with
>the same value. And you will get LOTS of bug reports from the compiler
>users!


Only if there is a hardware divide instruction. I modified gcc on the IBM
RT to use shift for division by a power of 2. At the same time, I changed
the divide function to round down when dividing a negative number by a
positive number. The divide function uses a series of divide-step
instructions; it takes about the same amount of code to implement either
rounding mode.


The IBM RS/6000 has an equally good solution to the problem: the divide
instructions rounds to zero, but the shift instruction sets the carry bit
if the result is negative so a shift follwed by an add-with-carry
instruction is equivalent to a round-to-zero divide.
--
        John Carr (jfc@athena.mit.edu)
--

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