Re: C code .vs. Assembly code for Microcontrollers/DSPs ?

Robert A Duff <bobduff@world.std.com> wrote:


>This is a language problem, IMHO. Languages like C and Ada and Pascal
>seem to think that integer data types have something to do with the
>underlying hardware. That's wrong. The size in bits of the result
>should be determined by the size of the operands, and not their
>"type". No casts should be needed.


Unless you move away from a static typed language, the C model has a
great advantage. If the result of an operation is defined to have
infinite precision a long expression gains bits for every operator and
the code to implement it is big and slow.


To get the exact result for


(x + y) * (a + b) / d


where all variables are 32 bits requires 66 bit arithmetic. Addition
adds 1 bit and multiplication doubles the number of bits. The quotient
has the same number of bits as the dividend.




--
        John Carr (jfc@mit.edu)


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