Re: Unsafe Optimizations (WAS: Compiler Design in C How about it?)

In article <1990Jun14.032437.5554@esegue.segue.boston.ma.us> cwitty@csli.Stanford.EDU (Carl Witty) writes:


      ... For example, on many machines, integer arithmetic
      really is commutative and associative.


I take Carl's point, and he is right about commutativity, but integer
arithmetic is not associative if overflow matters. If all you care about is
the result mod the word length (i.e., you do not have overflow trapping
enabled) then Carl is correct. There are some languages that detect and report
overflow where this would make a difference; it does not make a difference in
C. (I guess on the MIPS chip they always use the "unsigned" (no overflow)
instructions regardless of whether the operands are C "int" or "unsigned".)
Yours in nit picking :-) .....
--


J. Eliot B. Moss, Assistant Professor
Department of Computer and Information Science
Lederle Graduate Research Center
University of Massachusetts
Amherst, MA 01003
(413) 545-4206; Moss@cs.umass.edu
[The ANSI C standard has a specific example pointing this out. -John]
--

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