Re: Writing fast compilers... RS/6000

>|>[Optimizing compilers won't do as much on well-written code because
>|> of e.g., common subexpression elimination in the source.]


roger@gimli.inmos.co.uk (Roger Shepherd) writes:
>Well, it depends on what you mean by ``well written code''.


Ah, yes, that reminds me of a code fragment that multiplies every
element of an array by either zero or one, then performs the desired
computation on the array. Result values that are zero are discarded
implicitly because sum+0==sum.


        /* Does this. */
        for (i=0; i<N; ++i)
            sum += compute(a[i]) * zero_or_one[i];


        /* Instead of this. */
        for (i=0; i<N; ++i)
            if (cond[i])
sum += compute(a[i]);


On `normal' machines it runs slower, on a Cray it runs faster. Ok,
who's to decide what `well written code' means?


;-D oN ( Well, written at any rate ) Pardo
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