smart compilers

[John Gilmore]

> James Giles says:
>               My first version was wrong.  But 'Y' is a loop invariant, the
> correct optimization is:
> 
>       IF (Y.GT.0) THEN
>          TEMP=SQRT(Y)
>          DO 100 I=1,10
>             X(I)=TEMP
> 100      CONTINUE
>       ENDIF
> 
> This has the advantage of not going through the loop at all if the
> assignments would all have been skipped.  All these examples show the point
> I originally made: these optimizations are unsafe unless you have a REALLY
> GOOD data flow analyzer.

You're absolutely right!  Had you had a good data flow analyzer, it
would have detected that your optimization had changed the value of 'I'
on exit from this piece of code.