smart compilers

[cliff at unmvax.UUCP]

> > However, the IBM H Level FORTRAN Programmer's Guide does warn that code such
> > as
> > 
> > 	DO 100 I = 1 TO 10
> > 	IF (Y .GT. 0) X(I) = SQRT(Y)
> >     100 CONTINUE
> > 
> > (with X appropriately declared as an array of course) can cause an error
> > because it tries to take the square root of a negative number.  The optimizer
> > compiles this as if it were
> > 
> > 	TEMP = SQRT(Y)
> > 	DO 100 I = 1 TO 10
> > 	IF (Y .GT. 0) X(I) = TEMP
> >     100 CONTINUE
> 
> This particular example should be optimized as
> 
>       IF ( Y.GT.0 ) TEMP=SQRT(Y)
>       DO 100 I = 1,10         !THERE ARE NO 'TO' DELIMITERS IN FORTRAN
>          X(I)=TEMP            !FOLLOW INDENTATION STANDARDS PLEASE
> 100   CONTINUE

C Not quite.  You will wind up setting X(I) to zero or the last value of TEMP
C when Y is less than or equal to zero.  How about something that would really
C be correct, like:
C
	IF (Y.LE.0) GO TO 100
	TEMP = SQRT(Y)
	DO 100 I = 1, 10
	    X(I) = TEMP
100	CONTINUE
C
C The greatest derangement of the mind is to believe in something
C because one wishes to be Louis Pasteur.
C
C 	--Cliff [Matthews]
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