Fortran computes cosine 300 times faster than C (on Sun3)

[Chris Torek]

In article <765 at uceng.UC.EDU> achhabra at uceng.UC.EDU (atul k chhabra) writes:
>I chanced upon a segment of code that runs approximately 300 times faster in
>FORTRAN than in C. I have tried the code on Sun3(OS3.5) and on Sun4(OS4.0)
>(of course, on Sun4 the -f68881 flag was not used.) The results are similar
>on both machines. Can anyone enlighten me on this bizzare result?

`COS' is an intrisinc function in Fortran.  This means that the compiler
is required to know about it.  It is typically provided as an external
function in C, so that the compiler knows nothing of it.  Thus:

>    for(i=0;i<262144;i++)
>	tmp=cos(2.5)*cos(2.5)*cos(2.5)*cos(2.5);

makes the compiler call `cos' (262144*4) times, each time with the same
argument, and multiply all those values together.  The compiler does not
`guess at' the function and assume that, since its value is not used
the first 262143 times, eliminate the call, because `cos' might print
`hello world'.

On the other hand, given

>	do 10 i=1,262144
>		tmp=cos(2.5)*cos(2.5)*cos(2.5)*cos(2.5)
>10	continue

the Fortran compiler can be certain that COS(2.5) does nothing but
compute cosines, and can change the code to

	TMP = 4.0 * COS(2.5)
10	CONTINUE

possibly even replacing the COS(2.5) with the constant -.8011436155....
(Actually, since in both fragment, tmp is unused, both versions can
elide the assignment to tmp and the C version can elide the four multiplies
per iteration.  It cannot, however, replace the four calls wtih a single
call.)

Now, if Sun had a pANS-conformant compiler, they could make <math.h>
do something like

	#define cos(x) __intrinsic_cos(x)

and recognise calls to `__intrinsic_cos'.  This sort of optimisation
does have a real effect on real code (as opposed to silly examples like
calling cos four times with the same constant in a loop that runs 262144
times, then throwing away the result).
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris