XL Fortran Enterprise Edition for AIX : User's Guide - IBM

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Code that might cause an exception is not moved unless the exception is certain
to occur anyway. In the following example, the program could evaluate the
expression N/K before the loop because the result is the same for each iteration of
the loop:
DO 10 J=1,N
...
IF (K .NE. 0) M(J)=N/K
...
10 END
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However, it is not moved because K might be 0, and computing N/K results in
an exception where none occurs in the unoptimized program.
The rules for IEEE arithmetic are followed more closely than otherwise. 3 For
example, X+0.0 is not folded to X, because IEEE rules require that -0.0+0.0 be 0,
making X+0 equal to -X in this one case.
Floating-point calculations are not considered associative. For example, XL
Fortran evaluates X*Y*Z left-to-right, even though the program might already
have computed Y*Z, because the results might not be identical.
As the optimization level increases, these restrictions are relaxed where there is an
opportunity for a performance improvement:
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Calculations like N/K in the previous example and floating-point operations may
be moved or rescheduled because they are unlikely to cause exceptions.
IEEE conformance is not enforced for rules that are unlikely to be needed. The
sign of zero might not be correctly preserved, as in the preceding example.
However, this might only be a problem in an extreme case, such as multiplying
the wrongly signed zero by infinity and ending up with an infinity of the wrong
sign. Floating-point operations that might cause an exception may be moved,
rescheduled, or processed so they do not produce an exception.
Floating-point expressions might be reassociated, so that results might not be
identical.
When you specify the highest levels of optimization, XL Fortran assumes that you
are requesting speed even at the possibility of some risk, as already explained. If
you want as much optimization as possible without the resulting risk, you must
add the -qstrict compiler option.
The early XL family of compilers adopted a conservative approach to optimization.
This was intended to make an optimized program work exactly the same as an
unoptimized one, even in extreme cases unlikely to occur in real life. For example,
the array reference A(N) might not be optimized, because N might be a huge
number so that the program causes a segmentation violation when the address is
referenced, and this behavior would be “preserved”. With the industry in general
favoring a less conservative approach, XL Fortran’s highest optimization levels
now emphasize performance over identical execution between optimized and
unoptimized programs.
The different levels of the -O option incorporate various optimization techniques
that are expected to improve performance for many different kinds of programs.
The specialized optimization options, such as -qipa, -qhot, and -Q, can improve
performance in some kinds of programs but degrade it in others. Therefore, they
may require experimentation to determine whether they are appropriate for any
given program.
3. If IEEE compliance is a concern for you, you should also specify either the -qfloat or -qstrict option.
306 XL Fortran Enterprise Edition for AIX : User’s Guide