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

can also use the SNAPSHOT directive to create additional program points for
storage visibility by flushing registers to memory.
Other Program Behavior Options
The precision of compiler analyses is significantly affected by instructions that can
read or write memory. Aliasing pertains to alternate names for things, which in
this context are references to memory. A reference to memory can be direct, as in
the case of a named symbol, or indirect, as in the case of a pointer or dummy
argument. A function call might also reference memory indirectly. Apparent
references to memory that are false, that is, that do not actually reference some
location assumed by the compiler, constitute barriers to compiler analysis.
Fortran defines a rule that dummy argument references may not overlap other
dummy arguments or externally visible symbols during the execution of a
subprogram.
The compiler performs sophisticated analyses, attempting to refine the set of
possible aliases for pointer dereferences and calls. However, a limited scope and
the absence of values at compile time constrain the effectiveness of these analyses.
Increasing the optimization level, in particular, applying interprocedural analysis
(that is, compiling with -qipa), can contribute to better aliasing.
Programs that violate language aliasing rules, as summarized above, commonly
execute correctly without optimization or with low optimization levels, but can
begin to fail when higher levels of optimization are attempted. The reason is that
more aggressive optimizations take better advantage of aliasing information and
can therefore expose subtly incorrect program semantics.
Options related to these issues are -qstrict and -qalias. Their behaviors are
summarized in the table below.
Program behavior options
Option
Description
-qstrict, -qnostrict
Allows the compiler to reorder floating-point calculations and
potentially excepting instructions. A potentially excepting
instruction is one that may raise an interrupt due to erroneous
execution (for example, floating-point overflow, a memory access
violation). The default is -qstrict with -qnoopt and -O2; -qnostrict
with -O3, -O4, and -O5.
-qalias
Other Performance Options
Allows the compiler to assume that certain variables do not refer
to overlapping storage. The focus is on the overlap of dummy
arguments and array assignments in Fortran.
Options are provided to control particular aspects of optimization. They are often
enabled as a group or given default values when a more general optimization
option is enabled.
Selected compiler options for optimizing performance
Option
Description
320 XL Fortran Enterprise Edition for AIX : User’s Guide