3.4.112 –xalias[=keywords]
Specify degree of aliasing to be assumed by the compiler.
Some non-standard programming techniques can introduce situations
that interfere with the compiler’s optimization strategies. The use
of overindexing, pointers,
and passing global or non-unique variables as subprogram arguments, can introduce
ambiguous aliasing situations that could result code that does not work as
expected.
Use the -xalias flag to inform the compiler about
the degree to which the program deviates from the aliasing requirements of
the Fortran standard.
The flag may appear with or without a list of keywords. The keywords list is comma-separated, and each keyword indicates an aliasing
situation present in the program.
Each keyword may be prefixed by no% to indicate
an aliasing type that is not present.
The aliasing keywords are:
Table 3–10
-xalias Option Keywords
|
keyword
|
meaning
|
|
dummy
|
Dummy (formal) subprogram parameters can alias each other and global
variables.
|
|
no%dummy
|
(Default). Usage of dummy parameters follows the Fortran standard and
do not alias each other or global variables.
|
|
craypointer
|
(Default). Cray pointers can point at any global variable or a local
variable whose address is taken by the LOC() function.
Also, two Cray pointers might point at the same data. This is a safe assumption
that could inhibit some optimizations.
|
|
no%craypointer
|
Cray pointers point only at unique memory addresses, such as obtained
from malloc(). Also, no two Cray pointers point at
the same data. This assumption enables the compiler to optimize Cray pointer
references.
|
|
actual
|
The compiler treats actual subprogram arguments as if they were global
variables. Passing an argument to a subprogram might result in aliasing through
Cray pointers.
|
|
no%actual
|
(Default) Passing an argument does not result in further aliasing.
|
|
overindex
|
-
A reference to an element in a COMMON block might refer to
any element in a COMMON block or equivalence group.
-
Passing any element of a COMMON block or equivalence group
as an actual argument to a subprogram gives access to any element of that
COMMON block or equivalence group to the called subprogram.
-
Variables of a sequence derived type are treated as if they
were COMMON blocks, and elements of such a variable might alias other elements
of that variable.
-
Individual array bounds may be violated, but except as noted
above, the referenced array element is assumed to stay within the array. Array
syntax, WHERE, and FORALL statements
are not considered for overindexing. If overindexing occurs in these constructs,
they should be rewritten as DO loops.
|
|
no%overindex
|
(Default) Array bounds are not violated. Array references do not reference
other variables.
|
|
ftnpointer
|
Calls to external functions might cause Fortran pointers to point at
target variables of any type, kind, or rank.
|
|
no%ftnpointer
|
(Default) Fortran pointers follow the rules of the standard.
|
Specifying -xalias without a list gives the best
performance for most programs that do not violate Fortran aliasing rules,
and corresponds to:
no%dummy,no%craypointer,no%actual,no%overindex,no%ftnpointer
To be effective, -xalias should be used when
compiling with optimization levels -xO3 and higher.
The compiler default, with no -xalias flag specified,
assumes that the program conforms to the Fortran 95 standard except for Cray
pointers:
no%dummy,craypointer,no%actual,no%overindex,no%ftnpointer
Examples of various aliasing situations and how to specify them with -xalias are given in the Porting chapter of the Fortran
Programming Guide.
