Aliasing (computing)
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In computing, aliasing is a term that generally means that one variable or some reference, when changed, has an indirect (usually unexpected) effect on some other data.
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[edit] Examples
[edit] Array bounds checking
For example, the C programming language does not perform array bounds checking. One can then exploit the implementation of the programming language by the compiler, plus the computer architecture's assembly language conventions, to achieve aliasing effects.
If an array is created on the stack, with a variable laid out in memory directly beside that array, one could index outside that array and then directly change that variable by changing the relevant array element. For example, if we have a int array of size ten (for this example's sake, calling it vector), next to another int variable (call it i), vector[10] would be aliased to i if they are adjacent in memory.
This is possible in some implementations of C because an array is in reality a pointer to some location in memory, and array elements are merely offsets off that memory location. Since C has no bounds checking, indexing and addressing outside of the array is possible. Note that the aforementioned aliasing behaviour is implementation specific. Some implementations may leave space between arrays and variables on the stack, for instance, to minimize possible aliasing effects. C programming language specifications do not specify how data is to be laid out in memory.
[edit] Aliased pointers
Another variety of aliasing can occur in any language that can refer to one location in memory with more than one name (for example, with pointers). See the C example of the xor swap algorithm that is a function; it assumes the two pointers passed to it are distinct, but if they are in fact equal (or aliases of each other), the function fails. This is a common problem with functions that accept pointer arguments, and their tolerance (or the lack thereof) for aliasing must be carefully documented, particularly for functions that perform complex manipulations on memory areas passed to them.
[edit] Specified aliasing
Controlled aliasing behaviour may be desirable in some cases (that is, aliasing behaviour that is specified, unlike that relevant to memory layout in C). It is common practice in FORTRAN. The Perl programming language specifies, in some constructs, aliasing behaviour, such as in foreach loops. This allows certain data structures to be modified directly with less code. For example,
my @array = (1, 2, 3);
foreach my $element (@array) {
# Increment $element, thus automatically
# modifying @array, as $element is *aliased*
# to one of @array's elements.
$element++;
}
print "@array\n";
will print out "2 3 4" as a result. If one would want to bypass aliasing effects, one could copy the contents of the index variable into another and change the copy.
[edit] Conflicts with optimization
Many times optimizers have to make conservative assumptions about variables in the presence of pointers. For example, a constant propagation process which knows that the value of variable x is 5 would not be able to keep using this information after an assignment to another variable (for example, *y = 10) because it could be that *y is an alias of x. This could be the case after an assignment like y = &x.
As an effect of the assignment to *y, the value of x would be changed as well, so propagating the information that x is 5 to the statements following *y = 10 would be potentially wrong (if *y is indeed an alias of x). However, if we have information about pointers, the constant propagation process could make a query like: can x be an alias of *y? Then, if the answer is no, x = 5 can be propagated safely.
Another optimization that is impacted by aliasing is code reordering; if the compiler decides that x is not an alias of *y, then code that uses or changes the value of x can be moved before the assignment *y = 10, if this would improve scheduling or enable more loop optimizations to be carried out.
In order to enable such optimizations to be carried out in a predictable manner, the C99 edition of the C programming language specifies that it is illegal (with some exceptions) for pointers of different types to reference the same memory location. This rule, known as "strict aliasing", allows impressive increases in performance, but has been known to break some legacy code.
[edit] External links
- Understanding Strict Aliasing - Mike Acton
