Within computer science, a use-definition chain (or UD chain) is a data structure that consists of a use U, of a variable, and all the definitions D of that variable that can reach that use without any other intervening definitions. A UD Chain generally means the assignment of some value to a variable.
A counterpart of a UD Chain is a definition-use chain (or DU chain), which consists of a definition D of a variable and all the uses U reachable from that definition without any other intervening definitions.
Both UD and DU chains are created by using a form of static code analysis known as data flow analysis. Knowing the use-def and def-use chains for a program or subprogram is a prerequisite for many compiler optimizations, including constant propagation and common subexpression elimination.
Making the use-define or define-use chains is a step in liveness analysis, so that logical representations of all the variables can be identified and tracked through the code.
Consider the following snippet of code:
Notice that x
is assigned a value at three points (marked A, B, and C). However, at the point marked "1", the use-def chain for x
should indicate that its current value must have come from line B (and its value at line B must have come from line A). Contrariwise, at the point marked "2", the use-def chain for x
indicates that its current value must have come from line C. Since the value of the x
in block 2 does not depend on any definitions in block 1 or earlier, x
might as well be a different variable there; practically speaking, it is a different variable - call it x2
.
The process of splitting x
into two separate variables is called live range splitting. See also static single assignment form.
The list of statements determines a strong order among statements.
For a variable, such as v, its declaration is identified as V (italic capital letter), and for short, its declaration is identified as . In general, a declaration of a variable can be in an outer scope (e.g., a global variable).
When a variable, v, is on the LHS of an assignment statement, such as, then is a definition of v. Every variable (v) has at least one definition by its declaration (V) (or initialization).
If variable, v, is on the RHS of statement, there is a statement, with i < j and, that it is a definition of v and it has a use at (or, in short, when a variable, v, is on the RHS of a statement, then v has a use at statement).
Consider the sequential execution of the list of statements,, and what can now be observed as the computation at statement, j:
|A(i)|
This example is based on a Java algorithm for finding the gcd. (It is not important to understand what this function does.)
To find out all def-use-chains for variable d, do the following steps:
In this case it is "" (l.7)
In this case it is ""
In this case it is:
Repeat these steps in the following style: combine each write access with each read access (but NOT the other way round).
The result should be:
For example: From line 7 down to line 13 in the source code, is not redefined / changed.At line 14, could be redefined. This is why you have to recombine this write access on with all possible read accesses which could be reached.In this case, only the code beyond line 10 is relevant. Line 7, for example, cannot be reached again. For your understanding, you can imagine 2 different variables :
As a result, you could get something like this. The variable would be replaced by
With this algorithm, two things are accomplished: