In compiler theory, dependence analysis produces execution-order constraints between statements/instructions. Broadly speaking, a statement S2 depends on S1 if S1 must be executed before S2. Broadly, there are two classes of dependencies--control dependencies and data dependencies.
Dependence analysis determines whether it is safe to reorder or parallelize statements.
Control dependency is a situation in which a program instruction executes if the previous instruction evaluates in a way that allows its execution.
A statement S2 is control dependent on S1 (written ) if and only if S2's execution is conditionally guarded by S1. The following is an example of such a control dependence:
S1 if x > 2 goto L1 S2 y := 3 S3 L1: z := y + 1
Here, S2 only runs if the predicate in S1 is false.
See data dependencies for more details.
A data dependence arises from two statements which access or modify the same resource.
A statement S2 is flow dependent on S1 (written ) if and only if S1 modifies a resource that S2 reads and S1 precedes S2 in execution. The following is an example of a flow dependence (RAW: Read After Write):
S1 x := 10 S2 y := x + c
A statement S2 is antidependent on S1 (written ) if and only if S2 modifies a resource that S1 reads and S1 precedes S2 in execution. The following is an example of an antidependence (WAR: Write After Read):
S1 x := y + c S2 y := 10
Here, S2 sets the value of
y but S1 reads a prior value of
The term 'antidependence' widely used in literature is a misnomer because 'anti' means opposite. The correct term should be 'ante' means before. Hence the correct word should be antedependence.
A statement S2 is output dependent on S1 (written ) if and only if S1 and S2 modify the same resource and S1 precedes S2 in execution. The following is an example of an output dependence (WAW: Write After Write):
S1 x := 10 S2 x := 20
Here, S2 and S1 both set the variable
A statement S2 is input dependent on S1 (written ) if and only if S1 and S2 read the same resource and S1 precedes S2 in execution. The following is an example of an input dependence (RAR: Read-After-Read):
S1 y := x + 3 S2 z := x + 5
Here, S2 and S1 both access the variable
x. This dependence does not prohibit reordering.
The problem of computing dependencies within loops, which is a significant and nontrivial problem, is tackled by loop dependence analysis, which extends the dependence framework given here.
- Cooper, Keith D.; Torczon, Linda. (2005). Engineering a Compiler. Morgan Kaufmann. ISBN 1-55860-698-X.
- Kennedy, Ken; Allen, Randy. (2001). Optimizing Compilers for Modern Architectures: A Dependence-based Approach. Morgan Kaufmann. ISBN 1-55860-286-0.
- Muchnick, Steven S. (1997). Advanced Compiler Design and Implementation. Morgan Kaufmann. ISBN 1-55860-320-4.