Variations that reduce the number of Φ functions
#MINITAB 17 TUTORIAL PDF CODE#
Note: in the code above, an immediate predecessor of node n is any node from which control is transferred to node n, and idom(b) is the node that immediately dominates node b (a singleton set). For example, consider the following control-flow graph:Ĭhanging the name on the left hand side of "x ← Register allocation – optimize how the limited number of machine registers may be used for calculationsĬonverting ordinary code into SSA form is primarily a matter of replacing the target of each assignment with a new variable, and replacing each use of a variable with the "version" of the variable reaching that point.replace integer multiply or divide by powers of 2 with the potentially less expensive shift left (for multiply) or shift right (for divide). Strength reduction – replacing expensive operations by less expensive but equivalent ones, e.g.
Constant propagation – conversion of computations from runtime to compile time, e.g.But if the program is in SSA form, both of these are immediate:Ĭompiler optimization algorithms that are either enabled or strongly enhanced by the use of SSA include: A program would have to perform reaching definition analysis to determine this. Humans can see that the first assignment is not necessary, and that the value of y being used in the third line comes from the second assignment of y. For example, consider this piece of code: The primary usefulness of SSA comes from how it simultaneously simplifies and improves the results of a variety of compiler optimizations, by simplifying the properties of variables. 3 Variations that reduce the number of Φ functions.2.1 Computing minimal SSA using dominance frontiers.So optimizations and transformations formulated in terms of one immediately apply to the other. SSA is formally equivalent to a well-behaved subset of CPS excluding non-local control flow, which does not occur when CPS is used as intermediate representation. One can expect to find SSA in a compiler for Fortran, C or C++, whereas in functional language compilers, such as those for Scheme and ML, continuation-passing style (CPS) is generally used. Ron Cytron, Jeanne Ferrante and the previous three researchers at IBM developed an algorithm that can compute the SSA form efficiently. In SSA form, use-def chains are explicit and each contains a single element. Existing variables in the original IR are split into versions, new variables typically indicated by the original name with a subscript in textbooks, so that every definition gets its own version. In compiler design, static single assignment form (often abbreviated as SSA form or simply SSA) is a property of an intermediate representation (IR), which requires that each variable be assigned exactly once, and every variable be defined before it is used.