1 / 15

CS 201 Compiler Construction

CS 201 Compiler Construction. Lecture 9 Static Single Assignment Form. Program Representations. Why develop Advanced Program Representations? To develop faster algorithms To develop more powerful algorithms Superior representation for Data Flow Static Single Assignment Form (SSA Form)

nonnie
Download Presentation

CS 201 Compiler Construction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. CS 201Compiler Construction Lecture 9 Static Single Assignment Form

  2. Program Representations Why develop Advanced Program Representations? • To develop faster algorithms • To develop more powerful algorithms Superior representation for Data Flow Static Single Assignment Form (SSA Form) superior to def-use chains Superior representation for Control Flow Control Dependence Graph superior to control flow graph

  3. SSA-Form A program in SSA-form satisfies the following two properties: • A use of a variable is reached by exactly one definition of that variable. • The program is augmented with ϕ–nodes that distinguish values of variables transmitted on distinct incoming control flow edges.

  4. Example K11 L11 Repeat K2ϕ(K1,K5) L2ϕ(L1,L6) if (P) then if (Q) then L32 else L43 L5ϕ(L3,L4) K3K2+1 else K4K2+2 K5ϕ(K3,K4) L6ϕ(L2,L5) Until (T) K  1 L  1 Repeat if (P) then if (Q) then L2 else L3 KK+1 else KK+2 Until (T)

  5. SSA-Form Observations: • SSA-form has def-use information textually embedded in it. Given a use, we know where the definition comes from. • SSA-form is more compact representation of def-use chains. • Def-use chains: #defs x #uses – O(n2) • SSA-form: 2 x #defs or #uses – O(n)

  6. Constructing SSA-Form Step 1: Introduce functions at certain points in the program -- v  ϕ (v,v,….) where of operands equals number of control predecessors and ith operand corresponds to ith predecessor.

  7. Contd.. Step 2: Each variable v is given several new names v1, v2, …. Such that every name appears exactly once on the left hand side of an assignment.

  8. Step 1: Introducing ϕ-functions Node Z needs a ϕ-function for variable V if Z is the first node common to two non-null paths that originate at two different nodes each containing: an assignment to V; or a ϕ-function for V.

  9. Step 1 Contd.. Definition: X strictly dominates Y ≅ X dominates Y & X != Y. Definition: Immediate dominator of a node is its closest strict dominator. Notation: X = idom(Y). Definition: Dominance Frontier DF(X) = {Y: there exists P εpred(Y) such that X dominates P & X does not strictly dominate Y} ϕ-functions are placed at nodes in DF nodes of nodes with assignments.

  10. Step 1 Contd.. Y ε DF(X) Strict Domination Does Not Strictly Domination Dominate

  11. Step 1 Contd.. Observation: if Y εDF(X) then there may or may not be a direct edge from X to Y.

  12. Step 1 Contd.. Computing Dominance Frontier: for each Y εsucc(X) do if idom(Y) != X then DF(X) = DF(X) U {Y} for each Z εChidren(X) in the dominator tree do for each Y εDF(Z) do if idom(Y) != X then DF(X) = DF(X) U {Y} Compute bottom-up order According to dominator tree

  13. Step 1 Contd.. Dominator Tree

  14. Step 1 Contd.. Dominance Frontier of a Set of Nodes S DF(S) = DF(X) Iterated Dominance Frontier DF+(S): DF1 = DF(S) DFi+1 = DF(S U DFi) S – set of nodes which assign to variable V DF+(S) – set of nodes including those where ϕ-functions must be placed.

  15. Step 2: Rename the Variables Step 2: For each variable v rename its left hand side occurrences as v1, v2, …. Perform reaching definition analysis to identify names to use in the right hand side occurrences of v.

More Related