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Control Dependencies

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  1. Control Dependencies • A node (basic block) Y is control-dependent on another X iff X determines whether Y executes • there exists a path from X to Y s.t. every node in the path other than X and Y is post-dominated by Y • X is not post-dominated by Y

  2. Example

  3. Example

  4. Control Dependence Graph • Control dependence graph: Y descendent of X iff Y is control dependent on X • label each child edge with required condition • group all children with same condition under region node • Program dependence graph: super-impose dataflow graph (in SSA form or not) on top of the control dependence graph

  5. Example

  6. Example

  7. Another example

  8. Another example

  9. Another example

  10. Summary of Control Depence Graph • More flexible way of representing control-depencies than CFG (less constraining) • Makes code motion a local transformation • However, much harder to convert back to an executable form

  11. Course summary so far • Dataflow analysis • flow functions, lattice theoretic framework, optimistic iterative analysis, precision, MOP • Advanced Program Representations • SSA, CDG, PDG • Along the way, several analyses and opts • reaching defns, const prop & folding, available exprs & CSE, liveness & DAE, loop invariant code motion • Next: dealing with procedures

  12. Interprocedural analyses and optimizations

  13. Costs of procedure calls • Up until now, we treated calls conservatively: • make the flow function for call nodes return top • start iterative analysis with incoming edge of the CFG set to top • This leads to less precise results: “lost-precision” cost • Calls also incur a direct runtime cost • cost of call, return, argument & result passing, stack frame maintainance • “direct runtime” cost

  14. Addressing costs of procedure calls • Technique 1: try to get rid of calls, using inlining and other techniques • Technique 2: interprocedural analysis, for calls that are left

  15. Inlining • Replace call with body of callee • Turn parameter- and result-passing into assignments • do copy prop to eliminate copies • Manage variable scoping correctly • rename variables where appropriate

  16. Call graph nodes are procedures edges are calls, labelled by invocation counts/frequency Hard cases for builing call graph calls to/from external routines calls through pointers, function values, messages Where in the compiler should inlining be performed? Program representation for inlining

  17. Inlining pros and cons (discussion)

  18. Inlining pros and cons • Pros • eliminate overhead of call/return sequence • eliminate overhead of passing args & returning results • can optimize callee in context of caller and vice versa • Cons • can increase compiled code space requirements • can slow down compilation • recursion? • Virtual inlining: simulate inlining during analysis of caller, but don’t actually perform the inlining

  19. Which calls to inline (discussion) • What affects the decision as to which calls to inline?

  20. Which calls to inline • What affects the decision as to which calls to inline? • size of caller and callee (easy to compute size before inlining, but what about size after inlining?) • frequency of call (static estimates or dynamic profiles) • call sites where callee benefits most from optimization (not clear how to quantify) • programmer annotations (if so, annotate procedure or call site? Also, should the compiler really listen to the programmer?)

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