22 - Interprocedural Analysis Dealing with Procedures...

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Interprocedural Analysis Course so far: Control Flow Representation Dataflow Representation SSA form Classic DefUse and UseDef Chains Optimizations Scheduling Register Allocation Just-In-Time Compilation GC Alias analysis All of these have been for an individual procedure - intraprocedural analysis. Today: Interprocedural analysis (across/between procedures) Brief overview CS 380C Lecture 22 1 Interprocedural Analysis Dealing with Procedures Terminology int a, e // globals procedure foo(var b, c) // formal args b := c end program main int d // locals foo(a, d) // call site with end // actual args In procedure body formals and/or globals may be aliased (two names refer to same location) formals may have constant value At procedure call global vars may be modified or used actual args may be modified or used Conservative assumptions for procedures may significantly inhibit optimization CS 380C Lecture 22 2 Interprocedural Analysis
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Dealing with Procedures Aliasing example int a foo(a, a) ... procedure foo(var b, c) a := 1 // what is b, c? b := 2 // what is a, c? c := 3 // what is a, b? d := a + b // what is d? end Side e ff ect example a := 1 foo(a) // what is used/killed? b := a // what is b? Constants example foo(1) ... procedure foo(a) if (a = 1) // what is a’s value? ... CS 380C Lecture 22 3 Interprocedural Analysis Characterizing an Analysis Definiteness May captures possibility optimizations “must account for” Must captures certainty optimizations “may rely on” Flow sensitivity Sensitive (consider control flow) Determine value for each program point Requires data-flow analysis Precise Insensitive (ignore control flow) Determine value for whole procedure Can compute in linear time Imprecise CS 380C Lecture 22 4 Interprocedural Analysis
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Characterizing an Analysis Context sensitivity Sensitive (aka polyvariant analysis ) Determine values for each calling context Must re-analyze callee for each caller Insensitive (aka monovariant analysis ) Summarize values for all calling contexts Cheap(ish) but imprecise Recall : Procedure body and procedure call int a, b, c; main() s1: f(a); s2: f(b); foo(x) c = x; return; CS 380C Lecture 22 5 Interprocedural Analysis Why don’t many compilers use IP analysis? Inlining - poor woman’s interprocedural analysis. Is inlining an alternative, or is it part of IP analysis and transformation? Separate compilation with IP analysis requires recompilation analysis . IP analysis can be expensive. e.g., Flow-sensitive kill is intractable. Benefits of IP analysis on optimization have not been well explored, especially for scalar machines. Constants: Grove/Torczon’93, Metzer/Stoud’93 Dependence analysis: Paek et al.’98 Li et al.‘90, Havlak/Kennedy’90 Parallelizing Compilers: Hall et al.’95, McKinley’93 It is hard – solutions can be complex.
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