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Using Data Groups to Specify and Check Side Effects

Using Data Groups to Specify and Check Side Effects. K. Rustan M. Leino, Arnd Poetzsch-Heffter, and Yunhong Zhou Presented by Jonathan Aldrich. Outline. The Problem Checking effects in a modular and extensible way Data Groups Aliasing Restrictions Discussion. Effect Specifications.

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Using Data Groups to Specify and Check Side Effects

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  1. Using Data Groups to Specify and Check Side Effects K. Rustan M. Leino, Arnd Poetzsch-Heffter, and Yunhong Zhou Presented by Jonathan Aldrich

  2. Outline • The Problem • Checking effects in a modular and extensible way • Data Groups • Aliasing Restrictions • Discussion

  3. Effect Specifications proc p(x,y) modifies x.f,y.g { ... } • Describes the variables p might modify • Uses • Optimizations • Bug finding • Semantics

  4. Specification Challenges proc p(x,y) modifies x.f,y.g { ... } • Information hiding • Don’t want to reveal fields f and g • Extensibility • proc subcls.p(x,y) modifies x.f,y.g,x.h • Soundness • Does p(x,y) modify y.f?

  5. Outline • The Problem • Data Groups • Aliasing Restrictions • Discussion

  6. Data Group • A set of variables and nested data groups • Membership defined incrementally • A field/group can be part of multiple groups // class Collection group state group elems in state field array in elems // class Vector extends Collection field cnt in elems group state group elems field array field cnt

  7. Effect Specifications proc add(coll,o) modifies coll.elems • Information hiding • List data groups rather than fields • Can hide fields behind module interface • Extensibility • Subclasses add new fields to existing groups • So, Vector.add can modify size • What about soundness, expressiveness?

  8. Pivot Fields // class Stack group contents field vec maps elems into contents proc push(stk,o) modifies stk.contents • Provides hierarchy • Effect on vec specified through stk’s group

  9. Outline • The Problem • Data Groups • Aliasing Restrictions • Discussion

  10. Aliasing Problem #1 proc m(st, r) modifies r.obj impl q() { var st := new(); var result := new(); m(st,result); var v := result.obj; var n := v.cnt; push(st, 3); assert (n=v.cnt); } impl m(st, r) { r.obj := st.vec } Modifies v.contents (including v.cnt) if v == st

  11. Pivot Uniqueness • Three restrictions • Pivot fields can only be assigned new or null • Can’t assign a pivot field to anything • This avoid the previous problem • Can pass as parameter • But can’t assign to/from formal parameters • Aliasing Invariant • Pivot fields are unique • Except for formal parameter aliases

  12. Aliasing Problem #2 proc w(st, v) modifies st.contents impl w(st,v) { var n := v.cnt; push(st, 3); assert (n=v.cnt); } w(st, st.vec) Problem: st.vec is available through both st and v in w, but this alias is not obvious in w

  13. Owner Exclusion • Suppose f maps x into g • If p(s) can modify t.g, then s != t.f • In particular, since w(st, v) can modify st.contents, then v != st.vec • Question: how do they verify owner exclusion? • What about w(st1, st2.vec) • where st1 == st2? • I think they would conservatively reject this, even in cases where st1 != st2 • To do better you’d have to track more (non-)aliasing

  14. Outline • The Problem • Data Groups • Aliasing Restrictions • Discussion

  15. Extensible Specifications • Data groups • subclasses add new groups, new fields to existing groups • Ownership • subclasses add new domains, new objects to existing domains • Typestate • subclasses add new typestates, new predicates over added fields

  16. Ownership vs. Data Groups • Data groups can overlap • Allows a field to hold state that is conceptually part of two groups • Ownership is more flexible • Can express iterator, observer idioms • How to specify effects for these idioms?

  17. Ownership for Effect Specs • Some work in this area already • [Clarke & Drossopolou] • Ownership could support iterator effects • Call add on an Iterator in the c.iters domain • Effect is c.elems

  18. S/W Arch. and Effect Specs • Global Effect Problem • Notify callbacks can have large, arbitrary effects • The Achilles' heel of existing effect systems • Specifying all effects does not scale • Specifying global effects causes coupling • Solution: Local Effect Specs? • Specify only the “locally visible” effect within a component • All effects on local and shared data • All calls to external functions • Document assumptions about locally-visible effects of external functions • Component composition must respect assumptions • Global effects computed from local specs + architecture

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