Add alpha equivalence check for Expr, and use it when lambda lifting

6 files changed, 308 insertions, 43 deletions

diff --git a/Source/Core/AbsyExpr.cs b/Source/Core/AbsyExpr.cs
index bc603ea5..7e71453b 100644
--- a/Source/Core/AbsyExpr.cs
+++ b/Source/Core/AbsyExpr.cs
@@ -1893,7 +1893,19 @@ namespace Microsoft.Boogie {
       this.Type = type;

     }

 

-    public string/*!*/ FunctionName {

+    public override bool Equals(object obj) {

+      TypeCoercion other = obj as TypeCoercion;

+      if (other == null) {

+        return false;

+      } else {

+        return object.Equals(Type, other.Type);

+      }

+    }

+

+

+

+    public

+      string/*!*/ FunctionName {

       get {

         Contract.Ensures(Contract.Result<string>() != null);

 

@@ -2110,7 +2122,7 @@ namespace Microsoft.Boogie {
         return false;

 

       NAryExpr other = (NAryExpr)obj;

-      return object.Equals(this.Fun, other.Fun) && object.Equals(this.Args, other.Args);

+      return this.Args.ListEquals(other.Args) && object.Equals(this.Fun, other.Fun); 

     }

     [Pure]

     public override int GetHashCode() {

diff --git a/Source/Core/AbsyQuant.cs b/Source/Core/AbsyQuant.cs
index c9e8f210..9425f698 100644
--- a/Source/Core/AbsyQuant.cs
+++ b/Source/Core/AbsyQuant.cs
@@ -57,7 +57,6 @@ namespace Microsoft.Boogie {
       Contract.Invariant(Body != null);

     }

 

-

     public BinderExpr(IToken/*!*/ tok, List<TypeVariable>/*!*/ typeParameters,

                       List<Variable>/*!*/ dummies, QKeyValue kv, Expr/*!*/ body)

       : base(tok)

@@ -77,26 +76,45 @@ namespace Microsoft.Boogie {
       get;

     }

 

+    protected static bool CompareAttributesAndTriggers = false;

+

+    public static bool EqualWithAttributesAndTriggers(object a, object b) {

+      CompareAttributesAndTriggers = true;

+      var res = object.Equals(a, b);

+      Contract.Assert(CompareAttributesAndTriggers);

+      CompareAttributesAndTriggers = false;

+      return res;

+    }

+

     [Pure]

     [Reads(ReadsAttribute.Reads.Nothing)]

     public override bool Equals(object obj) {

-      if (obj == null)

+      return BinderEquals(obj);

+    }

+

+    public bool BinderEquals(object obj) {

+      if (obj == null) {

         return false;

+      }

       if (!(obj is BinderExpr) ||

-          this.Kind != ((BinderExpr)obj).Kind)

+          this.Kind != ((BinderExpr) obj).Kind) {

         return false;

+      }

 

-      BinderExpr other = (BinderExpr)obj;

-      // Note, we consider quantifiers equal modulo the Triggers.

-      return object.Equals(this.TypeParameters, other.TypeParameters)

-             && object.Equals(this.Dummies, other.Dummies)

-             && object.Equals(this.Body, other.Body);

+      var other = (BinderExpr) obj;

+

+      var a = this.TypeParameters.ListEquals(other.TypeParameters);

+      var b = this.Dummies.ListEquals(other.Dummies);

+      var c = !CompareAttributesAndTriggers || object.Equals(this.Attributes, other.Attributes);

+      var d = object.Equals(this.Body, other.Body);

+

+      return a && b && c && d;

     }

 

     [Pure]

     public override int GetHashCode() {

       int h = this.Dummies.GetHashCode();

-      // Note, we consider quantifiers equal modulo the Triggers.

+      // Note, we don't hash triggers and attributes

       h ^= this.Body.GetHashCode();

       h = h * 5 + this.TypeParameters.GetHashCode();

       h *= ((int)Kind + 1);

@@ -335,6 +353,22 @@ namespace Microsoft.Boogie {
     public override Absy StdDispatch(StandardVisitor visitor) {

       return visitor.VisitQKeyValue(this);

     }

+

+    public override bool Equals(object obj) {

+      var other = obj as QKeyValue;

+      if (other == null) {

+        return false;

+      } else {

+        return Key == other.Key && object.Equals(Params, other.Params) &&

+               (Next == null

+                 ? other.Next == null

+                 : object.Equals(Next, other.Next));

+      }

+    }

+

+    public override int GetHashCode() {

+      throw new NotImplementedException();

+    }

   }

 

   public class Trigger : Absy {

@@ -432,6 +466,20 @@ namespace Microsoft.Boogie {
       Contract.Ensures(Contract.Result<Absy>() != null);

       return visitor.VisitTrigger(this);

     }

+

+    public override bool Equals(object obj) {

+      var other = obj as Trigger;

+      if (other == null) {

+        return false;

+      } else {

+        return this.Tr.ListEquals(other.Tr) && 

+          (Next == null ? other.Next == null : object.Equals(Next, other.Next));

+      }

+    }

+

+    public override int GetHashCode() {

+      throw new NotImplementedException();

+    }

   }

 

   public class ForallExpr : QuantifierExpr {

@@ -720,6 +768,15 @@ namespace Microsoft.Boogie {
       }

     }

 

+    public override bool Equals(object obj) {

+      var other = obj as QuantifierExpr;

+      if (other == null) {

+        return false;

+      } else {

+        return this.BinderEquals(obj) && 

+          (!CompareAttributesAndTriggers || object.Equals(Triggers, other.Triggers));

+      }

+    }

   }

 

 

diff --git a/Source/Core/AlphaEquality.cs b/Source/Core/AlphaEquality.cs
new file mode 100644
index 00000000..1d4a1d95
--- /dev/null
+++ b/Source/Core/AlphaEquality.cs
@@ -0,0 +1,162 @@
+//-----------------------------------------------------------------------------

+//

+// Copyright (C) Microsoft Corporation.  All Rights Reserved.

+//

+//-----------------------------------------------------------------------------

+

+using System.ComponentModel;

+

+namespace Microsoft.Boogie

+{

+

+  using System;

+  using System.IO;

+  using System.Collections;

+  using System.Collections.Generic;

+  using System.Diagnostics;

+  using System.Diagnostics.Contracts;

+

+  public class AlphaEquality : IEqualityComparer<Expr>

+  {

+    private readonly DeBruijnRenamer deBruijn = new DeBruijnRenamer();

+

+    bool IEqualityComparer<Expr>.Equals(Expr x, Expr y) {

+      var nx = deBruijn.Rename(x);

+      var ny = deBruijn.Rename(y);

+      return BinderExpr.EqualWithAttributesAndTriggers(nx, ny);

+    }

+

+    int IEqualityComparer<Expr>.GetHashCode(Expr obj) {

+      return 0; 

+      // Best we can do because GetHashCode for Expression don't respect its equality.

+      // When it does, we can instead use: 

+      // return deBruijn.Rename(obj).GetHashCode(); 

+    }

+

+    // Renames expressions into deBruijn indicies, such as

+    //     (lambda x : int :: x + a) 

+    // into

+    //     (lambda bv#0 : int :: bv#0 + fv#0) 

+    // It does not handle type variables yet, but it could be added.

+    //

+    // This class could be made public, but it is not since the Rename method 

+    // could then leak FreeVariables out of here.

+    private class DeBruijnRenamer : Duplicator

+    {

+

+      // Maps from index positions and types to new variables

+      private readonly TypeDict<BoundVariable> boundVars =

+        new TypeDict<BoundVariable>("bv", ti => new BoundVariable(Token.NoToken, ti));

+

+      private readonly TypeDict<FreeVariable> freeVars =

+        new TypeDict<FreeVariable>("fv", ti => new FreeVariable(ti));

+

+      // These three variables are reset at the beginning of every renaming

+      private int boundVarCount, freeVarCount;

+      private Dictionary<Variable, FreeVariable> freeVarMap;

+

+      // Cached, previous results

+      private readonly Dictionary<Expr, Expr> cache = new Dictionary<Expr, Expr>();

+

+      public Expr Rename(Expr e) {

+        Expr ne;

+        if (!cache.TryGetValue(e, out ne)) {

+          boundVarCount = 0;

+          freeVarCount = 0;

+          freeVarMap = new Dictionary<Variable, FreeVariable>();

+

+          ne = VisitExpr(e);

+          cache[e] = ne;

+#if DEBUG_ALPHA_RENAMING

+          var wr = new TokenTextWriter("<console>", Console.Out, true);

+          Console.Write("nm( ");

+          e.Emit(wr);

+          Console.WriteLine(" )");

+          Console.Write("  = ");

+          ne.Emit(wr);

+          Console.WriteLine("");

+          Console.WriteLine("h = " + ne.GetHashCode());

+#endif

+        }

+        return ne;

+      }

+

+      public override BinderExpr VisitBinderExpr(BinderExpr node) {

+        var subst = new Dictionary<Variable, Expr>();

+        var newBound = new List<Variable>();

+        foreach (var bv in node.Dummies) {

+          var bvNew = boundVars[boundVarCount++, bv.TypedIdent.Type];

+          newBound.Add(bvNew);

+          subst[bv] = new IdentifierExpr(Token.NoToken, bvNew);

+        }

+        node.Dummies = this.VisitVariableSeq(newBound);

+        node.Body = this.VisitExpr(Substituter.Apply(Substituter.SubstitutionFromHashtable(subst), node.Body));

+        return node;

+      }

+

+      public override Variable VisitVariable(Variable node) {

+        FreeVariable fv;

+        var bv = node as BoundVariable;

+        if (boundVars.ContainsValue(bv)) {

+          return node;

+        } else if (freeVarMap.TryGetValue(node, out fv)) {

+          return fv;

+        } else {

+          return freeVarMap[node] = freeVars[freeVarCount++, node.TypedIdent.Type];

+        }

+      }

+

+      public override Expr VisitIdentifierExpr(IdentifierExpr node) {

+        var ie = (IdentifierExpr) base.VisitIdentifierExpr(node);

+        // Need to fix up the name, since IdentifierExpr's equality also checks the name

+        ie.Name = ie.Decl.TypedIdent.Name;

+        return ie;

+      }

+

+      private class TypeDict<A>

+      {

+        private readonly Dictionary<Tuple<int, Type>, A> vars = new Dictionary<Tuple<int, Type>, A>();

+

+        private readonly string Prefix;          // either "bv" or "fv"

+        private readonly Func<TypedIdent, A> Mk; // either new BoundVar or new FreeVar

+

+        public TypeDict(string prefix, Func<TypedIdent, A> mk) {

+          Prefix = prefix;

+          Mk = mk;

+        }

+

+        // For debugging purposes, we create unique names when types differ, but the index are the same.

+        private int created = 0;

+

+        // Make sure that this index and this type is always mapped to the same variable

+        public A this[int i, Type t] {

+          get {

+            A v;

+            if (!vars.TryGetValue(Tuple.Create(i, t), out v)) {

+              v = Mk(new TypedIdent(Token.NoToken, Prefix + i + "#" + created++, t));

+              vars[Tuple.Create(i, t)] = v;

+            }

+            return v;

+          }

+        }

+

+        public bool ContainsValue(A a) {

+          return vars.ContainsValue(a);

+        }

+      }

+

+      private class FreeVariable : Variable

+      {

+        public FreeVariable(TypedIdent ti) : base(Token.NoToken, ti) {}

+

+        public override bool IsMutable {

+          get { throw new cce.UnreachableException(); }

+        }

+

+        public override void Register(ResolutionContext rc) {

+          throw new cce.UnreachableException();

+        }

+      }

+    }

+  }

+}

diff --git a/Source/Core/Core.csproj b/Source/Core/Core.csproj
index 881daa7f..27f4eea7 100644
--- a/Source/Core/Core.csproj
+++ b/Source/Core/Core.csproj
@@ -164,6 +164,7 @@
     <Compile Include="AbsyExpr.cs" />

     <Compile Include="AbsyQuant.cs" />

     <Compile Include="AbsyType.cs" />

+    <Compile Include="AlphaEquality.cs" />

     <Compile Include="InterProceduralReachabilityGraph.cs" />

     <Compile Include="CommandLineOptions.cs" />

     <Compile Include="DeadVarElim.cs" />

diff --git a/Source/Core/LambdaHelper.cs b/Source/Core/LambdaHelper.cs
index c884be3c..45c68fdd 100644
--- a/Source/Core/LambdaHelper.cs
+++ b/Source/Core/LambdaHelper.cs
@@ -52,6 +52,9 @@ namespace Microsoft.Boogie {
     }

 

     private class LambdaVisitor : StandardVisitor {

+      private readonly Dictionary<Expr, FunctionCall> liftedLambdas = 

+        new Dictionary<Expr, FunctionCall>(new AlphaEquality());

+

       internal List<Expr/*!*/>/*!*/ lambdaAxioms = new List<Expr/*!*/>();

       internal List<Function/*!*/>/*!*/ lambdaFunctions = new List<Function/*!*/>();

       [ContractInvariantMethod]

@@ -148,43 +151,61 @@ namespace Microsoft.Boogie {
           }

         }

 

+        var sw = new System.IO.StringWriter();

+        var wr = new TokenTextWriter(sw, true);

+        lambda.Emit(wr);

+        string lam_str = sw.ToString();

+

+        FunctionCall fcall;

         IToken tok = lambda.tok;

         Formal res = new Formal(tok, new TypedIdent(tok, TypedIdent.NoName, cce.NonNull(lambda.Type)), false);

-        Function fn = new Function(tok, "lambda@" + lambdaid++, freshTypeVars, formals, res, "auto-generated lambda function",

-          Substituter.Apply(Substituter.SubstitutionFromHashtable(substFnAttrs), lambdaAttrs));

-        lambdaFunctions.Add(fn);

 

-        FunctionCall fcall = new FunctionCall(new IdentifierExpr(tok, fn.Name));

-        fcall.Func = fn;  // resolve here

+        if (liftedLambdas.TryGetValue(lambda, out fcall)) {

+          if (CommandLineOptions.Clo.TraceVerify) {

+            Console.WriteLine("Old lambda: {0}", lam_str);

+          }

+        } else {

+          if (CommandLineOptions.Clo.TraceVerify) {

+            Console.WriteLine("New lambda: {0}", lam_str);

+          }

+          Function fn = new Function(tok, "lambda#" + lambdaid++, freshTypeVars, formals, res, "auto-generated lambda function",

+            Substituter.Apply(Substituter.SubstitutionFromHashtable(substFnAttrs), lambdaAttrs));

+

+          fcall = new FunctionCall(new IdentifierExpr(tok, fn.Name));

+          fcall.Func = fn;  // resolve here

+          liftedLambdas[lambda] = fcall;

 

-        List<Expr/*!*/> selectArgs = new List<Expr/*!*/>();

-        foreach (Variable/*!*/ v in lambda.Dummies) {

-          Contract.Assert(v != null);

-          selectArgs.Add(new IdentifierExpr(v.tok, v));

-        }

-        NAryExpr axcall = new NAryExpr(tok, fcall, axCallArgs);

-        axcall.Type = res.TypedIdent.Type;

-        axcall.TypeParameters = SimpleTypeParamInstantiation.From(freeTypeVars, fnTypeVarActuals);

-        NAryExpr select = Expr.Select(axcall, selectArgs);

-        select.Type = lambdaBody.Type;

-        List<Type/*!*/> selectTypeParamActuals = new List<Type/*!*/>();

-        List<TypeVariable> forallTypeVariables = new List<TypeVariable>();

-        foreach (TypeVariable/*!*/ tp in lambda.TypeParameters) {

-          Contract.Assert(tp != null);

-          selectTypeParamActuals.Add(tp);

-          forallTypeVariables.Add(tp);

+          List<Expr/*!*/> selectArgs = new List<Expr/*!*/>();

+          foreach (Variable/*!*/ v in lambda.Dummies) {

+            Contract.Assert(v != null);

+            selectArgs.Add(new IdentifierExpr(v.tok, v));

+          }

+          NAryExpr axcall = new NAryExpr(tok, fcall, axCallArgs);

+          axcall.Type = res.TypedIdent.Type;

+          axcall.TypeParameters = SimpleTypeParamInstantiation.From(freeTypeVars, fnTypeVarActuals);

+          NAryExpr select = Expr.Select(axcall, selectArgs);

+          select.Type = lambdaBody.Type;

+          List<Type/*!*/> selectTypeParamActuals = new List<Type/*!*/>();

+          List<TypeVariable> forallTypeVariables = new List<TypeVariable>();

+          foreach (TypeVariable/*!*/ tp in lambda.TypeParameters) {

+            Contract.Assert(tp != null);

+            selectTypeParamActuals.Add(tp);

+            forallTypeVariables.Add(tp);

+          }

+          forallTypeVariables.AddRange(freeTypeVars);

+          select.TypeParameters = SimpleTypeParamInstantiation.From(lambda.TypeParameters, selectTypeParamActuals);

+

+          Expr bb = Substituter.Apply(Substituter.SubstitutionFromHashtable(subst), lambdaBody);

+          NAryExpr body = Expr.Eq(select, bb);

+          body.Type = Type.Bool;

+          body.TypeParameters = SimpleTypeParamInstantiation.EMPTY;

+          Trigger trig = new Trigger(select.tok, true, new List<Expr> { select });

+

+          lambdaFunctions.Add(fn);

+          lambdaAxioms.Add(new ForallExpr(tok, forallTypeVariables, dummies,

+            Substituter.Apply(Substituter.SubstitutionFromHashtable(subst), lambdaAttrs),

+            trig, body));

         }

-        forallTypeVariables.AddRange(freeTypeVars);

-        select.TypeParameters = SimpleTypeParamInstantiation.From(lambda.TypeParameters, selectTypeParamActuals);

-

-        Expr bb = Substituter.Apply(Substituter.SubstitutionFromHashtable(subst), lambdaBody);

-        NAryExpr body = Expr.Eq(select, bb);

-        body.Type = Type.Bool;

-        body.TypeParameters = SimpleTypeParamInstantiation.EMPTY;

-        Trigger trig = new Trigger(select.tok, true, new List<Expr> { select });

-        lambdaAxioms.Add(new ForallExpr(tok, forallTypeVariables, dummies,

-          Substituter.Apply(Substituter.SubstitutionFromHashtable(subst), lambdaAttrs),

-          trig, body));

 

         NAryExpr call = new NAryExpr(tok, fcall, callArgs);

         call.Type = res.TypedIdent.Type;

diff --git a/Source/Core/Util.cs b/Source/Core/Util.cs
index 75cb25aa..1b8353e1 100644
--- a/Source/Core/Util.cs
+++ b/Source/Core/Util.cs
@@ -66,6 +66,18 @@ namespace Microsoft.Boogie {
     {

       return source1.Zip(source2, (e1, e2) => new Tuple<TSource1, TSource2>(e1, e2));

     }

+

+    public static bool ListEquals<A>(this List<A> xs, List<A> ys) {

+      var equal = xs.Count == ys.Count;

+      for (int i = 0; equal && i < xs.Count; i++) {

+        equal = object.Equals(xs[i], ys[i]);

+      }

+      return equal;

+    }

+

+    public static int ListHash<A>(this List<A> xs) {

+      return xs.Aggregate(xs.Count, (current, x) => current ^ xs.GetHashCode());

+    }

   }

 

   public class TokenTextWriter : IDisposable {