Foundations of Separation Logic Chapter: "WPsem". Author: Arthur Charguéraud. License: CC-by 4.0.

Chapter in a rush

In previous chapters, we have introduced the notion of Separation Logic triple, written triple t H Q. In this chapter, we introduce the notion of "weakest precondition" for Separation Logic triples, written wp t Q. The intention is for wp t Q to be a heap predicate (of type hprop) such that H ==> wp t Q if and only if triple t H Q holds. The benefits of introducing weakest preconditions is two-fold:

• the use of wp greatly reduces the number of structural rules required, and thus reduces accordingly the number of tactics required for carrying out proofs in practice;
• the predicate wp will serve as guidelines for setting up in the next chapter a "characteristic formula generator", which is the key ingredient at the heart of the implementation of the CFML tool.

This chapter presents:

• the notion of weakest precondition, as captured by wp,
• the reformulation of structural rules in wp-style,
• the reformulation of reasoning rules in wp-style,
• (optional) alternative, equivalent definitions for wp, and alternative proofs for deriving wp-style reasoning rules.

Notion of weakest precondition

We next introduce a function wp, called "weakest precondition". Given a term t and a postcondition Q, the expression wp t Q denotes a heap predicate wp t Q such that, for any heap predicate H, the entailment H ==> wp t Q is equivalent to triple t H Q. The notion of wp usually sounds fairly mysterious at first sight. It will make more sense when we describe the properties of wp.

The wp t Q is called "weakest precondition" for two reasons: because (1) it is a precondition, and (2) it is the weakest one, as we explain next. First, wp t Q is always a "valid precondition" for a triple associated with the term t and the postcondition Q.

Second, wp t Q is the "weakest" of all valid preconditions for the term t and the postcondition Q, in the sense that any other valid precondition H, i.e. satisfying triple t H Q, is such that H entails wp t Q.

In other words, wp t Q is the "smallest" H satisfying triple t H Q with respect to the order on heap predicates induced by the entailment relation ==>. There are several equivalent ways to define wp, as we show in the optional contents of this chapter. It turns out that the equivalence (H ==> wp t Q) ↔ (triple t H Q) fully characterizes the predicate wp, and that it is all we need to carry out formal reasoning. For this reason, we postpone to further on in this chapter the description of alternative, direct definitions for wp.

Structural rules in weakest-precondition style

We next present reformulations of the frame rule and of the rule of consequence in "weakest-precondition style".

The frame rule

The frame rule for wp asserts that (wp t Q) \* H entails wp t (Q \*+ H). This statement can be read as follows: if you own both a piece of state satisfying H and a piece of state in which the execution of t produces (a result and an output value satisfying) Q, then you own a piece of state in which the execution of t produces Q \*+ H, that is, produces both Q and H.

The lemma is proved by exploiting the frame rule for triples and the equivalence that characterizes wp.

The connection with the frame might not be totally obvious. Recall the frame rule for triples.
    triple t H₁ Q →
    triple t (H₁ \* H) (Q \*+ H)

Let us replace the form triple t H Q with the form H ==> wp t Q. We obtain the following statement.

If we exploit transitivity of entailment to eliminate H₁, then we obtain exactly wp_frame, as illustrated by the proof script below.

The rule of consequence

The rule of consequence for wp materializes as a covariance property: it asserts that wp t Q is covariant in Q. In other words, if one weakens Q, then one weakens wp t Q. The corresponding formal statement appears next.

The connection with the rule of consequence is, again, not totally obvious. Recall the rule of consequence for triples.
    triple t H₁ Q₁ →
    H₂ ==> H₁ →
    Q₁ ===> Q₂ →
    triple t H₂ Q₂

Let us replace the form triple t H Q with the form H ==> wp t Q. We obtain the following statement:

If we exploit transitivity of entailment to eliminate H₁ and H₂, then we obtain exactly wp_conseq, as illustrated below.

The extraction rules

The extraction rules triple_hpure and triple_hexists have no specific counterpart with the wp presentation. Indeed, in a weakest-precondition style presentation, the extraction rules for triples correspond exactly to the extraction rules for entailment. To see why, consider for example the rule triple_hpure.

Replacing the form triple t H Q with H ==> wp t Q yields the following statement.

The above implication is just a special case of the extraction lemma for pure facts on the left on an entailment, named himpl_hstar_hpure_l, and whose statement is as follows.
    (P → (H ==> H')) →
    (\[P] \* H) ==> H'.

Instantiating H' with wp t Q proves triple_hpure_with_wp.

A similar reasoning applies to the extraction rule for existentials.

Reasoning rules for terms, in weakest-precondition style

Rule for values

Recall the rule triple_val which gives a reasoning rule for establishing a triple for a value v.

If we rewrite this rule in wp style, we obtain the rule below.
    H ==> Q v →
    H ==> wp (trm_val v) Q.

By exploiting transitivity of entailment, we can eliminate H. We obtain the following statement, which reads as follows: if you own a state satisfying Q v, then you own a state from which the evaluation of the value v produces Q.

We can verify that, when migrating to the wp presentation, we have not lost any expressiveness. To that end, we prove that triple_val is derivable from wp_val.

Rule for sequence

Recall the reasoning rule for a sequence trm_seq t₁ t₂.

Replacing triple t H Q with H ==> wp t Q throughout the rule gives the statement below.
      H ==> (wp t₁) (fun v ⇒ H₁) →
      H₁ ==> (wp t₂) Q →
      H ==> wp (trm_seq t₁ t₂) Q.

This entailment holds for any H and H₁. Let us specialize it to H₁ := (wp t₂) Q and H := (wp t₁) (fun v ⇒ (wp t₂) Q). This leads us to the following statement, which reads as follows: if you own a state from which the evaluation of t₁ produces a state from which the evaluation of t₂ produces the postcondition Q, then you own a state from which the evaluation of the sequence t₁;t₂ produces Q.

Exercise: 2 stars, standard, optional (triple_seq_from_wp_seq)

Check that wp_seq is just as expressive as triple_seq, by proving that triple_seq is derivable from wp_seq and from the structural rules for wp and/or the structural rules for triple.

Additional contents

Other reasoning rules for terms

Rule for functions

Recall the reasoning rule for a term trm_fun x t₁, which evaluates to the value val_fun x t₁.

The rule for functions follow exactly the same pattern as for values.

A similar rule holds for the evaluation of a recursive function.

Rule for conditionals

Recall the reasoning rule for a term triple_if b t₁ t₂.

Replacing triple using wp entailments yields:
    H ==> wp (if b then t₁ else t₂) Q →
    H ==> wp (trm_if (val_bool b) t₁ t₂) Q.

which simplifies by transitivity to:
    wp (if b then t₁ else t₂) Q ==> wp (trm_if (val_bool b) t₁ t₂) Q.

This statement corresponds to the wp-style reasoning rule for conditionals. The proof appears next.

Rule for let-bindings

Recall the reasoning rule for a term trm_let x t₁ t₂.

The rule of trm_let x t₁ t₂ is very similar to that for trm_seq, the only difference being the substitution of x by v in t₂, where v denotes the result of t₁.

Rule for function applications

Recall the reasoning rule for an application (val_fun x t₁) v₂.

The corresponding wp rule is stated and proved next.

A similar rule holds for the application of a recursive function.

Bonus contents (optional reading)

A concrete definition for weakest precondition

The lemma wp_equiv captures the characteristic property of wp, that is, (H ==> wp t Q) ↔ (triple t H Q) . However, it does not give evidence that there exists a predicate wp satisfying this equivalence. We next present one possible definition. The idea is to define wp t Q as the predicate \∃ H, H \* \[triple t H Q], which, reading litterally, is satisfied by "any" heap predicate H which is a valid precondition for a triple for the term t and the postcondition Q.

First, let us prove that wp t Q is itself a valid precondition, in the sense that triple t (wp t Q) Q always holds (as asserted by the lemma wp_pre). To establish this fact, we have to prove: triple t (\∃ H, H \* \[triple t H Q]) Q. Applying the extraction rule for existentials gives: ∀ H, triple t (H \* \[triple t H Q]) Q. Applying the extraction rule for pure facts gives: ∀ H, (triple t H Q) → (triple t H Q), which is true. Second, let us demonstrate that the heap predicate wp t Q is entailed by any precondition H that satisfies triple t H Q, as asserted by the lemma wp_weakest. Assume triple t H Q. Let us prove H ==> wp t Q, that is H ==> \∃ H, H \* \[triple t H Q]. Instantiating the H on the right-hand side as the H from the left-hand side suffices to satisfy the entailment. Recall that the properties wp_pre and wp_weakest were derivable from the characteristic equivalence triple t H Q ↔ H ==> wp Q. Thus, to formalize the proofs of wp_pre and wp_weakest, all we have to do is to establish that equivalence.

Exercise: 2 stars, standard, recommended (wp_equiv)

Prove that the definition wp_high satisfies the characteristic equivalence for weakest preconditions.

Equivalence between all definitions of wp

We next prove that the equivalence (triple t H Q) ↔ (H ==> wp t Q) defines a unique predicate wp. In other words, all possible definitions of wp are equivalent to each another. Thus, it really does not matter which concrete definition of wp we consider: they are all equivalent. Concretely, assume two predicates wp₁ and wp₂ to both satisfy the characteristic equivalence. We prove that they are equal.

Recall that both wp_pre and wp_weakest are derivable from wp_equiv. Let us also that, reciprocally, wp_equiv is derivable from the conjunction of wp_pre and wp_weakest. In other words, the property of "being the weakest precondition" also uniquely characterizes the definition of wp.

An alternative definition for weakest precondition

The concrete definition for wp given above is expressed in terms of Separation Logic combinators. In contrast to this "high level" definition, there exists a more "low level" definition, expressed directly as a function over heaps. In that alternative definition, the heap predicate wp t Q is defined as a predicate that holds of a heap h if and only if the execution of t starting in exactly the heap h produces the post-condition Q. Technically, wp t Q can be defined as: fun (h:heap) ⇒ triple t (fun h' ⇒ h' = h) Q. In other words, the precondition requires the input heap to be exactly h.

One can prove that this definition of wp also satisfies the characteristic equivalence H ==> wp Q ↔ triple t H Q.

The details of the proofs are given for the reference but are beyond the scope of this course. Technically, the proof exploits the lemma triple_named_heap which was established as an exercise in the appendix of the chapter SLFHimpl.)

Extraction rule for existentials

Recall the extraction rule for existentials.

Replacing triple t H Q with H ==> wp t Q yields the lemma stated below.

Exercise: 1 star, standard, optional (triple_hexists_in_wp)

Prove the extraction rule for existentials in wp style.

In other words, in the wp presentation, we do not need a specific extraction rule for existentials, because the extraction rule for entailment already does the job.

Combined structural rule

Recall the combined consequence-frame rule for triple.

Let us reformulate this rule using wp, replacing the form triple t H Q with the form H ==> wp t Q.

Exercise: 2 stars, standard, recommended (wp_conseq_frame_trans)

Prove the combined structural rule in wp style. Hint: exploit wp_conseq_trans and wp_frame.

The combined structural rule for wp can actually be stated in a more concise way, as follows. The rule reads as follows: if you own a state from which the execution of t produces (a result and a state satisfying) Q₁ and you own H, and if you can trade the combination of Q₁ and H against Q₂, the you own a piece of state from which the execution of t produces Q₂.

Exercise: 2 stars, standard, recommended (wp_conseq_frame)

Prove the concise version of the combined structural rule in wp style. Many proofs are possible.

Alternative statement of the rule for conditionals

We have established the following rule for reasoning about conditionals using wp.

Equivalently, the rule may be stated with the conditional around the calls to wp t₁ Q and wp t₂ Q.

Exercise: 1 star, standard, optional (wp_if')

Prove the alternative statement of rule wp_if, either from wp_if or directly from triple_if.

Definition of wp directly from hoare

Let's take a step back and look at our construction of Separation Logic so far. 1. We defined Hoare triples (hoare) with respect to the big-step judgment (eval). 2. We defined Separation Logic triples (triple) in terms of Hoare triples (hoare), through the definition: \∀ H', hoare t (H \* H') (Q \*+ H'). 3. We then defined Separation Logic weakest-preconditions (wp) in terms of Separation Logic triples (triple). Through the construction, we established reasoning rules, first for Hoare triples (hoare), then for Separation Logic triples (triple), and finally for weakest-preconditions (wp). One question that naturally arises is whether there is a more direct route to deriving reasoning rules for weakest preconditions. In other words, can we obtain the same end result through simpler proofs? The notion of Hoare triple is a key abstraction that enables conduction further proofs without manipulating heaps (of type heap) explicitly. Experiments suggest that it is beneficial to introduce the Hoare logic layer. In other words, it is counterproductive to try an prove Separation Logic reasoning rules, whether for triple or for wp, directly with respect to the evaluation judgment eval. Thus, the only question that remains is whether it would have some interest to derive the reasoning rules for weakest preconditions (wp) directly from the the reasoning rules for Hoare triples (hoare), that is, by bypassing the statement and proofs for the reasoning rules for Separation Logic triples (triple). In what follows, we show that if one cares only for wp-style rules, then the route to deriving them straight from hoare-style rules may indeed be somewhat shorter.

Recall the definition of triple in terms of hoare.
    Definition triple (t:trm) (H:hprop) (Q:val→hprop) : Prop :=
      ∀ (H':hprop), hoare t (H \* H') (Q \*+ H').

In what follows, we conduct the proofs by assuming a concrete definition for wp, namely wp_high, which lends itself better to automated proofs.

First, we check the equivalence between triple t H Q and H ==> wp t Q. This proof is the same as wp_equiv from the module WpHighLevel given earlier in this chapter.

Second, we prove the consequence-frame rule associated with wp. It is the only structural rule that is needed for working with weakest preconditions.

The proof leverages the consequence rule for hoare triples, and the frame property comes from the ∀ H' quantification baked in the definition of triple.

Third and last, we establish reasoning rules for terms in wp-style directly from the corresponding rules for hoare triples. The proof details are beyond the scope of this course. The point here is to show that the proofs are fairly concise.

Remark: it is technically possible to bypass even the definition of triple and specify all functions directly using the predicate wp. However, using triple leads to better readability of specifications, thus it seems preferable to continue using that style for specifying functions. (See discussion in chapter SLFWand, appendix on "Texan triples".)