Iterators: where folds fail Sylvie Boldo
To cite this version: Sylvie Boldo. Iterators: where folds fail. Workshop on High-Consequence Control Verification, Jul 2016, Toronto, Canada. �hal-01377155�
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Iterators: where folds fail Sylvie Boldo∗† Inria, Universit´e Paris-Saclay, F-91893 Palaiseau LRI, CNRS & Univ. Paris-Sud, F-91405 Orsay This paper is about the formal verification of a floating-point case study where the common iterators fold_left and fold_right have not the wanted behaviors. We then had to define other iterators, which are very similar in most cases, but that do behave well in our case study.
Our case study: floating-point expansions This is about floating-point arithmetic, defined by the IEEE-754 standard [7, 4]. It has been formalized in the Coq proof assistant by the Flocq library . Our case study is a way to increase the precision of a computation, in order to get more accuracy on critical systems. To retain efficiency, we rely on the floating-point unit (FPU) of theP processor. The idea of these floating-point expansions is to consider x as x = xi with the xi being floating-point numbers, see Figure 1. Using exact addition and multiplication [3, 6], we can build correct operations on these expansions such as addition, multiplication, division, and so on [3, 8, 9, 1]. We are in the progress of proving operations on expansions. It is important as there may be intermediate zeroes in these expansions that may endanger the algorithms. Moreover, the condition of use of these algorithm: allowed overlap, underflow, overflow, and so on are hardly known, and not formally verified.
10011001011 111 Figure 1: Example of a floating-point expansion.
∗ This work was supported by the FastRelax (ANR-14-CE25-0018-01) project of the French National Agency for Research (ANR). † Email: [email protected]
Iterators Here is the Coq definition of fold_left: Fixpoint fold_right ( l : list B ) : A := match l with | nil = > a0 | cons b t = > f b ( fold_right t ) end . In particular, it relies on the definition of a0:A that stands for the zero of A. For real numbers, we can consider fold_right Rplus R0 (1::2::3::nil) that will be simplified in 1 + (2 + (3 + 0)). This is perfect as Rplus is the mathematical + on R with R0 = 0 is the identity element. If you consider f as the floating-point addition, it is fine as 0 is also an identity element for this function. Now consider the function errf (x, y) = ◦(x + y) − (x + y), with ◦ being a rounding to nearest. Then f computes the error of the floating-point addition, which is also a floating-point number when x and y are floating-point numbers, and that may be computed using only floating-point operations [3, 6]. Then fold_right errf R0 (a::b::c::nil) is (errf a (errf b (errf c 0))). As errf (x, 0) = 0, this value is 0 whatever a, b and c. And this is not what we want as we wished to have (errf a (errf b c)). This is not a toy example as such an iteration of errf on a sequence is the first part of a renormalization algorithm .
Solution Of course, this problem with fold_right also applies to fold_left. We have therefore defined two other iterators: Definition my_iter_l f l := match l with | nil = > 0 | x :: l = > fold_left f l x end . Definition my_iter_r f l := match l with | nil = > 0 | _ = > fold_right f ( last l 0) ( removelast l ) end . They have the expected behavior on the previous example. The correctness of my_iter_r is given by the following theorem: Lemma m y _ i t e r _ r _ f o l d _ r i g h t : forall ( f : R ->R ->R ) b l , ( forall x , f x b = x ) -> ( l <> nil \/ b = 0) -> my_iter_r f l = fold_right f b l . 2
If there exists an identity element b, then the two iterators have the same result, providing either b = 0 or the list is not empty. A similar result for my_iter_l is useless as it simplifies immediately on fold_left as soon as the list is non empty. Several useful lemmas have also been proved, such as my_iter_l f l = my_iter_r (fun x y =>f y x) (rev l). As a conclusion, we have defined other iterators that comply with our needs. In particular, they do not expect an identity element on the function, but are more difficult to handle in the proofs. They could have been made more generic, in the returned value when the list is empty (here 0), and in the input type (here R). It is nevertheless surprising that this problem has not arisen before on other applications.
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