Version 1.3 of ai12s/ai12-0280-2.txt

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!standard 6.1.1(24/3)          19-06-15 AI12-0280-2/03
!standard 6.1.1(26.4/4)
!standard 6.1.1(39/5)
!class Amendment 18-05-15
!status work item 18-05-15
!status received 18-05-14
!priority Low
!difficulty Medium
!subject Making 'Old more sensible
!summary
X'Old may be used in a potentially unevaluated subexpression so long as the condition controlling the subexpression can be evaluated on entry yielding the same result.
!problem
The restrictions on the use of 'Old in potentially unevaluated subexpressions can make it hard to properly express complex postconditions, or can make the evaluation of such postconditions expensive (by requiring the saving of a large object rather than a single, smaller, part).
Additionally, the requirement to evaluate all 'Old prefixes can be expensive (for example, if the object is large or has controlled parts), especially if some of them are in mutually exclusive subexpressions of the postcondition.
!proposal
We define the term "known on entry subexpression" to describe expressions that can be evaluated at the point of evaluation of 'Old, getting the same value without side-effects that would be determined when evaluated as part of the postcondition expression.
We then use that to define the term "determined to be unevaluated" to be used on subexpressions in a postcondition that are not going to evaluated given the value of the known on entry subexpression. (Note: which subexpressions are determined to be unevaluated is calculated at runtime; this term is referring to a Dynamic Semantics property.)
Finally, we use the above terms to define when parts of the postcondition are evaluated and when the prefix of Old attribute references is not evaluated.
!wording
Add after 6.1.1(19/3):
A subexpression of a postcondition expression is known on entry if it is any of:
* a static subexpression (see 4.9);
* a literal whose type does not have any Integer_Literal, Real_Literal, or String_Literal aspect specified, or, the function specified by such an attribute has aspect Global specified to be null;
AARM Reason: We mention literals explicitly in case they are not static (as when their subtype is not static, they are the literal null, etc.). We exclude literals of types with the aspects that are not Global => null those cause a user-written subprogram with possible side-effects to be called.
* a name statically denoting a full constant declaration of a type for which all views are constant (see 3.3);
AARM Ramification: Constants of types with immutably limited or controlled parts are not allowed by this rule. Generic formal in objects are allowed by this rule (as they are defined to be full constant declarations).
AARM Reason: We only want things that cannot be changed. We can't just say "constant" since that includes views of variables in some cases (for instance, a dereference of an access to constant object can be a view of a variable). There are other things we could have allowed (like a loop parameter), but having a subprogram declaration where those could be used (for instance, inside of a loop) seems unusual enough to not be worth defining.
* a name statically denoting a non-aliased /in/ parameter of an elementary type;
AARM Ramification: All such parameters are by-copy, so the value won't change during the execution of the expression.
* an Old attribute_reference;
* a predefined operator where all of the operands are known on entry;
* a function call where the function has aspect Global => null where all of the actual parameters are known on entry;
AARM Reason: Such a function can only depend on the values of its parameters.
* a selected component of a known on entry prefix;
* an indexed component of a known on entry prefix where all index expressions are known on entry;
* a parenthesized known on entry expression;
* a qualified expression or type conversion whose operand is a known on entry expression;
* a conditional expression where all of the conditions, selecting_expressions, choice_conditions, and dependent_expressions are known on entry.
AARM Discussion: It's OK if such an expression raises an exception, so long as every evaluation of the expression raises the same exception.
Add before 6.1.1(20/3): A subexpression of a postcondition is always evaluated if it is not potentially unevaluated. ...
Modify 6.1.1(21/3):
* any part of an if_expression other than the first condition{, unless all of the conditions are known on entry and then entire if_expression is always evaluated};
AARM Ramification: If all of the conditions are known on entry, none of the dependent_expressions or conditions are potentially unevaluated.
AARM Reason: We require all of the conditions to be known on entry for simplicity of description and implementation: either a conditional expression has to be known on entry or it does not. If we allowed only the first condition to be known on entry, we would have a mixture of parts that are and are not potentially unevaluated. That's possible but difficult to describe.
Modify 6.1.1(22/3):
* a dependent_expression of a case_expression{, unless the selecting_expression or all of the choice_conditions are known on entry and the entire case_expression is not potentially unevaluated};
AARM Ramification: If the selecting_expression is known on entry, or all of the choice_conditions are known on entry, none of the dependent_expressions are potentially unevaluated.
AARM Reason: We require all of the choice_conditions to be known on entry as a case_expression without a selecting_expression requires exactly one to be true. A mix could cause whether the expression raises an exception to change.
[Author's note: See AI12-0214-2 for choice conditions.]
Modify 6.1.1(23/3):
* the right operand of a short-circuit form{, unless the left operand is known on entry and the entire short-circuit form is always evaluated};
Add after 6.1.1(24/3): [Author's Note: These following rules are talking about a dynamic condition; this is all lumped together here in 6.1.1.]
If an if_expression is always evaluated and all of its conditions are known on entry, then:
* each dependent_expression whose associated condition is False is determined to be unevaluated;
* each condition and dependent_expression that follows the first condition that evaluates to True is determined to be unevaluated};
If a case_expression with a selecting_expression is always evaluated and has a known on entry selecting_expression, then each dependent_expression other than the one selected by the selecting expression is determined to be unevaluated.
If a case_expression without a selecting_expression is always evaluated and all of the choice_conditions are known on entry, then each dependent_expression other than the one corresponding to the (single) choice_condition that evaluated to True is determined to be unevaluated. [Author's note: See AI12-0214-2.]
If a short-circuit form is always evaluated and the left operand of the form is known on entry, then:
* if the left operand is True for an /or else/ form, the right operand is determined to be unevaluated;
* if the left operand is False for an /and then/ form, the right operand is determined to be unevaluated;
Modify 6.1.1.(26/4):
Each X'Old in a postcondition expression {other than those that occur in subexpressions that are determined to be unevaluated} that is enabled denotes a constant that is implicitly declared at the beginning of the subprogram body, entry body, or accept statement.
AARM Ramification: If X'Old occurs in a subexpression that is determined to be unevaluated, then there is no associated constant, and no evaluation of the prefix takes place. In general, this will require evaluating one or more known on entry subexpressions before creating and initializing any X'Old constants. Note that any 'Old in a known on entry subexpression evaluated this way represents the current value of the prefix (the 'Old itself can be ignored).
Add after 6.1.1(39/5):
Implementation Permissions
An implementation may evaluate known on entry subexpression of a postcondition expression of an entity at the place where X'Old constants are created for the entity, with the normal evaluation of the postcondition expression, or both.
AARM Reason: We allow the evaluation of known on entry subexpressions when they might be needed to determine whether to create a particular 'Old constant. We allow them to be evaluated later as well, or for the results to be saved somehow. This permission shouldn't matter, as the results ought to be same wherever they are evaluated and there should not be any side-effects. The main effect of the permission is to determine when any exceptions caused by such subexpressions may be raised. We never require waiting to determine the value of such subexpressions, even if they aren't used to determine the creation of a constant for 'Old.
!discussion
This proposal addresses some of the same problems that the Contract_Cases aspect does. Contract_Cases is not a good fit for Ada since it really requires a complex static check that its alternatives cover the precondition (provided by SPARK and GNATProve). Without such a check, it modifies the precondition, making it difficult for the caller to be sure what the precondition actually is.
Rather than introducing a new construct that has the needed properties, we instead look to improve the existing description of postconditions to better match the sorts of problems that Contract_Cases handles. We do this in two parts, first, by changing the evaluation of 'Old to avoid extra evaluations and the Legality Rules needed to make those make sense (this AI), and by expanding the abilities of case expressions (and statements), in AI12-0214-2.
!example
Consider:
procedure Proc (A : access Integer) with Post => (if A /= null then (A.all'Old > 1 and A.all > 1));
In Ada 2012, the reference A.all'Old is illegal as the prefix does not statically denote an object (required as the "then" dependent_expression is potentially unevaluated).
With the proposed AI, since "A /= null" meets the requirements of a known on entry expression (A being an in parameter of an elementary type), there is no restriction on the use of 'Old, and if A = null, then the 'Old reference is not evaluated at all. This allows the author to write the postcondition they want with less likelihood of tripping over these rules (and as a side-effect, requires less evaluation of 'Old prefixes that are never going to be used).
!ASIS
No ASIS effect.
!ACATS test
ACATS C-Tests are needed to check that the new capabilities are supported, specifically that (A) the restriction on 'Old prefixes is not enforced on a dependent expression that is not potentially unevaluated by these rules; (B) that the prefix of 'Old is not evaluated when it is determined to be unevaluated.
!appendix

From: Randy Brukardt
Sent: Saturday, April 14, 2018  7:22 PM

...
> Alternative proposals are always of interest, but (as I'm sure you'd
> agree) ya gotta give me something more concrete than that.

I had an idea when I woke up this morning. So here's a quick outline of the
ideas (two).

---------

For the 'Old evaluation problem:

The basic issue is that in general, the value of conditionals aren't known at
the start of the subprogram, so we have to unconditionally evaluate 'Old
prefixes. Obviously, there is a subset where it *is* possible to know. In that
case, we don't need to evaluate 'Old unconditionally, and we can drop the
restrictions.

That would look something like (some details to be worked out):

For a postcondition expression, a subexpression of it is "pre-evaluable" if is
any of:

   (1) A declared constant object of a type that doesn't change (see 3.3.1);
   (2) A literal;
   (3) An in parameter of a by-copy type;
   (4) An 'Old attribute_reference;
   (5) A predefined operator where all of the operands are pre-evaluable;
   (6) A function with global = null ("pure function") where all of the operands
       are pre-evaluable;

For a conditional expression that is not itself potentially unevaluated, if the
selector expression or all of the conditions are pre-evaluable, then the parts of
the expression are *not* potentially unevaluated. Rather, the selector or
conditions are evaluated when the subprogram is entered, and only the 'Old
expressions in the dependent_expression that is selected are evaluated and saved
in the normal manner. (This needs a bit of wording improvement to properly
evaluate if conditions only if the preceding ones are False, but I hope you get
the idea.) We might want a term to describe this kind of expression.

We probably would want to do something similar for short circuit forms.

The rule would be applied recursively (for nested if statements could be subject
to it).

This eliminates the "potentially unevaluated" checks for whatever part of the
postcondition can be pre-evaluated -- one only needs to make the conditions
pre-evaluable. And it eliminates overhead for copying objects that will never be
used.

A compiler could evaluate the pre-evaluable expressions at the start of the
subprogram and somehow save the answers, or it could just evaluate them twice
(and let the compiler optimizer deal with the duplication - they tend to be good
at things like that).

The advantage we have over SPARK here is that we (the ARG) can get rid of the
Legality Rules in the existing construct, whereas for SPARK to do that, they
have to invent a new construct. It's better to file down the bump than to start
over...

[The rest of the message pertains to a proposal found in AI12-0214-2 - Editor.]

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