CVS difference for ai05s/ai05-0144-2.txt
--- ai05s/ai05-0144-2.txt 2009/10/30 05:50:04 1.2
+++ ai05s/ai05-0144-2.txt 2009/11/04 06:26:38 1.3
@@ -1,4 +1,4 @@
-!standard 6.02 (11) 09-06-07 AI05-0144-2/01
+!standard 6.02 (11) 09-10-30 AI05-0144-2/02
!class Amendment 09-06-07
!status work item 09-06-07
!status received 09-06-07
@@ -27,7 +27,10 @@
+The following rules eliminate the most obvious side effects that can cause evaluation
+order problems. These rules are checked statically. Unlike most static rules, which
+are conservative, these rules are liberal in that they do not attempt to prevent all
+evaluation order problems, just ones that are certain to be problems.
@@ -59,8 +62,9 @@
known to denote the same object, and the subtypes denoted by S1 and S2
- AARM Discussion: This is determined statically. If the name contains
- some dynamic portion other than a dereference, indexed_component, or
+ AARM Discussion: Whether or not names or prefixes are known to denote the
+ same object is determined statically. If the name contains some dynamic
+ portion other than a dereference, indexed_component, or
slice, it is not "known to denote the same object". [We could also
use the same rules for indexes for the bounds of slices that have
explicit bounds, although it doesn't seem very likely to occur and
@@ -226,7 +230,8 @@
The usual concern about the use of *in out* parameters in functions begins something
-Imagine writing an expression like:
+Imagine writing (in a future version of Ada that allows *in out* parameters) an
if F3(O1) = F3(F2(O1)) then
@@ -236,7 +241,7 @@
(the right operand is still (C => 1), but now the left operand is (C => 2), and O1.C
is still 2 afterwards).
-This is usually used as a reason to not allow *in out* parameters on functions.
+This is usually used as a reason to disallow *in out* parameters on functions.
If you have to use access parameters, then the expression is:
if F3(O1) = F3(F1(O1'access)) then
@@ -257,8 +262,8 @@
We have the same order dependency, but there is no sign of a red flag here.
-This is all Ada 95 code, but Ada 2005 makes this situation worse by adding prefix
-views and implicit 'access. If A_Rec is tagged, we can write:
+All of these calls can be written in Ada 95, but Ada 2005 makes this situation worse
+by adding prefix views and implicit 'access. If A_Rec is tagged, we can write:
if O1.F3 = O1.F1.F3 then
@@ -468,5 +473,375 @@
[This was version /01 of the AI. This was the version discussed at the
Brest ARG meeting. This was made as a separate alternative so the
original lengthy discussion could be preserved without rewriting it.]
+From: Bob Duff
+Sent: Saturday, June 13, 2009 11:30 AM
+> Here is the version I am proposing.
+I agree with your proposal. Mostly editorial comments below (maybe that's premature?).
+> (See wording.)
+I think a summary of the proposal should go here. Such as:
+The following rules eliminate the most obvious side effects that can cause
+evaluation order problems. These rules are checked statically. Unlike most static
+rules, which are conservative, these rules are liberal in that they do not attempt
+to prevent all evaluation order problems.
+Otherwise, one gets lost in the detailed definitions, wondering what the point is.
+Or else put some similar text *before* the detailed wording, either as an AARM
+annotation, or a [bracketed introductory paragraph].
+> AARM Discussion: This is determined statically. If the name
+> some dynamic portion other than a dereference, indexed_component, or
+> slice, it is not "known to denote the same object". [We could also
+> use the same rules for indexes for the bounds of slices that have
+> explicit bounds, although it doesn't seem very likely to occur and
+> the wording is messy.]
+It's hard to know what "this" refers to above. I suggest moving the AARM
+annotation above the bullets, and change "This" to "This property". Or else
+keep it here, and spell it out: "Whether or not names or prefixes are known
+to denote the same object is determined statically. ..."
+> Two names N1 and N2 are *known to refer to the same object* if N1 and
+> N2 are known to denote the same object, or if N1 is known to denote a
+> subcomponent of the object denoted by N2, or vice-versa.
+> If a construct C has two or more direct constituents that are names or
+> expressions whose evaluation may occur in an arbitrary order, at least
+> one of which contains a function call with an in out, out, or
+> access-to-variable parameter, then the construct is legal only if:
+"access-to-variable parameter" seems confusing; I think you mean it to include
+named access types, as well as access parameters. How about "parameter of an
+> * For each name N that is passed as a parameter of mode in out or out
+> to some inner function call C2 (not including the construct C
+> itself), there is no other name anywhere within a direct constituent
+> of the construct C other than the one containing C2, that is known
+> to refer to the same object; and
+> * For each name N'Access or N'Unchecked_Access that is passed as an
+> access-to-variable parameter to some inner function call C2 (not
+> including the construct C itself), there is no other name anywhere
+> within a direct constitutent of the construct C other than the one
+> containing C2, that is known to refer to the same object as N.
+> For the purposes of checking this rule on an array aggreagate, an
+> expression associated with a discrete_choice_list that has two or more
+> discrete choices, or that has a nonstatic range, is considered as two
+> or more separate occurrences of the expression. Similarly for a
+> record aggregate, the expression of a record_component_association is
+> considered to occur once for each associated component.
+> AARM Reason: This prevents obvious cases of dependence on the order of
+Another dangling "this".
+> evaluation of names or expressions. Such dependence is usually a bug,
+> and in any case, is not portable to another implementation (or even
+> another optimization setting).
+> The third bullet does not check for uses of the prefix, since the
+> access type
+Which third bullet?
+[Editor's note: The one Tucker deleted. ;-) He apparently didn't update
+> and the designated object are not the same, and "known to denote the
+> same prefix" does not include dereferences anyway.
+> The usual concern about the use of *in out* parameters in functions
+> begins something
+It would be useful to mark each of the following with a comment "-- OK" or
+"-- ERROR:" showing whether the proposed rules outlaw it. The proposed rules
+do not outlaw all the cases below, I think -- e.g. the cases that pass A1.
+Which is OK with me.
+> Imagine writing an expression like:
+> if F3(O1) = F3(F2(O1)) then
+> This expression has an evaluation order dependency: if the expression
+> is evaluated left-to-right, the result is True (both values have (C =>
+> 1) and O1.C is set to 2 afterwards), and if the expression is
+> evaluated right-to-left, the result is False (the right operand is
+> still (C => 1), but now the left operand is (C => 2), and O1.C is still 2 afterwards).
+> This is usually used as a reason to not allow *in out* parameters on functions.
+"to not allow" --> "not to allow" or "to disallow"
+> If you have to use access parameters, then the expression is:
+> if F3(O1) = F3(F1(O1'access)) then
+> and the use of 'access and aliased on the declaration of O1 should
+> provide a red flag about the possible order dependence.
+> However, this red flag only occurs some of the time. First of all,
+> access objects are implicitly converted to anonymous access types, so
+> no red flag is raised when using them:
+> if F3(A1.all) = F3(F1(A1)) then
+> Perhaps the .all on the left-hand argument could be considered a red
+> flag. But of course that doesn't apply if that function also takes an access parameter:
+> if F4(A1) = F3(F1(A1)) then
+> We have the same order dependency, but there is no sign of a red flag here.
+> This is all Ada 95 code, ...
+No, it's not -- I see calls to functions with 'in out' params above.
+[I'd don't see any such calls other than in a single 'straw man' case.]
+>...but Ada 2005 makes this situation worse by adding prefix views and
+>implicit 'access. If A_Rec is tagged, we can write:
+> We can also get an order dependence from a single call (even in Ada 83):
+> P2 (O1, O1);
+> but only if there are multiple parameters that can modify an object,
+I think I know what you mean, but it's confusing -- parameters don't modify things.
+[Editor's note: neither do calls or anything else. They just assigned back
+as needed, apparently by some magical force. :-) Writing two extra sentences
+to be pedantic about topics such as this don't help understanding.]
+> interestingly, only if the parameters are passed by-copy. (If A_Rec is
+> tagged, for example, the value of O1.C will be increased by 3, no
+> matter what order the parameters are evaluated in.)
+> Survey of solutions
+It is very useful to have this "Survey" attached to this AI, for posterity!
+> It fairly obvious that that order dependencies are a problem in Ada,
+"that that" --> "that"
+> have been getting worse with each version of the language (even
+> without *in out* parameters in functions). Moreover, it has been used
+> as the primary reason for leaving something natural and useful (*in
+> out* parameters for functions) out of the language.
+> One obvious solution would be to define the order of evaluation,
+> eliminating the problem at the source. Java, for instance, requires
+> left-to-right evaluation. However, that would encourage tricky code
+> like the various examples by making it portable.
+I doubt if I'll convince anyone, but I think that's a bogus argument.
+People do, in fact, depend on eval order all the time, either by accident, or because they don't know the language rules. And there's nothing any language definition can say to stop it. A language definition can say, "It is considered bad style to depen
d on evaluation order (of...).
+Don't do that." That would have just as much effect as leaving the order undefined -- i.e. it puts people on notice, but doesn't entirely stop the problem.
+>... Moreover, Ada compilers have been using this flexibility for
+>decades; trying to remove it from compilers (particularly from
+>optimizers) could be very difficult. Note that this is the only
+>solution that actually could eliminate dependencies on side-effects inside of functions.
+> But defining the order of evaluation was considered for both Ada 83
+>and Ada 95 and was deemed not worth it -- it's hard to see what has changed.
+> Another option would be to increase the visibility of parameters with
+> side-effects. This sounds somewhat appealing (after all, it seems to
+> be the basis on which access parameters are deemed OK and *in out*
+> parameters are not). One possibility would be to add ordering symbols to named notation:
+> Param <- <expr> for an *in* parameter (this includes access); Param ->
+> <expr> for an *out* parameter; and Param <-> <expr> for an *in out* parameter.
+Ada 80 (or thereabouts) used the symbols :=, =:, and :=: for this.
+I suggest you use this notation (before shooting it down below).
+And eliminate the "That's especially bad..." sentence below.
+> However, for compatibility, old code that don't use the symbols would
+> have to be allowed. That's especially bad because the symbol for
+> ordinary named parameters (=>) looks like the symbol for an *out*
+> parameter; while it usually will be an *in* parameter. Moreover, this
+> solution does nothing for positional parameters in calls nor for the
+> prefixes of prefix notation. And it is misleading for access
+> parameters, whose mode is officially "in", but still might cause side-effects.
+> One could argue that positional parameters are already unsafe and
+> requiring named notation to be safe is not much of an imposition. But
+> the prefix and access issues are not so easily explained away.
+> Additionally, putting the mode into calls in some way makes
+> maintenance of programs harder: changing the mode of a call is going
+> to make many calls illegal, while today most calls will remain legal (all will if the mode is changed from "in out" to "in").
+I think that last part is bogus -- it's like saying the full coverage rules for aggregates make maintenance harder.
+> Another syntax suggestion that was made recently was to (optionally)
+> include the parameter mode as part of the call. That would look something like:
+> if F3(in O1) = F3(in F2(in out O1)) then
+Shirley, you'd leave out the 'in's!
+> This could be applied to positional calls as well, but still provides
+> no help for prefix calls nor for access parameters.
+> One could imagine requiring the syntax for calls of functions with *in
+> out* parameters and making it optional elsewhere. That might placate
+> *in out* parameter opponents, but otherwise doesn't seem to do much for the language.
+If we were to do any of the above "marking [in]out params" syntax, we should also define
+a Restriction that forces it on all calls (not 'in' params, of course).
+Worth mentioning, even though we're not going this route.
+> Finally, we come to some sort of legality rules and/or runtime checks
+> for preventing such order dependencies. It is important to note that
+> making such rules too simple (and strong) only would mean that
+> temporaries have to be introduced in some expressions. That would be
+> an annoyance, but surely not as bad as the current ticking time-bomb.
+> The easiest option is to blame all of the problems on functions with
+> *in out* parameters and make them stand alone. The rule would be something like:
+> A call of a function with an *in out* parameter must be the only call in
+> an expression.
+> That would mean that
+> if F2(O1) then
+> would be legal (assuming F2 returned type Boolean), but
+> if F2(O1) = (C => 1) then
+> would not be. Obviously, this is too strict.
+I agree it's too strict, but I don't think it's "obviously" too strict.
+ Blah : T := Func(...);
+which is one of the more useful cases. Especially when T is something like String.
+>...Amazingly, it also not strict
+> Some_Array(F3(O1)) := F2(O1);
+> would be allowed. (An assignment statement is *not* an expression!)
+Well, then obviously the wording of the rule should not be "expression".
+It should be something like (in this rejected alternative):
+ If a construct has two or more constituents whose evaluation may occur in an
+ arbitrary order, and contains a call to a function with an [in]out param,
+ then it shall contain no other calls.
+> A call of a function with an *in out* parameter must be the only call in
+> an expression or statement;
+> If a call of a function with an *in out* parameter is the source expression
+> of an assignment_statement, the target variable_name shall not include a
+> call, an indexed component, a slice, or a dereference.
+> So we could modify the definition of "potentially the same object" to:
+> Two objects are considered to be "potentially the same object" one has
+> the type
+> of a part of the other, or one has a part whose type is a partial view, unless:
+> * one object is part of a stand-alone object, and the other object is part
+> Such rules would only reject calls where it is clear that parts of the
+> same object are involved. That eliminates the complications of private
+> types (we won't look in them), arrays (we won't try to determine if they are the same), and so on.
+"arrays" --> "array components", I think you mean.
+> These are the rules proposed in the !wording section above.
+> None of the proposed rules do anything about side-effects totally
+> inside of functions. One way to deal with that would be to require an
+> expression to contain only a single function call unless all of the functions are *strict*.
+I don't think "strict" is the right term, here. In computer science, "strict"
+means "arguments are evaluated at the call", as opposed to "lazy" and "non-strict" (which
+are almost, but not quite the same thing -- Haskell is non-strict), and "call by name".
+I'd consider extending "pure" to this concept. Or else don't define a term, just talk about
+"such functions" in this part.
+[Editor's note: I tried that and people hated it. A new term seemed better. Doesn't
+matter, because we're not doing any of this.]
+>... A strict
+> function exposes all of its side effects in its specification, meaning
+>it does not read or write global variables, call non-strict functions,
+>write hidden parts of parameters, etc. Most of the language-defined
+>functions are strict (that would have to be declared somehow).
+> Strict functions have several other nice properties: they don't need
+> elaboration checking (freezing is sufficient to prevent
+> access-before-elaboration problems); they can be used in symbolic
+> reductions; and they can use the optimizations allowed for functions in pure packages.
+> However, such a requirement would be quite incompatible. Moreover,
+> strict functions would be rather limiting by themselves.
+> An alternative that has been suggested is to allow Global_In and
+> Global_Out annotations on subprograms, which would declare the global
+> side-effects of a subprogram. Such annotations could not be a lie
+> (they'd have to be checked in some way), and thus would fill the role
+> of strict functions more flexibly. But it would still be too
+> incompatible to ban dangerous side-effects in functions (although separate tools
+> or non-Ada operating modes could make such checks).
+Or pragma Restrictions. Same applies to the "strict" discussion above.
+From: Tucker Taft
+Sent: Saturday, June 13, 2009 4:36 PM
+Thanks for reviewing this. We approved
+the intent, after deleting the bullet relating to access parameters, and generalizing
+the copy-back rules to apply to not only elementary types, but in fact any type that
+is not tagged or immutably limited. Basically, unless it is bloody obvious/guaranteed
+the type will be passed by reference, then we disallow having two out parameters that denote
+the same object in a single call.
+We didn't really review the discussion or examples, but it sounds like they will need
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