!standard D.7(8) 09-11-19 AI05-0190-1/02 !class amendment 09-11-03 !status work item 09-11-03 !status received 09-11-03 !priority Low !difficulty Easy !subject Global storage pool controls !summary Provide a means to force the use of user-defined pools, and a means to specify a particular pool to be used by default. !problem Some applications need to keep tight control over heap allocation. For example, it is common for object-oriented applications to have many access-to-class-wide types, but few used for allocation. Another example is an embedded system for which is is inappropriate to rely on the implementation-provided pools. This can be done by appplying "Storage_Size use 0" to all types that should not have allocators, and explicitly specifying a Storage_Pool for the few others. But this is error prone; one might forget the "Storage_Size use 0". Another problem is that it is technically erroneous to deallocate from the "wrong" pool. But it is implementation defined which pool is used for each access type! Many Ada programmers don't know about this rule, and most Ada implementations use a single global heap by default, so it is common to write code that does "new" for one type, converts to another type, and does Unchecked_Deallocation. This common coding practise could be erroneous on some implementations. It is uncomfortable for erroneousness to be implementation defined in this way. !proposal (see summary) !wording Syntax The form of a pragma No_Default_Storage_Pool or Default_Storage_Pool is as follows: pragma No_Default_Storage_Pool; pragma Default_Storage_Pool(*storage_pool*_name); A pragma No_Default_Storage_Pool is a configuration pragma. A pragma Default_Storage_Pool is allowed at the place where a declarative_item is allowed. Name Resolution Rules The *storage_pool*_name is expected to be of type Root_Storage_Pool'Class. Legality Rules The *storage_pool*_name shall denote a variable. A Default_Storage_Pool shall not be within the immediate scope of another Default_Storage_Pool. [???Or should the inner/later one override the other? Sounds complicated, wording-wise and implementation-wise.] Static Semantics Within the immediate scope of a pragma Default_Storage_Pool, the Storage_Pool attribute of any access type to which no Storage_Pool nor Storage_Size clause applies is defined to be the pool denoted by the *storage_pool*_name. If No_Default_Storage_Pool applies to a given compilation_unit, then any access type within that compilation_unit that has neither a specified Storage_Pool nor Storage_Size is defined by the language to have a Storage_Size of zero. [Therefore, an allocator for such a type is illegal.] AARM: This implies that Default_Storage_Pool trumps No_Default_Storage_Pool. !discussion Expected usage scenarios are: - No_Default_Storage_Pool as a configuration pragma applying to the whole program. To use an allocator for an access type, you have to apply a Storage_Pool or Storage_Size pragma to that type, or else put a Default_Storage_Pool pragma such that the type is within the pragma's immediate scope. - Default_Storage_Pool in the spec of the root package of a program or subsystem. This default pool is used within the package and its children, but you can override it with a Storage_Pool or Storage_Size clause. !example !ACATS test ACATS B and C tests are needed. !appendix From: Bob Duff Sent: Thursday, November 19, 2009 12:52 PM New version of AI05-0190-1. [This is version /02 of this AI. - ED] The previous version used a Restriction to specify "no allocators allowed by default". I added pragma Default_Storage_Pool to specify a pool to be used by default. Having done that, it seemed better to use a pragma No_Default_Storage_Pool instead of that Restriction. I changed the !subject accordingly. **************************************************************** From: Bob Duff Sent: Sunday, November 22, 2009 1:16 PM I wrote: > Provide a means to force the use of user-defined pools, and a means to > specify a particular pool to be used by default. One interesting question, which comes from a private conversation I had with Laurent Guerby, is: What happens if pragma Default_Storage_Spool applies to an instantiation? Does it apply to access types in the instance? If so, does that provide a way to control the storage pool used for instances of Ada.Constainers.Vectors and friends? **************************************************************** From: Randy Brukardt Sent: Tuesday, November 24, 2009 12:28 AM It's clear that No_Default_Storage_Pool had better apply to instances, so that any allocators in the generic units are detected. So I suppose from that it implies that a similar capability exists in the other direction. But such a capability would be next to useless. There is not (and better not be, IMHO) any requirements on how the containers packages use the (default) storage pool (except of course for the bounded forms, where using a pool at all is disallowed). In particular, a container can request storage from the pool with any size and any alignment, and as part of any operation. I suppose a program tied to a particular implementation of the containers might be able to gain some advantage from a custom pool, but such code is unlikely to be portable. Note that this is true of any generic that allocates from the default pool. Unless you are willing to "break privacy" by looking into the body of the generic, you can't assume anything about how the pool will be called. But almost all interesting user-defined storage pools make some assumptions about the alignment and size that will be requested. Such pools would not work on instances. It would of course be possible to create a monitoring pool that passes the actual allocation requests to the default pool, but that is not going to be very helpful in managing storage use. **************************************************************** From: Randy Brukardt Sent: Tuesday, November 24, 2009 12:52 AM > !wording Minor issue - you didn't recommend some place to put this new wording. It can't be floating in space!! The old positioning of D.7(8) related specifically to it being a restriction, so that doesn't work. So some other suggestion is needed. **************************************************************** From: Bob Duff Sent: Tuesday, November 24, 2009 7:01 AM Right. I intended to suggest someplace in chap 13, but I forgot. So I'd say it should be 13-dot-. I'm presuming there was no deliberate intent to max this "optional annex" -- it's just where the Restrictions happen to live. Actually, I guess it should be 13.11.3, "Default Storage Pools", and bump "Pragma Controlled" up to 13.11.4. What about the proposal overall? Do people like it? It's radically different from the previous version. **************************************************************** From: Tucker Taft Sent: Tuesday, November 24, 2009 7:36 AM > But such a capability would be next to useless. I don't agree this would be next to useless for a container instance. Clearly you would have to provide a general purpose storage pool if you are going to apply it by default to your entire system. But often all that is wanted is an ability to replace the system-defined default storage pool with a project-specific default storage pool. Unlike your experience, most of the storage pools I create are general purpose in the sense that they handle any size or alignment, but are more specific in when/if any automatic reclamation occurs. **************************************************************** From: Randy Brukardt Sent: Tuesday, November 24, 2009 5:45 PM > I don't agree this would be next to useless for a container instance. > Clearly you would have to provide a general purpose storage pool if > you are going to apply it by default to your entire system. But often > all that is wanted is an ability to replace the system-defined default > storage pool with a project-specific default storage pool. Not sure that makes much sense (unless your vendor's pool is particularly bad, or if you need additional monitoring capabilities). > Unlike your experience, most of the storage pools I create are general > purpose in the sense that they handle any size or alignment, but are > more specific in when/if any automatic reclamation occurs. Fine, but those aren't very necessary with the containers, which already are handling storage management (they're required not to leak, after all). So you are essentially repeating the existing storage management. Moreover, the containers (other than the bounded ones) are using controlled objects, and freeing storage in that case makes your program erroneous (and might very well cause it to crash hard, depending on how controlled objects are implemented). The only time it is safe to free storage is if the instance has gone away, and in that case, the "no leak" rule means that the storage has already been freed. So while I could see that this capability might be useful with your own generics, it wouldn't have any real value with the unbounded/indefinite containers. One could imagine trying to constrain implementation choices for the unbounded containers so that pools could be useful (for instance, we'd have to ban reusing nodes in a different object after they're deleted -- a technique I was planning to use). But that comes uncomfortably close to requiring the containers to have a particular body, something that was previously rejected. Anyway, I still think this capability would necessarily have to be provided in Bob's proposal (it doesn't make sense otherwise), but I don't think it should be suggested that it has any particular utility for the language-defined packages. **************************************************************** From: Tucker Taft Sent: Tuesday, November 24, 2009 7:39 AM I don't understand why No_Default_Storage_Pool is made into its own pragma, rather than relying on a restriction. All you said was: ... it seemed better to use a pragma No_Default_Storage_Pool instead of that Restriction. Could you provide a bit more rationale? It smells and feels like a restriction, so I don't see the advantage of not using the Restrictions pragma. I also find the wording for No_Default_Storage_Pool a bit confusing, as it says that an access type that ... has neither a specified Storage_Pool nor Storage_Size ... ends up with Storage_Size 0. But it isn't clear from the pragma Default_Storage_Pool that this has the effect of giving an access type a *specified* Storage_Pool. I think from a language point of view it is still unspecified, but it picks up the Default_Storage_Pool precisely because it is unspecified. Hence I would recommend that you be more explicit and say that any access type that is "not within the scope of a Default_Storage_Pool nor has a specified Storage_Pool nor a specified Storage_Size..." ends up with a Storage_Size of zero. It is somewhat annoying that Default_Storage_Pool can't be a configuration pragma, but I understand the problem. It would be nice if you could just specify it once and have it apply "everywhere." I suppose if you organize your library units into a small number of large subsystems, then you would only need one per top-level library package. For example, in the RTS you would only need it in the specs for Ada, Interfaces, and System. Of course creating a storage pool object that can live in a Pure package would be a bit of a challenge! Especially if it has to depend on System.Storage_Pools... ;-) **************************************************************** From: Randy Brukardt Sent: Tuesday, November 24, 2009 5:50 PM > I don't understand why No_Default_Storage_Pool is made into its own > pragma, rather than relying on a restriction. Bob might have a better reason, but to me it seems that "No_Default_Storage_Pool" is just another form of specifying the default storage pool. Perhaps it would be better to just use one pragma: pragma Default_Storage_Pool (null); to mean no default pool, although that would require rather annoying resolution and legality rules. In that sense it isn't really a restriction, it is just the absence of a default storage pool. (Every access type must have a pool before an allocator can be used - Storage_Size = 0 could be thought of as the same as not having a pool. I assume Bob worded it to the other way to minimize the wording changes needed.) **************************************************************** From: Tucker Taft Sent: Tuesday, November 24, 2009 7:17 PM I don't think this accomplishes what Bob (and I want). You want this to be a configuration pragma, and probably be one of those that you give all by itself in a file, so it applies to the whole library. That's why I think it makes the most sense as a restriction. And yes, this is for cases where you want to monitor and/or control *all* allocation, because you don't trust or you haven't certified the vendor-provided default storage pool, or you simply have special requirements relating to storage management in the given target environment. **************************************************************** From: Randy Brukardt Sent: Wednesday, November 25, 2009 12:38 PM I was assuming that this would be a configuration pragma. I'm not sure why you assumed otherwise. Of course, you couldn't give an actual pool in that case because none would be visible. But there would be no problem with giving "null" in that usage. So only one pragma is needed, and there is no need to worry about what happens when they conflict. ****************************************************************