[foreign] RFR 8218052: Don't throw an exception when encountering a type with a flexible array member
Jorn Vernee
jbvernee at xs4all.nl
Tue Feb 26 14:33:29 UTC 2019
Uhh... I just realized the MultiplexingVisitor I made is totally
pointless... because Trees implement the visitor pattern. (probably time
to take a break :-) )
Updated webrev:
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218052/webrev.06/
Jorn
Jorn Vernee schreef op 2019-02-26 14:20:
> After some private discussion, here is the updated webrev that that
> implements the check as an actual visitor instead.
>
> Updated webrev:
> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218052/webrev.05/
>
> Jorn
>
> Maurizio Cimadamore schreef op 2019-02-26 02:24:
>> Thanks this looks much simpler.
>>
>> And also points at another possible simplification - why don't we
>> generate such warning in the parser? After all, is the parser that
>> knows when a StructTree will be created, so it is in an ideal
>> situation to intercept that event and log a warning accordingly (and,
>> the parser already has logging support). Btw, I know that the actual
>> StructTree creation occurs in TreeMaker - we could either add context
>> to TreeMaker, or just peek for a struct cursor in Parser.
>>
>> But overall, adding the check on earlier visitors might be better than
>> using peek().
>>
>> Maurizio
>>
>> On 26/02/2019 00:30, Jorn Vernee wrote:
>>> Hi Maurizio,
>>>
>>> I think you're right. I replaced the exception with a warning since
>>> the 2 are similar, but throwing an exception/warning at that point is
>>> probably wrong in the first place, like you say.
>>>
>>> Based on your suggestion, I've implemented the warning separately as
>>> a .peek() operation on the stream of Trees.
>>>
>>> Updated webrev:
>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218052/webrev.04/
>>>
>>> Jorn
>>>
>>> Maurizio Cimadamore schreef op 2019-02-26 00:34:
>>>> Hi Jorn,
>>>> while in principle making context immutable is the right to do on
>>>> paper, I believe that we probably ended up curing a symptom and not
>>>> the disease :-)
>>>>
>>>> Which means: I find it very strange to see that a layout computer -
>>>> that is, a function which takes a clang Cursor and emits a Layout is
>>>> now suddenly starting to give side-effects. Given I've worked on
>>>> javac
>>>> for many years, I can assure you that this will almost surely lead
>>>> to
>>>> problems down the road. For instance, if this apparently harmless
>>>> stateless function is called twice on the same cursor, we would get
>>>> two warnings; another issue is that the code will fail to scale if
>>>> new
>>>> warnings will need to be added - e.g. if record layout computer
>>>> needs
>>>> to check for other problematic stuff. And I'm not even mentioning
>>>> adding logic to enable/disable specific classes of warnings, which
>>>> is
>>>> another thing that might happen, at some point.
>>>>
>>>> What works in these contexts, is to separate the logic that does the
>>>> stateless computation (e.g. layout computation, in this case), from
>>>> the logic that issues the warning. For instance, we could have a
>>>> separate visitor in jextract which looks at the clang cursor,
>>>> looking
>>>> for things that are suspicious or not well supported (such as
>>>> incomplete arrays) and report a warning _only once_.
>>>>
>>>> This visitor will probably write itself, and, longer term would be a
>>>> much more maintainable option than inlining the check in the layout
>>>> computer function itself.
>>>>
>>>> After which, the decision as to whether make the context mutable or
>>>> not is largely orthogonal - we could go there, although it's not
>>>> clear
>>>> if the extra visitor I'm proposing would significantly switch the
>>>> balance one way or another (probably not, as we have plenty other
>>>> visitors which do logging).
>>>>
>>>> Maurizio
>>>>
>>>>
>>>> On 25/02/2019 23:16, Jorn Vernee wrote:
>>>>> I've split this into 2 to try and make reviewing easier.
>>>>>
>>>>> Here's the part that makes Context immutable (this required quite a
>>>>> bit of refactoring in Main):
>>>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/context/webrev.00/
>>>>>
>>>>> Here's the original part that returns a layout reference when
>>>>> encountering a type with a flexible array (applies on top of the
>>>>> first one):
>>>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218052/webrev.03/
>>>>>
>>>>> Hope that helps,
>>>>> Jorn
>>>>>
>>>>> Jorn Vernee schreef op 2019-02-25 23:06:
>>>>>> I talked a bit with Sundar;
>>>>>>
>>>>>> We can not create a global static method to return the Context
>>>>>> since
>>>>>> there could be more than 1 Context, due to multiple instances of
>>>>>> Jextract being created through the ToolProvider interface. But, as
>>>>>> long as Context is Immutable, passing it down to other code should
>>>>>> be
>>>>>> OK.
>>>>>>
>>>>>> I have made Context immutable by adding a Context.Builder, and
>>>>>> creating the Context in a separate method in Main (createContext).
>>>>>> Some places in that code were prematurely exiting and returning an
>>>>>> error code. This is replaced by throwing a custom exception
>>>>>> instead.
>>>>>>
>>>>>> Update webrev:
>>>>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218052/webrev.02/
>>>>>>
>>>>>> Thanks,
>>>>>> Jorn
>>>>>>
>>>>>> Jorn Vernee schreef op 2019-02-16 15:30:
>>>>>>> In other words; we can't get the layout for a type with a
>>>>>>> flexible
>>>>>>> array from libclang at all. A good way to resolve that is
>>>>>>> probably to
>>>>>>> submit a patch for it to libclang. I've done a little bit of
>>>>>>> searching, and there doesn't seem to be a bug report in their bug
>>>>>>> database for it either:
>>>>>>> https://bugs.llvm.org/buglist.cgi?quicksearch=flexible%20array so
>>>>>>> we
>>>>>>> could submit a bug about it as well. But, currently you need to
>>>>>>> manually request an account in order to post bugs, so this might
>>>>>>> take
>>>>>>> a while. I've also send an email just now to the cfe-dev mailing
>>>>>>> list,
>>>>>>> maybe someone there can offer some help.
>>>>>>>
>>>>>>> In the meantime we can work around structs with flexible arrays.
>>>>>>> Currently an exception is thrown, but this makes the whole
>>>>>>> extracted
>>>>>>> interface unusable, so emitting an undefined layout as a default
>>>>>>> value
>>>>>>> and printing a warning seems like a better option to me until the
>>>>>>> issue is fixed in libclang.
>>>>>>>
>>>>>>> There's also the idea of introducing a jextract option to
>>>>>>> manually
>>>>>>> override a descriptor, e.g. `--patch-descriptor
>>>>>>> struct:MyStruct=[i32(x)i32(y)]`. We could use something like that
>>>>>>> to
>>>>>>> manually provide the layout descriptor for structs with flexible
>>>>>>> array
>>>>>>> members in the meantime.
>>>>>>>
>>>>>>> Jorn
>>>>>>>
>>>>>>> Jorn Vernee schreef op 2019-02-16 09:07:
>>>>>>>>>> Now to the real discussion about incomplete array support,
>>>>>>>>>> instead of undefined layout, I prefer to have limited support,
>>>>>>>>>> we can either strip that field or generate a 0-length array
>>>>>>>>>> for now. >> For jextract, C only allow such field at end of
>>>>>>>>>> struct, and sizeof() operator simply ignore that trailing
>>>>>>>>>> array field. This should give us a good match as first step.
>>>>>>>>> Agree
>>>>>>>>
>>>>>>>> Sure, that would be preferable. But, as discussed before [1],
>>>>>>>> libclang
>>>>>>>> does not handle incomplete arrays properly, that's why it was
>>>>>>>> changed
>>>>>>>> to emit an exception in the first place.
>>>>>>>>
>>>>>>>> It is not possible to filter out structs with any jextract
>>>>>>>> option, so
>>>>>>>> if you have an incomplete array in a header file that is
>>>>>>>> otherwise
>>>>>>>> usable, you're out of luck.
>>>>>>>>
>>>>>>>> Jorn
>>>>>>>>
>>>>>>>> [1] :
>>>>>>>> https://mail.openjdk.java.net/pipermail/panama-dev/2019-January/003975.html
>>>>>>>> Maurizio Cimadamore schreef op 2019-02-16 01:29:
>>>>>>>>> On 16/02/2019 00:00, Henry Jen wrote:
>>>>>>>>>> Now to the real discussion about incomplete array support,
>>>>>>>>>> instead of undefined layout, I prefer to have limited support,
>>>>>>>>>> we can either strip that field or generate a 0-length array
>>>>>>>>>> for now. For jextract, C only allow such field at end of
>>>>>>>>>> struct, and sizeof() operator simply ignore that trailing
>>>>>>>>>> array field. This should give us a good match as first step.
>>>>>>>>> Agree
>>>>>>>>>>
>>>>>>>>>> Move on to more general support, where incomplete array can be
>>>>>>>>>> in-between layouts. Before that, we probably need to validate
>>>>>>>>>> some assumption,
>>>>>>>>>>
>>>>>>>>>> Any incomplete array must have length specified in the same
>>>>>>>>>> struct before the incomplete array. I believe this will pretty
>>>>>>>>>> much cover most cases if not all.
>>>>>>>>>
>>>>>>>>> To reinforce this point, I believe most compilers even give
>>>>>>>>> error if
>>>>>>>>> the incomplete array is the only member of the struct, or if
>>>>>>>>> it's
>>>>>>>>> followed by other stuff:
>>>>>>>>>
>>>>>>>>> $ cat testInc.h
>>>>>>>>>
>>>>>>>>> struct A {
>>>>>>>>> int arr[];
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> struct B {
>>>>>>>>> int l;
>>>>>>>>> int arr[];
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> struct C {
>>>>>>>>> int l;
>>>>>>>>> int arr[];
>>>>>>>>> int l2;
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> $ gcc -c testInc.h
>>>>>>>>>
>>>>>>>>> testInc.h:2:8: error: flexible array member in a struct with no
>>>>>>>>> named members
>>>>>>>>> int arr[];
>>>>>>>>> ^~~
>>>>>>>>> testInc.h:12:8: error: flexible array member not at end of
>>>>>>>>> struct
>>>>>>>>> int arr[];
>>>>>>>>> ^~~
>>>>>>>>>
>>>>>>>>> $ clang -c testInc.h
>>>>>>>>> testInc.h:2:8: error: flexible array member 'arr' not allowed
>>>>>>>>> in otherwise empty
>>>>>>>>> struct
>>>>>>>>> int arr[];
>>>>>>>>> ^
>>>>>>>>> testInc.h:12:8: error: flexible array member 'arr' with type
>>>>>>>>> 'int []' is not at
>>>>>>>>> the end of struct
>>>>>>>>> int arr[];
>>>>>>>>> ^
>>>>>>>>> testInc.h:13:8: note: next field declaration is here
>>>>>>>>> int l2;
>>>>>>>>> ^
>>>>>>>>> 2 errors generated.
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> With that, I think following should work well enough,
>>>>>>>>>
>>>>>>>>> What you describe is what I've dubbed 'dependent layout'
>>>>>>>>> approach -
>>>>>>>>> e.g. have one or more values in a struct provide more info for
>>>>>>>>> certain
>>>>>>>>> layout elements in same struct. This is fairly frequent
>>>>>>>>> business with
>>>>>>>>> message protocols - almost all representation for
>>>>>>>>> variable-sized data
>>>>>>>>> is expressed as length + data array (sometimes compressed, as
>>>>>>>>> in
>>>>>>>>> protobuf's VarInt).
>>>>>>>>>
>>>>>>>>> I agree that's where we need to land, longer term. Short term
>>>>>>>>> it feels
>>>>>>>>> like the best move would be to just strip the array. Creating a
>>>>>>>>> 0-length array might be a move with subtle consequences: the
>>>>>>>>> array
>>>>>>>>> occurs within a region with some boundaries (e.g. a struct
>>>>>>>>> region).
>>>>>>>>> The boundaries of the enclosing region are usually computed
>>>>>>>>> using
>>>>>>>>> sizeof(enclosing type). Meaning that the enclosing region won't
>>>>>>>>> be
>>>>>>>>> 'big enough' to host anything but a zero-length array. If we
>>>>>>>>> cast the
>>>>>>>>> array to something else, what we get back is an array whose
>>>>>>>>> boundaries
>>>>>>>>> would exceed those of the enclosing struct - so if you try e.g.
>>>>>>>>> to
>>>>>>>>> write to the array, the operation would fail.
>>>>>>>>>
>>>>>>>>> To do this stuff properly in Panama you would need to allocate
>>>>>>>>> a
>>>>>>>>> bigger chunk of memory, of the desired size (pretty much as you
>>>>>>>>> would
>>>>>>>>> in C), and then cast the memory to the struct type - now you
>>>>>>>>> have a
>>>>>>>>> memory region that is big enough to do the struct + the array.
>>>>>>>>>
>>>>>>>>> The alternative, which does look simpler, is to just allocate a
>>>>>>>>> struct
>>>>>>>>> (with array stripped) followed by an array of desired size in
>>>>>>>>> the same
>>>>>>>>> scope - e.g. compare this:
>>>>>>>>>
>>>>>>>>> try (Scope s : Scope.globalScope().fork()) {
>>>>>>>>>
>>>>>>>>> Pointer<Byte> slab = s.allocateArray(NativeTypes.UINT8,
>>>>>>>>> Struct.sizeOf(StructWithIncompleteArray.class) + 40);
>>>>>>>>> Pointer<StructWithIncompleteArray> pstruct =
>>>>>>>>> slab.cast(LayoutType.ofStruct(StructWithIncompleteArray.class));
>>>>>>>>> Array<Integer> data =
>>>>>>>>> pstruct.data$get().cast(NativeTypes.INT32.array(10));
>>>>>>>>>
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> With this:
>>>>>>>>>
>>>>>>>>> try (Scope s : Scope.globalScope().fork()) {
>>>>>>>>>
>>>>>>>>> StructWithoutIncompleteArray struct =
>>>>>>>>> s.allocateStruct(StructWithoutIncompleteArray.class);
>>>>>>>>> Array<Integer> data = s.allocateArray(NativeTypes.INT32,
>>>>>>>>> 10);
>>>>>>>>>
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> The code looks similar - and a client can, after the
>>>>>>>>> allocation, use
>>>>>>>>> the array and the struct at will. But there's a subtle
>>>>>>>>> difference
>>>>>>>>> between the two: in the first snippet, the array is allocated
>>>>>>>>> immediately after the struct bits - that's how allocation
>>>>>>>>> happened. In
>>>>>>>>> the second snippet there's no guarantee that the array will be
>>>>>>>>> after
>>>>>>>>> the struct; in fact, the native scope might have run out of
>>>>>>>>> space with
>>>>>>>>> the struct and needed to allocate a new slab of native memory
>>>>>>>>> via
>>>>>>>>> unsafe before the array allocation happens.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Which seems to suggest that the right way of approaching the
>>>>>>>>> problem,
>>>>>>>>> even if more verbose, is the first one.
>>>>>>>>>
>>>>>>>>> Maurizio
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