Dan H, Thank you for the suggestion. I need to study this in more detail.
Would it work for you if we finalize the latest Condy JVMS that Dan S sent out and look into recursion handling in more detail as part of a future cycle? http://mail.openjdk.java.net/pipermail/valhalla-spec-experts/2018-January/000505.html <http://mail.openjdk.java.net/pipermail/valhalla-spec-experts/2018-January/000505.html> thanks, Karen > On Jan 19, 2018, at 3:11 PM, Dan Smith <daniel.sm...@oracle.com> wrote: > > We had the following discussion in the Dec 20 meeting: > >> On Jan 3, 2018, at 9:11 AM, Karen Kinnear <karen.kinn...@oracle.com >> <mailto:karen.kinn...@oracle.com>> wrote: >> >> Dan S: Cycle handling >> Array types can have cycles >> - rather than relying on ordering - perhaps tell tree depth - e.g. >> component type & dimensions - all in the CP >> John: perhaps modify to include the depth indicators in an attribute >> (separate proof of acyclicity) >> Remi: but if you insert a condy … >> John: would need to discard the attribute >> Dan H: concern about a constant array for each dimension >> ? maybe some adhoc compression techniques >> Remi: why do you want to be cycle free? >> Dan S: hard to verify if types cyclical? >> Remi: why do you need to check in the verifier? >> John: maybe a structural constraint? >> Remi: if value - ok, but not for an array of values? >> e.g. Object[] with first entity as itself >> Dan: want array type convertible to finite string, benefit of attribute not >> tied to construct is it can evolve >> >> Dan H: Concerns about attributes - will this be as hard to maintain for e.g. >> class redefiners/bytecode generators as stackmaptables? >> No - you have to keep the CP indices in sync, but stackmaptables required >> abstract interpretation - transformer >> did not have the type information, and the compression mechanism was harder >> to implement and these >> would just require bumping indices. > > To flesh the attribute idea out, here's a possible spec. Mentions > CONSTANT_Dynamic for now, and would be modified in the future to name other > constant types that allow cycles. > > Is this a happy solution to the problems raised in previous discussions about > constant cycles? Any new problems introduced? > > —Dan > > > 4.7.28 The `RecursiveConstants` Attribute (new) > ----------------------------------------- > > The `RecursiveConstants` attribute is a variable-length attribute in the > `attributes` table of a `ClassFile` structure ([4.1]). The > `RecursiveConstants` attribute facilitates checks that a `CONSTANT_Dynamic` > entry in the `constant_pool` table does not refer, directly or indirectly, to > itself ([4.4.13]). > > ~~~~ > RecursiveConstants_attribute { > u2 attribute_name_index; > u4 attribute_length; > u2 num_recursive_constants; > { u2 constant_index; > u2 max_recursion_depth; > } recursive_constants[num_recursive_constants]; > } > ~~~~ > > The items in the `RecursiveConstants_attribute` structure are as follows: > > `attribute_name_index` > > : The value of the `attribute_name_index` item must be a valid index into > the `constant_pool` table. The `constant_pool` entry at that index must be a > `CONSTANT_Utf8_info` structure ([4.4.7]) representing the string > `"RecursiveConstants"`. > > `attribute_length` > > : The value of the `attribute_length` item indicates the length of the > attribute, excluding the initial six bytes. > > `num_recursive_constants` > > : The value of the `num_recursive_constants` item gives the number of entries > in the `recursive_constants` array. > > `recursive_constants` > > : Each entry in the `recursive_constants` table contains the following two > items: > > `constant_index` > > : The value of `constant_index` must be a valid index into the > `constant_pool` table. The `constant_pool` entry at that index must be a > `CONSTANT_Dynamic` structure ([4.4.13]). > > There may be at most one entry in the `recursive_constants` table for > each `CONSTANT_Dynamic` structure in the `constant_pool`. > > `max_recursion_depth` > > : The value of `max_recursion_depth` designates a _maximum recursion > depth_ for the referenced constant. > > > 4.4.13 The `CONSTANT_Dynamic_info` Structure (modified) > -------------------------------------------- > > The `CONSTANT_Dynamic_info` structure is used to describe a > _dynamically-computed constant_, an arbitrary primitive or reference value > produced by invocation of a _bootstrap method_ ([4.7.23]). The structure > specifies i) a bootstrap method handle with an optional sequence of _static > arguments_ and ii) a referenced name and type. > > ~~~~ > CONSTANT_Dynamic_info { > u1 tag; > u2 bootstrap_method_attr_index; > u2 name_and_type_index; > } > ~~~~ > > The items of the `CONSTANT_Dynamic_info` structure are as follows: > > `tag` > > : The `tag` item of the `CONSTANT_Dynamic_info` structure has the value > `CONSTANT_Dynamic` (17). > > `bootstrap_method_attr_index` > > : The value of the `bootstrap_method_attr_index` item must be a valid index > into the `bootstrap_methods` array of the bootstrap method table ([4.7.23]) > of this class file. > > **The _maximum recursion depth_ of a `CONSTANT_Dynamic` structure is the > value given by `max_recursion_depth` of an entry referencing the > `CONSTANT_Dynamic` structure in the `recursive_constants` table of the > `RecursiveConstants` attribute ([4.7.28]); or, if no such entry exists, 0. > Let _d_ be the maximum recursion depth of this `CONSTANT_Dynamic_info` > structure. For each item in the `bootstrap_arguments` array of the > `bootstrap_methods` entry referenced by `bootstrap_method_attr_index`, if the > item references a `CONSTANT_Dynamic_info` structure, the maximum recursion > depth of that structure must be less than (and not equal to) _d_.** > > > **This check prevents a `CONSTANT_Dynamic_info` structure from > referring to itself via one of its static arguments.** > > `name_and_type_index` > > : The value of the `name_and_type_index` item must be a valid index into the > `constant_pool` table. The `constant_pool` entry at that index must be a > `CONSTANT_NameAndType_info` structure ([4.4.6]) representing a name and field > descriptor ([4.3.2]). >
