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Extending exception tag blocks #5009

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vicuna opened this Issue Mar 29, 2010 · 2 comments

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vicuna commented Mar 29, 2010

Original bug ID: 5009
Reporter: @mmottl
Assigned to: @alainfrisch
Status: closed (set by @xavierleroy on 2015-12-11T18:25:18Z)
Resolution: fixed
Priority: normal
Severity: feature
Version: 3.11.2
Target version: 4.02.0+dev
Fixed in version: 4.02.0+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: sweeks @hcarty yminsky @Chris00 @yakobowski @mmottl

Bug description

We have recently run into a problem (efficient S-expression converters for exceptions) that could be easily solved with a presumably small change to the OCaml-runtime.

Currently exception values have a pointer as first field that points to a block whose address uniquely identifies the kind of exception (lets call it "exception tag block"), e.g. as required for local modules, functor instantiations, etc. This block only stores a pointer to the string representing the exception constructor.

This makes it seemingly impossible to look up a converter for an exception in less than linear time, which may not be good enough for applications that e.g. instantiate a large number of functors or create many local modules in a loop.

The reason is that the address of the exception tag block is the only thing unique about it and it can change due to garbage collection, which prohibits the use of lookup tables/maps.

By extending the exception tag block, which currently only holds the pointer to the constructor string, by another word (or maybe two on 32bit architectures), it would be possible to store another, non-volatile piece of information that could uniquely identify the kind of exception. This would allow us to create the lookup tables required for O(log(N)) lookups.

A global 64bit counter in the OCaml runtime, for example, could then be used to generate unique ids at acceptably low cost. Exception values would not suffer any penalty. Only new instantiations of exceptions (as with functors, local modules, possibly first-class modules in the future, etc.) would see a tiny amount of extra cost.

Do you think this extension to exception tag blocks could make it into the OCaml runtime?

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vicuna commented Oct 23, 2013

Comment author: @alainfrisch

Commit 14239 on the raise_variants branch modifies the representation of exception slots, reusing the existing unique id machinery from object values.

Commit 14240 introduces a function Printexc.exn_slot_id (exn -> int) returning the unique id of the constructor used to create an exception value.

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vicuna commented Nov 13, 2013

Comment author: @alainfrisch

raise_variants branch has been merged into trunk (rev 14289).

@vicuna vicuna closed this Dec 11, 2015

@vicuna vicuna added this to the 4.02.0 milestone Mar 14, 2019

@vicuna vicuna added the feature-wish label Mar 20, 2019

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