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[Help-smalltalk] ReadOnly

From: Gwenaël Casaccio
Subject: [Help-smalltalk] ReadOnly
Date: Wed, 06 Nov 2013 10:25:06 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0


  The current implementation of the read-only support is not finished
a read-only object can change the value of an instance variable.
So I've made two implementations of the read-only the first with
a write barrier using sigsegv and the other checking the read-only

  The write barrier implementation uses the fixed space as a
read-only space after the allocation and the copy of the object
to the space the pages are marked read only and registered to
a read_ony_space_handler. Before the STORE_RECEIVER_ operation in vm.def
the interpreter state is exported. Once a write is done oldspace_sigsegv_handler is called and a check is done to see it's a read-only page or not (using sigsegv library) the read-only space handler is called and does a long jump in the interpreter. I send
a message to _gst_self.

The other implementation uses the bit check, nothing really special here :) I've made a simple "benchmark" and the difference between both approaches is not really noticeable. Frankly speaking I don't know what is the best approach to use (using write barrier or not). I think it could be nice to use an other heap for the read only objects: there are less wrong
positive with grey pages.

I see at least two issues the first with C programs and the write barrier that uses libgst and write an object directly the handler is not correct in that case. Second issue - in both cases - with the bytecode set for instance if it's the bc 80 and I send an tryToWriteAt::put: message and if the code simply returns it's
going to crash.


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