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no_scan_tag and int array
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Date: -- (:)
From: Damien Doligez <damien.doligez@i...>
Subject: Re: [Caml-list] no_scan_tag and int array
Hello,

On 2010-03-06, at 10:26, ygrek wrote:

> So, as expected, setting No_scan_tag on the array of integers prevents GC from uselessly 
> scanning the huge chunk of memory. Looks like polymorphic array functions still work fine and
> GC correctly reclaims array memory when it is not referenced anymore.
> Apparantly this trick is not allowed for float array as they have a special tag set.

The trick is not needed for float arrays, the GC already doesn't scan
them.

> The question is - how safe is this?

It's safe, and will be in the forseeable future.

BUT: you should use Abstract_tag and not No_scan_tag.  Abstract_tag means
"don't make assumptions about the contents of this block", while
No_scan_tag is just the min of all the tags that the GC is not supposed
to scan.  Right now they are equal, but a future version of OCaml might
have No_scan_tag = Double_array_tag, which would break your code.

> And even more, could the compiler itself set this tag?

This is a bit tricky because you have to make sure that the static
type of the array is "int array".  Unlike floats, a run-time test
at allocation will not work.

You should enter this as a feature wish in the BTS.

-- Damien