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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.arch,comp.lang.ada
Subject: Re: Have the Itanium critics all been proven wrong?
Date: Tue, 21 Aug 2012 18:32:18 -0400
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Date: 2012-08-21T18:32:18-04:00
List-Id: <comp.lang.ada>

Niklas Holsti <niklas.holsti@tidorum.invalid> writes:

> As far as I know, the only Ada systems with automatic garbage collection
> are those that compile Ada to Java byte-code and run on the JVM.

Or .NET.

>...I have
> heard that some people have used the Boehm "conservative" GC with Ada,
> but I'm not sure if it works reliably (because some Ada compilers play
> tricks with array base addresses).

I have used the Boehm GC with Ada compiled with GNAT.  It works.
But I wouldn't use it in a real-time/embedded/safety-critical
program.

Boehm GC doesn't work very well if the amount of memory you use
is a large fraction of the virtual address space.  This is because
of the way "blacklisting" works.  If the GC runs across the value
12345000, which might be a pointer to a heap object, it checks whether
something has been allocated at that address.  If not, it black-lists
that page of virtual memory, which means it refuses to ever allocate
any heap object on that page.  That means 12345000 can never be
mistaken for a pointer.  So large chunks of virtual memory get
black listed.

I had problems with that years ago, on 32-bit machines with heap sizes
in the hundreds of megabytes.  But on a 64-bit machine, it's not a
problem -- nobody can afford enough RAM to make up (say) 10% of
the virtual address space.

Anyway, this isn't Ada specific -- Boehm GC works that way with all
languages.

> Unchecked_Deallocation is used a lot.

Another alternative in Ada is storage pools (sometimes called
"region-based memory management).  Not 100% safe in Ada, but much safer
and much more efficient than Unchecked_Deallocation (in cases where
storage pools are appropriate).

- Bob