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From: Robert Dewar <dewar@gnat.com>
Subject: Re: Subverting 'Access for Sub-programs
Date: 1999/08/13
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In article <37B1A8B5.219C9FE6@averstar.com>,
  Tucker Taft <stt@averstar.com> wrote:

> or using some sort of "trampoline" trick.  Creating a
> trampoline that has the right lifetime is a fair amount of
> wor as well, since the place of the 'Unchecked_Access is a bit
> late. I realize that GNAT has solved this, but I believe it
> takes significant advantage of work already performed as part
> of the GCC nested procedure support.

Actually I don't see any lifetime problems, as long as the
trampoline is placed in the stack frame of the enclosing
subprogram, as is done in GNAT.

However, there are real problems in trampolines -- first the
whole issue of cache coherency in Harvard style architectures,
and second the issue of write protection of the stack (Sun has
a configuration option to do this, but it is troublesome enough
that it is very rarely used).

GNAT has solved these problems yes, but there are efficiency
concerns on some architectures.

SO I would agree with Tuck that the possibility of using
trampolines should not be something that is factored in.
In other words if the ADa definition *requires* trampolines
then I think that is a bad choice.

The alternative is as Tuck notes, two words per procedure
access value, and an extra load on the call. How bad is that?
Hard to say, the trade off is this extra space and time,
compared to extra complexity in having a separate type.

Note that on some architectures, notably from Digital, where
all indirect calls go through a procedure descriptor in any
case, there is no additional overhead at all (if you like, it
is already incurred unconditionally!)

Robert Dewar
Ada Core Technologies


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