Re: Waiver question

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Larry said

<<I do not understand why you feel the default should be task-unsafe.
The general trend in Ada seems to be that safeguards are enabled by
default and disabled by explicit statement.>>

I don't think that's quite the right principle. Ada defaults to unsafe
if for a great majority of implementations, being safe would be too
costly or otherwise infeasible.

I actually think that the decision in Ada 95 is just right, and reflects

this principle.

For unpacked records, on most architectures, it is reasonable to default
to task safe -- yes it costs on the Alpha, but that is because the Alpha
made a bad architectural choice -- and that choice has a penalty here.
The judgment that the choice was a bad one seems to be borne out by the
fact that Digital is "fixing" it.

On the other hand, for packed records, on nearly all architectures,
guaranteeing task safety by default would pretty much wipe out the
effectiveness of packing. This was a major hole in Ada 83 (where
independence was required by the language even for packed records,
but luckily not tested for, and hence entirely ignored except by
language lawyers).

For an in-depth discussion of the independence issue, see Norman
Schulman's Thesis on "Semantics of Shared Variables in Programming
Languages" (or something like that -- from NYU -- I was his advisor).

Little information tidbit: Norman Schulman's wife is the well known
Canadian journalist who recently wrote the book "Red China Blues"
on her experiences in China.