Re: Modifies vs. Replaces

[brender at gemevn dot zko dot dec dot com (Ron 603-884-2088)]

>As I see it, the code which Ron presents is extremely common and 
>not at all unusual.  Using his method would mean that location
>information would need to be duplicated for virtually every use 
>of a global variable.  Instead of one relocation entry for the
>definition of a global variable, there would be one or more for
>each use of a global variable.

Not so -- this characterization of the cost is far too high. The cost
is not "each use" of the global variable, but "each non-defining declaration"
in the common case.

In the more common case, where it is valid for the value in the primary
location to be copied back to the global location arbitrarily by
the debugger, the cost will be exactly one extra entry in the
location list of each non-defining declaration (together with its
associated relocation), which is to say, one entry and relocation
per compilation unit. That is unlikely to be significant in the
presence of the remaining location entries that make it necessary to have
a location list for the non-defining declaration in the first place.

In the unusual case (possibly non-existant today?), the cost will
be higher, maybe even significantly so. But that cost describes
a case that cannot be described at all by the "appends" model.

There is a bit processing advantage in the "replaces" model as well.
Given that a non-defining declaration has been found that has its
own location attribute, there is no need to find and search two
location lists to form the set of locations that apply at the PC
of interest. I won't claim that is a big deal, however.