Re: [boost] Re: Virtual inheritance in exception hierarchies

From: Iain K. Hanson (ikh_at_[hidden])
Date: 2003-09-06 22:31:24

David Abrahams wrote:

>"Iain K. Hanson" <iain.hanson@[...].com>  writes:

>>  dynamic_cast needs rtti afaik, at least on some compilers e.g. Sun.
>>
>>  My mention of incurring cost was to MI over SI. a quick test shows that
>>  throwing a exception using MI and virtual bases with gcc 3.2.2.5 with
>>  -O4 produces an assemblers file more than twice the size of of the same
>>  program using SI exceptions only. With a line count difference of nearly
>>  exactly double.
>>
>>  That is a significant overhead for every exception.

>Where is the overhead?  Compile time?  Runtime?  Static data? Size of
>intermediate assembly file ;->  ?

hehe I'm an old hand assembler programmer and I probably assumed to much.

There is, of course a compile time overhead. If you generate twice as much code
then it will of course take longer. This is almost certainly I/O dominated and
in practice is negligable.

I have not done a full analysis of the output as this would take several hours
but form a quick read through there is a small ( relative to the Code segemnt )
 Data segment overhead for additional book-keeping and the rest is code 
( active instructions ). I have no idea without doing a full analysis what
proportion of the instructions actually execute and what are just code bloat.

Which ever way you cut it, virtually inheriting from std::exception has a real
cost to support an outside case of using MI for exceptions.

[snip]
>Your analysis was a bit too thin to be convincing.

Sorry. I was assuming far to much.

/ikh