Re: Overhead for type conversions?

["Bobby D. Bryant" <bdbryant@mail.utexas.edu>]

On Sat, 26 Jul 2003 12:01:43 -0400, Warren W. Gay VE3WWG wrote:

> "Nick Roberts" <nickroberts@blueyonder.co.uk> wrote in message news:bfu83g$ibodl$1@ID-25716.news.uni-berlin.de...
>> "Bobby D. Bryant" <bdbryant@mail.utexas.edu> wrote in message
>> news:pan.2003.07.25.01.01.43.5699@mail.utexas.edu...
>>
>> > Given these declarations -
>> >
>> >    type f1 is new float;
>> >    type f2 is new float;
>> >
>> >    function "*"( left : in f1; right : in f2 ) return f1 is
>> >    begin
>> >       return( left * f1( right ) );
>> >    end "*";
>> >
> ...
>> I would also suggest that a function like this should be declared inline,
>> since it is likely that the eliminated call and return code would be bigger
>> (as well as much slower) than the intrinsic code for the multiplication
>> anyway.

I actually used the inline pragma in the test program I wrote before
posting, but decided to delete it from the post in order to avoid raising
an issue that I thought irrelevant to the basic question.  (But of course
it isn't really irrelevant, since the question of inlining wouldn't be
raised at all if I just used floats instead of the two derived types.)


> I once suggested something like this to Simon Wright regarding
> some procedure/function calls within the Booch Components. He
> did some profiling, and discovered that inlining that I suggested
> actually worsened the performance slightly under Linux. I don't
> know if he did more investigations along this line, but for
> the examples that I suggested, it actually made things worse
> (due to instruction caching reasons I expect).
> 
> So inlining should probably be tested before the conclusion is
> made. [...]

Yes, I have profiled some code a time or two and discovered that some
"obvious" inlining didn't help, and in fact actually appeared to hurt a
little in one case.  So I now assert that the desirability of inlining is
an empirical issue, though in fact I still make the decision
heuristically more often than empirically.

-- 
Bobby Bryant
Austin, Texas