Re: Representation clauses and records

[stt@houdini.camb.inmet.com (Tucker Taft)]

David J. Fiander (davidf@mks.com) wrote:

: Pascal MALAISE <malaise@alphalink.com.au> writes:
: > As far as I know, this problem would not happen in full ADA because the
: > instanciation of unchecked_conversion will be compiled as a call to
: > something
: > like memcpy, a procedure which does not make any assumption about
: > alignment of arguments.
: > We loose performance but gain reliability.

: I've seen other comments that indicate that the runtime cost of a
: call to Unchecked_Conversion is going to be a function call of
: some sort, and I have just one question: "Why?"

I have never seen an implementation that imposes function call
overhead for unchecked-conversion.  This sounds like an
"urban myth" ;-).  Unchecked conversion does involve making a copy,
but not a function call.

: It would seem to me that if I have an object of some sort, and I
: call Unchecked_Conversion on it, if the type is small enough
: (in C terms converting a pointer to an int), why shouldn't the
: compiler just generate a "load ptr/store int" instruction
: sequence?  Now, for larger objects, the compiler will have to
: perform a block move to effect the conversion, either via a
: runtime library call or by open-coding the block move, but that
: depends entirely on the H/W support for block moves.

Right.  Whether block moves/copies involve out-of-line code is 
independent of the use of unchecked-conversion.

: - David

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA