Re: Ada and embedded applications

[John English <je@brighton.ac.uk>]

Martin Dowie wrote:
> A few compilers I've come across offer 'zero cost' exception handling. By
> this
> they mean that it will have no affect on %CPU usage if there are no
> exceptions.
> The price you pay for that tends to be *slightly* slower exception handling
> if/
> when it does happen.

Hmm, the "obvious" way to handle exceptions involves one extra word
in the stack frame (a pointer to the start of the exception handling
code) which costs (typically) 4 bytes per handled block and 1 extra
word to pop on exit, which may or may not cost nothing, but is certainly
pretty cheap. For example, i386 processors have POPA/POPAD (pop all)
instructions, 680x0 have MOVEM (move multiple registers) where the
cost of an extra register is an extra memory cycle, and Vaxen of
fond memory had a word for the exception handler as a standard part
of the stack frame created by CALL.

Detecting exceptions can be more expensive -- e.g. on a processor that
doesn't generate an exception on arithmetic overflow, checking for
constraint errors can add quite a bit to the overall code cost.

The other complication is controlled types and the sequence of object
destruction that block exit can entail.

Does anyone have any references to alternative ways of implementing
exception handling?

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