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Exception structure

October 23, 2001 21:08
Exception structure
Message ID:
    I couldn't help but notice that the current parrot interpreter
generates a divide-by-zero error when executing 'div I0,I1,0'. I'm
currently packaging up a set of patches that adds the following set of
features to parrot:

1) An exception stack (The stack prefix is 'E')
2) Instructions push_e, pop_e, set_e_i, set_i_e
3) Alters div_i_i_ic to raise an exception when dividing by the constant
4) Added t/op/exception.t to test the div_i_i_ic instruction.

  Having the virtual machine die was, to me, unacceptable. Skipping the
'div' instruction was also unacceptable, at least without signaling an
exception. One way to signal an exception would be to create a bitfield
of exceptions, where bit position 1 would signal 'divide exception' on.
This would limit the number of exceptions to the number of bits in the
register, which would be fixed at the time of creation of the register.
Limited and inflexible.

  Another solution was to write a special set of integers onto the int
stack which would signal an exception. Also bad because it would take a
long time to test for the special set of integers. Pushing a special
string onto the string stack would have the same problem. Having a
separate exception stack keeps the special information out of bounds,
and assigning one integer to each exception (Say, 1 = DIVIDE_BY_ZERO, 2
= NAN_I0, 35 = SOCKET_CLOSED) means that we have an effectively
unlimited number of exceptions available.

  Because it's an exception stack, the user can preserve previous
exceptions at will. For instance, catching a divide-by-zero error might
look as follows:

pushe # Save previous exceptions
div I0,I1,0 # The bogus operation
pope # No error occurred, go on with life

CONTINUE: add I0,I1,3 # Go on with life.

Of course, divide-by-zero is a simple error to catch. However, this
technique can be extended to mathematical errors like square roots of a
negative number, I/O errors like writing to an illegal filehandle, and
it should be able to handle IEEE math, with any luck.

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