a note WRT exception handlers

[Leopold Toetsch]

Yesterday on IIRC coke pasted[1] the following piece of code, I'll snip
here some irrelevant parts:

push_eh catch
$P1 = find_global "Spam", "q"
clear_eh

In combination with the resume functionality below in the "catch"
handler, this will do the following:

1) global found, pop the handler (ok)
2) gobal not found, resume at C<clear_eh>), try to pop the handler:
2a) there isn't an outer handler (ok)
2b) there is an outer exception handler - pop em (err)

Therefore such a construct can't be used reliably.

push_eh catch
store_lex 3, "q", $P1
clear_eh

The C<store_lex> does again throw an exception. But the exception
constructed in this opcode isn't resumable (it has a NULL C<ret_addr>
argument). Therefore it segfaults in the line C<invoke P4>.

catch:
P4 = P5["_invoke_cc"]
invoke P4
.end

I have recommended the following snippet to handle exceptions:

push_eh except_N
# code that might fail
clear_eh
resume_N:
...
except_N:
$S0 = P5["_message"] # get and
printerr $S0 # print
printerr "\n" # exception message
gpto resume_N

This corresponds to HLL exception code with try{} / except. If no
message is to be printed, the resume label should be directly after the
clear_eh:

push_eh except_N
# code that might fail
clear_eh
except_N:

(the _N denotes a unique number possibly created by the compiler)

leo

[1] http://nopaste.snit.ch:8001/2808

[Leopold Toetsch]

Leopold Toetsch wrote:
> Yesterday on IIRC

oops, too man TLAs and ETLAs and severe lack of coffee.
leo