[type-annos-observers] Comments on annotations on exception parameters

[Alex Buckley]

Werner Dietl suggested:

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In "3.3.8 Exception parameters" the JSR 308 design document from Nov.
7 2012 states that annotations on an exception parameter (e.g. ...
catch (@A Exception e) ...) are stored as an exception table index.

I'm wondering whether for uniformity this could be stored like a local
variable or resource variable, where we store the information for the
variables explicitly. Unifying this aspect would simplify both the 
specification and its implementation. Thoughts?
*****

The declaration of an exception parameter is syntactically similar to 
the declaration of a local variable or resource variable, but the JVM 
representation is different.

When an exception object is thrown, the JVM places a reference to the 
thrown object on top of the operand stack. The bytecode of an exception 
handler (i.e. a catch block) may or may not store this reference in a 
local variable. There is no compiler spec to govern the matter. For 
simple catch blocks, a compiler might be able to dup the top stack entry 
and avoid local variables entirely.

Even if a compiler stores the reference in a local variable, the live 
range of that local variable would be tricky to compute because there is 
no "end" to an exception handler. Multiple handlers for one try block 
may be compiled in such a way that they jump between themselves to share 
code.

For these reasons, and to avoid duplicating the information in 
Code.exception_table, I think 3.3.8 should continue to index the 
exception_table.

Alex