[LLVMdev] RFC: Exception Handling Rewrite

[Bill Wendling]

On Aug 4, 2011, at 4:03 PM, Peter Lawrence wrote:

> Bill,
>       I believe the empty-exception-specification example is a red-herring,
> but that if you can construct an example where you think a landing-pad
> requires multiple filter lists then I think we can then have a productive
> conversation about it.
> 
> I believe we can only get multiple filter lists in a landing-pad if we attempt
> to merge exception-regions, and since filters are only an outer-function
> concept (not an inner try-statement concept) this can only come about
> from function inlining.
> 
> so say we're considering inlining "void foo() throw(A) { ... }"  into
> "void bar() throw (B) {...  foo(); ... ; frob(); ...}"
> 
> if you attempt to merge the landing pad for these functions (perhaps
> assuming it is going to be simple because there are no cleanups in either
> function that would complicate things) then...
> 
> there has to be a filter for A that will terminate the program if A is thrown,
> but you cannot terminate the program if exception A comes from frob()
> since it has not exception specification.
> 
> 
> my conclusion is that you can only merge landing-pads if they already
> have *identical* filters, so multiple filter lists isn't mathematically possible.
> 
> thoughts ?
> comments ?
> 
I think you might have the concept backwards. The call to foo() will call "unexpected" if anything *other* than A is thrown out of foo(). The EH tables can be set up to accommodate this situation. Let's say you have this example:

	void frob() { }
	void foo() throw (A) { }
	void bar() throw (B) { foo(); frob(); }

The exception table for "foo" will look something like this for the action (the indexes are negative offsets into the exception spec table):

  .byte 0x7f   # index into the exception specification table (-1)
  .byte 0      # index to the next "action"
...
  # Start of exception specification table
  .long _A     # Only "A" can be thrown
  .byte 0      # The end of this exception spec.

This is as you'd expect. The EH table for bar is similar:

  .byte 0x7f   # index into the exception specification table (-1)
  .byte 0      # index to the next "action"
...
  # Start of exception specification table
  .long _B     # Only "B" can be thrown
  .byte 0      # The end of this exception spec.

Now, when we inline "foo()" into "bar()", we get something that looks like this:

  .byte 0x7f   # index into the exception specification table (-1)
  .byte 0      # index to the next "action"
  .byte 0x7d   # index into the exception specification table (-3)
  .byte 0      # index to the next "action"
...
  # Start of exception specification table
  .long _A     # Only "A" can be thrown
  .byte 0      # The end of this exception spec.
  .long _B     # Only "B" can be thrown
  .byte 0      # The end of this exception spec.

Any call from foo will point to the ".byte 7f" action, which says that it can only throw "A" All calls from bar will point to the ".byte 0x7d" entry, which says that it can only throw "B". So in that way we can combine two functions with different filters.

> PS.  I believe the empty-exception-specification example is a
> red-herring because I believe you are making the same mistake that
> everyone does concerning "exception specifications", which is assuming
> they imply compile time correct information --  like "const" is enforced at
> compile time  --  which is false  (and why exception specifications are a
> bad design).
> 
> 
> __attribute__((nothrow))  <--- user guarantee that the function
> does not throw, the compiler is free to optimize based on this
> assertion, and if the program violates that guarantee then the
> the program is "undefined" and can "do anything".
> 
> throw()                                  <--- not a user guarantee of anything,
> rather a user requirement that the compiler generate runtime checking
> code to enforce that no exception is propagated out.


I'm aware of the semantics of "throw()". This is taken care of with the exception handling table. We would have an action of something like this:

  .byte 0x7f
  .byte 0

where the exception specification table is this:

  .byte 0x0

This indicates to the personality function won't find a matching exception specification (there are none to match) and then we call the unexpected dealy.

-bw