[LLVMdev] Getting exceptions to work?

Duncan Sands baldrick at free.fr
Thu May 21 01:16:47 PDT 2009

Hi Talin, if you change @throwSomething so that it prints a message
if the _Unwind_RaiseException call returns, then you will see that
it does in fact return.

define internal fastcc void @throwSomething() {
         %throwable = malloc %Throwable
         call fastcc void @Throwable.construct(%Throwable* %throwable)
         %unwindInfo = getelementptr %Throwable* %throwable, i32 0, i32 1
         call void @print(i8* bitcast ([6 x i8]* @str_throw to i8 *))
         %throw = call i32 @_Unwind_RaiseException(%UnwindInfo* %unwindInfo)
         call void @print(i8* bitcast ([6 x i8]* @str_abort to i8 *))
         ret void

This is because it unwound the stack without finding any exception
handlers.  It doesn't consider your exception handler to be a handler
because it is registered as a "cleanup" which apparently doesn't count.
(To force cleanups to be run in this situation you have to call the gcc
forced unwind routine after calling Unwind_RaiseException).

Of course you would probably like to have your handler be considered a
real handler.  Sadly you can't use __gcc_personality_v0 for this: it
only handles cleanups, and if it sees something else (eg: if you specify
a catch-all in the selector) then it tells the unwinder to keep on
unwinding, which is not what you want.

So indeed you either need to write your own personality routine or use
a more powerful personality function like the C++ personality, and use
an eh.selector call to specify a catch-all (use null for this).

In order to get correct invoke semantics, LLVM really needs its own
personality function.  The problem is that LLVM doesn't have its own
runtime library like libgcc.  This could be overcome by outputting the
LLVM personality into each assembler file that needs it, with weak
linkage so that multiple copies get collapsed to one at link time.



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