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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Having the number of arguments for each handler depend on the handler type feels a bit shaky to me, but I can’t think of an actually problem with it that wouldn’t
be handled by checking the type of each argument as we process it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">There are a couple of other pieces of information that need to end up in the catch handler array in the .xdata section, but I don’t think they have any reason
to be included here.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">So, yeah, I think this form for llvm.eh.actions is reasonable. We can always replace it later if we come up with a cleaner idea.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">-Andy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Reid Kleckner [mailto:rnk@google.com]
<br>
<b>Sent:</b> Thursday, March 05, 2015 3:54 PM<br>
<b>To:</b> LLVM Developers Mailing List; Kaylor, Andrew<br>
<b>Subject:</b> Making llvm.eh.actions work<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">llvm.eh.actions is basically a sequenced list of actions to take when an invoke throws to the landing pad containing the actions call. The prototype is just 'i8* llvm.eh.actions(...)', but it needs some structure. LLVM IR supports three
types of actions: cleanup, catch, and filter. __CxxFrameHandler3 doesn't support filters, so it isn't really relevant here.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Catch actions need three pieces of information:<o:p></o:p></p>
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<p class="MsoNormal">1. The typeinfo to catch. This can be null for catch-all handlers.<o:p></o:p></p>
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<p class="MsoNormal">2. The alloca that the exception object is stored into. This can be null if there is no catch parameter.<o:p></o:p></p>
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<p class="MsoNormal">3. The handler to run. This is required.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Cleanup actions only need one piece of information: the handler to run.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">There are two main ways I can think of to encode this list.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The first is that we have a constant i32 parameter of 0 or 1 to indicate if this is a cleanup or catch, followed by constants representing the information required. We'd pass 3 args for catches and 1 for cleanups.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The second way is that we invent some kind of not-quite-a-catch-all sentinel to use instead of the i32 argument. We could use i32 -1, i8* undef, or something else. This all pretty confusing though.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The i32 argument approach extends to handle filters pretty naturally. It's extensible, but it uses a lot of memory just to communicate a single bit.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The EH state number isn't encoded in the action list, it's computed later using the algorithm you described in the "RFC: Native Windows C++ exception handling" thread.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Does this seem reasonable?<o:p></o:p></p>
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