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Users have complained llvm.trap produce two ud2 instructions on Win64, one for the trap, and one for unreachable. This change fixes that. TrapUnreachable was added and enabled for Win64 in r206684 (April 2014) to avoid poorly understood issues with the Windows unwinder. There seem to be two major things in play: - the unwinder - C++ EH, _CxxFrameHandler3 & co The unwinder disassembles forward from the return address to scan for epilogues. Inserting a ud2 had the effect of stopping the unwinder, and ensuring that it ran the EH personality function for the current frame. However, it's not clear what the unwinder does when the return address happens to be the last address of one function and the first address of the next function. The Visual C++ EH personality, _CxxFrameHandler3, needs to figure out what the current EH state number is. It does this by consulting the ip2state table, which maps from PC to state number. This seems to go wrong when the return address is the last PC of the function or catch funclet. I'm not sure precisely which system is involved here, but in order to address these real or hypothetical problems, I believe it is enough to insert int3 after a call site if it would otherwise be the last instruction in a function or funclet. I was able to reproduce some similar problems locally by arranging for a noreturn call to appear at the end of a catch block immediately before an unrelated function, and I confirmed that the problems go away when an extra trailing int3 instruction is added. MSVC inserts int3 after every noreturn function call, but I believe it's only necessary to do it if the call would be the last instruction. This change inserts a pseudo instruction that expands to int3 if it is in the last basic block of a function or funclet. I did what I could to run the Microsoft compiler EH tests, and the ones I was able to run showed no behavior difference before or after this change. Differential Revision: https://reviews.llvm.org/D66980 llvm-svn: 370525
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