[LLVMdev] Splitting basic block results in unknown instruction type assertion

[Tyler Denniston]

Hello,

I would like to see if this issue is a result of a misunderstanding on  
my part before I file a bug. I am using LLVM 3.4, built from the  
source tarballs. My system's uname is "Darwin tyler-air 12.5.0 Darwin  
Kernel Version 12.5.0: Sun Sep 29 13:33:47 PDT 2013;  
root:xnu-2050.48.12~1/RELEASE_X86_64 x86_64".

All I'm trying to do is add a runtime check after all call  
instructions. To do this I would (eventually) like to add a branch to  
an "error" block that aborts the program. However, in trying to insert  
a new block after each call site, I encountered this issue. The  
problem I'm seeing is an InstVisitor assertion failure ("UNREACHABLE  
executed" and "Unknown instruction type encountered") after splitting  
a basic block.

Here is a method to reproduce the issue:

     // -------------------- Test.cpp
     #include "llvm/IR/Module.h"
     #include "llvm/Pass.h"
     #include "llvm/InstVisitor.h"

     using namespace llvm;

     namespace {
       class TestInstVisitor : public InstVisitor<TestInstVisitor> {
       public:
         void visitCallInst(CallInst &I) {
           Instruction *splitPoint = I.getNextNode();
           BasicBlock *head = splitPoint->getParent();
           BasicBlock *tail = head->splitBasicBlock(splitPoint);
         }
       };

       class TestPass : public ModulePass {
       public:
         static char ID;
         TestPass() : ModulePass(ID) {}
         virtual bool runOnModule(Module &M) {
           TestInstVisitor visitor;
           visitor.visit(M);
           return true;
         }
       };
     }
     char TestPass::ID = 0;
     static RegisterPass<TestPass> X("bugtest", "Bug test pass", false, false);

     // -------------------- buginput.cpp
     void foo() {}
     int main() {
       foo();
       return 0;
     }

If you run the "bugtest" pass on the buginput test program, you should  
see the following assertion failure:

     Unknown instruction type encountered!
     UNREACHABLE executed at  
/Users/tyler/research/llvm-3.4/include/llvm/InstVisitor.h:125!
     0  opt               0x0000000103bcc77e  
llvm::sys::PrintStackTrace(__sFILE*) + 46
     1  opt               0x0000000103bcca8b  
PrintStackTraceSignalHandler(void*) + 27
     2  opt               0x0000000103bccd99 SignalHandler(int) + 297
     3  libsystem_c.dylib 0x00007fff9455990a _sigtramp + 26
     4  opt               0x000000010367bbc5 llvm::AllocaInst**  
std::__uninitialized_copy_aux<llvm::AllocaInst* const*,  
llvm::AllocaInst**>(llvm::AllocaInst* const*, llvm::AllocaInst*  
const*, llvm::AllocaInst**, std::__true_type) + 37
     5  opt               0x0000000103bccabb raise + 27
     6  opt               0x0000000103bccb72 abort + 18
     7  opt               0x0000000103bb3386  
llvm::llvm_unreachable_internal(char const*, char const*, unsigned  
int) + 198
     8  Test.dylib        0x00000001060d9376  
llvm::InstVisitor<(anonymous namespace)::TestInstVisitor,  
void>::visit(llvm::Instruction&) + 198
     9  Test.dylib        0x00000001060d92a5 void  
llvm::InstVisitor<(anonymous namespace)::TestInstVisitor,  
void>::visit<llvm::ilist_iterator<llvm::Instruction>  
 >(llvm::ilist_iterator<llvm::Instruction>,  
llvm::ilist_iterator<llvm::Instruction>) + 101

(some stack frames omitted for brevity).

Is this due to some mistake on my part, or is this a bug?

Thanks,

Tyler