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

[Tyler Denniston]

Quoting John Criswell <jtcriswel at gmail.com>:
> I don't know for certain, but my guess is that one of the two basic  
> blocks after the split has an unreachable instruction (yes, LLVM has  
> an instruction called unreachable), and the InstVistor class doesn't  
> like it.
>
> To verify this, call the dump() method on head and tail to print the  
> two new basic blocks.

Hi John,

Thanks for your reply. I'm not sure how unreachable instructions are  
represented, but there don't appear to be any in the dump of the two  
blocks. Here is what I see on the "buginput" program from earlier,  
after the split:

Dump head:

entry:
   call void @_Z3foov()
   br label %0

Dump tail:

; <label>:0                                       ; preds = %entry
   ret i32 0

Tyler

>>
>> 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
>>
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