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

[John Criswell]

On 7/7/14, 2:45 PM, Tyler Denniston wrote:
> 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.

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.

If this is the problem, add code to change the terminator instruction of 
the errant basic block to the value that you want.

Regards,

John Criswell

>
> 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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