[LLVMdev] Dereferencing NULL pointer in IndVarSimplify.cpp? (original) (raw)

Duncan P. N. Exon Smith dexonsmith at apple.com
Fri Oct 24 08:28:21 PDT 2014

On 2014-Oct-23, at 20:32, Andrew Trick <atrick at apple.com> wrote:

On Oct 17, 2014, at 5:21 PM, Liang Wang <netcasper at gmail.com> wrote: Hi, Here is the code in IndVarSimplify.cpp. SmallVector<WeakVH, 16> DeadInsts;

while (!DeadInsts.empty()) if (Instruction *Inst = dyncastornull(&*DeadInsts.popbackval())) RecursivelyDeleteTriviallyDeadInstructions(Inst, TLI); Since DeadInsts.popbackval() is WeakVH which could hold a NULL pointer, the expression, &*DeadInsts.popbackval(), could be &*NULL. Then NULL pointer is dereferenced here. I wrote a small test case and it works just fine. But is this a well-defined behavior in the standard?

This is UB, but &*nullptr often "works" so I'm not surprised you couldn't expose it with a testcase.

Try clang-dev or a c++ list for questions about the standard. I think it would have been nicer to write (Value*)DeadInsts.popbackval() -Andy

+1 (or static_cast<Value *>(DeadInsts.pop_back_val())).

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