llvm-diff: Avoid crash with complex expressions

[ddcc]

Summary

With complex IR, the difference engine can enter an inconsistent state where the instruction and block difference engines return different results. Prevent a crash by refusing to pop beyond the end of the vector.

[ddcc]

I can trigger the runtime crash when comparing the _ZN13LaplaceSolver6SolverILi3EE22assemble_linear_systemERNS1_12LinearSystemE function from the SPEC2006 benchmark 447.dealII, but unfortunately even with llvm-extract, I haven't been able to generate a simple testcase for this.

[rjmccall]

This probably shouldn't happen, but okay, it's more defensive programming.

llvm/tools/llvm-diff/DifferenceEngine.cpp

void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
   TentativeValues.clear();
 
   SmallVectorImpl<char> &Path = Cur[NL].Path;
   BasicBlock::iterator LI = LStart, RI = RStart;
 
   DiffLogBuilder Diff(Engine.getConsumer());
 
   // Drop trailing matches.
-  while (Path.back() == DC_match)
+  while (Path.size() && Path.back() == DC_match)
     Path.pop_back();
 
   // Skip leading matches.
   SmallVectorImpl<char>::iterator
     PI = Path.begin(), PE = Path.end();
   while (PI != PE && *PI == DC_match) {
     unify(&*LI, &*RI);
     ++PI;