[CodeGen] Don't assume that a landing pad exists in setMustKeepGlobalVariables

[mstorsjo]

Summary

This fixes crashes (PR30535) when building C++ code (with exceptions enabled) for ARM/Windows with optimizations enabled.

[mstorsjo]

Ping @qcolombet or anyone else familiar with GlobalMerge.cpp.

[hans]

+majnemer who knows about these things.

mstorsjo: Please turn the IR reproducer on the PR into a test case and add to the patch.

[mstorsjo]

Added a test based on the pretty minimal IR that shows the issue.

[majnemer]

lib/CodeGen/GlobalMerge.cpp

513–521

This seems wrong. We should add cases for catchswitch & cleanuppad.

[mstorsjo]

This seems to have been fixed differently in SVN r284633.

lib/CodeGen/GlobalMerge.cpp

void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
   for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
        ++IFn) {
     for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
          IBB != IEndBB; ++IBB) {
       // Follow the invoke link to find the landing pad instruction
       const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
       if (!II) continue;
 
       const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
+      if (!LPInst) continue;
       // Look for globals in the clauses of the landing pad instruction
       for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
            Idx != NumClauses; ++Idx)
         if (const GlobalVariable *GV =
             dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
                                      ->stripPointerCasts()))
           MustKeepGlobalVariables.insert(GV);
     }
   }
 }
 
 bool GlobalMerge::doInitialization(Module &M) {
   if (!EnableGlobalMerge)
     return false;