[Local] Handle terminators with users in removeUnreachableBlocks.

[fhahn]

Summary

Terminators like invoke can have users outside the current basic block.
We have to replace those users with undef, before replacing the
terminator.

This fixes a crash exposed by rL373430.

[fhahn]

clang-format-diff

[asbirlea]

Thank you for this!
LGTM.

llvm/trunk/lib/Transforms/Utils/Local.cpp

for (auto *BB : DeadBlockSet) {
     for (BasicBlock *Successor : successors(BB)) {
       if (!DeadBlockSet.count(Successor))
         Successor->removePredecessor(BB);
       if (DTU)
         Updates.push_back({DominatorTree::Delete, BB, Successor});
     }
     BB->dropAllReferences();
     if (DTU) {
-      // Remove the terminator of BB to clear the successor list of BB.
-      if (BB->getTerminator())
-        BB->getInstList().pop_back();
+      Instruction *TI = BB->getTerminator();
+      assert(TI && "Basic block should have a terminator");
+      // Terminators like invoke can have users. We have to replace their users,
+      // before removing them.
+      if (!TI->use_empty())
+        TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
+      TI->eraseFromParent();
       new UnreachableInst(BB->getContext(), BB);
       assert(succ_empty(BB) && "The successor list of BB isn't empty before "
                                "applying corresponding DTU updates.");
     }
   }
 
   if (DTU) {
     DTU->applyUpdatesPermissive(Updates);

llvm/trunk/unittests/Transforms/Utils/LocalTest.cpp

std::unique_ptr<Module> M = parseIR(C,
         br label %bb0
       bb0:
         br label %bb0
       bb1:
         br label %bb2
       bb2:
         br label %bb1
       }
+
+      declare i32 @__gxx_personality_v0(...)
+
+      define void @invoke_terminator() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+      entry:
+        br i1 undef, label %invoke.block, label %exit
+
+      invoke.block:
+        %cond = invoke zeroext i1 @invokable()
+                to label %continue.block unwind label %lpad.block
+
+      continue.block:
+        br i1 %cond, label %if.then, label %if.end
+
+      if.then:
+        unreachable
+
+      if.end:
+        unreachable
+
+      lpad.block:
+        %lp = landingpad { i8*, i32 }
+                catch i8* null
+        br label %exit
+
+      exit:
+        ret void
+      }
+
+      declare i1 @invokable()
       )");
 
   auto runEager = [&](Function &F, DominatorTree *DT) {
     PostDominatorTree PDT = PostDominatorTree(F);
     DomTreeUpdater DTU(*DT, PDT, DomTreeUpdater::UpdateStrategy::Eager);
     removeUnreachableBlocks(F, &DTU);
     EXPECT_TRUE(DTU.getDomTree().verify());
     EXPECT_TRUE(DTU.getPostDomTree().verify());
   };
 
   auto runLazy = [&](Function &F, DominatorTree *DT) {
     PostDominatorTree PDT = PostDominatorTree(F);
     DomTreeUpdater DTU(*DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
     removeUnreachableBlocks(F, &DTU);
     EXPECT_TRUE(DTU.getDomTree().verify());
     EXPECT_TRUE(DTU.getPostDomTree().verify());
   };
 
   // Test removeUnreachableBlocks under Eager UpdateStrategy.
   runWithDomTree(*M, "br_simple", runEager);
   runWithDomTree(*M, "br_self_loop", runEager);
   runWithDomTree(*M, "br_constant", runEager);
   runWithDomTree(*M, "br_loop", runEager);
+  runWithDomTree(*M, "invoke_terminator", runEager);
 
   // Test removeUnreachableBlocks under Lazy UpdateStrategy.
   runWithDomTree(*M, "br_simple", runLazy);
   runWithDomTree(*M, "br_self_loop", runLazy);
   runWithDomTree(*M, "br_constant", runLazy);
   runWithDomTree(*M, "br_loop", runLazy);
+  runWithDomTree(*M, "invoke_terminator", runLazy);
 
   M = parseIR(C,
               R"(
       define void @f() {
       entry:
         ret void
       bb0:
         ret void