dead code elimination: Fixed a crash when trying to get the immediate post-dominating block in an infinite loop.

SR-7805
rdar://problem/40650953

Author: eeckstein

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -419,10 +419,12 @@ void DCE::propagateLiveness(SILInstruction *I) {
 SILBasicBlock *DCE::nearestUsefulPostDominator(SILBasicBlock *Block) {
   // Find the nearest post-dominator that has useful instructions.
   auto *PostDomNode = PDT->getNode(Block)->getIDom();
-  while (!LiveBlocks.count(PostDomNode->getBlock()))
+  while (PostDomNode && !LiveBlocks.count(PostDomNode->getBlock()))
     PostDomNode = PostDomNode->getIDom();
 
-  return PostDomNode->getBlock();
+  if (PostDomNode)
+    return PostDomNode->getBlock();
+  return nullptr;
 }
 
 // Replace the given conditional branching instruction with a plain
@@ -485,8 +487,11 @@ bool DCE::removeDead(SILFunction &F) {
       // We want to replace dead terminators with unconditional branches to
       // the nearest post-dominator that has useful instructions.
       if (isa<TermInst>(Inst)) {
-        replaceBranchWithJump(Inst,
-                              nearestUsefulPostDominator(Inst->getParent()));
+        SILBasicBlock *postDom = nearestUsefulPostDominator(Inst->getParent());
+        if (!postDom)
+          continue;
+  
+        replaceBranchWithJump(Inst, postDom);
         Inst->eraseFromParent();
         BranchesChanged = true;
         Changed = true;

test/SILOptimizer/dead_code_elimination.sil

@@ -243,3 +243,19 @@ bb0(%0 : $*Container):
   return %90 : $()
 }
 
+// Check that DCE does not crash with an infinite loop through a cond_br.
+// CHECK-LABEL: sil @deal_with_infinite_loop
+// CHECK: cond_br
+sil @deal_with_infinite_loop : $@convention(thin) () -> () {
+bb0:
+  br bb1
+
+bb1:
+  cond_br undef, bb2, bb3
+
+bb2:
+  br bb1
+
+bb3:
+  br bb1
+}