Index: lib/Analysis/ReachableCode.cpp
===================================================================
--- lib/Analysis/ReachableCode.cpp
+++ lib/Analysis/ReachableCode.cpp
@@ -58,6 +58,15 @@
   return false;
 }
 
+static bool isBuiltinUnreachable(const Stmt *S) {
+  if (const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(S)) {
+    const FunctionDecl *FDecl = dyn_cast<FunctionDecl>(DRE->getDecl());
+    return FDecl && FDecl->getIdentifier() &&
+           FDecl->getBuiltinID() == Builtin::BI__builtin_unreachable;
+  }
+  return false;
+}
+
 static bool isDeadReturn(const CFGBlock *B, const Stmt *S) {
   // Look to see if the current control flow ends with a 'return', and see if
   // 'S' is a substatement. The 'return' may not be the last element in the
@@ -574,8 +583,7 @@
 
   if (isa<BreakStmt>(S)) {
     UK = reachable_code::UK_Break;
-  }
-  else if (isTrivialDoWhile(B, S)) {
+  } else if (isTrivialDoWhile(B, S) || isBuiltinUnreachable(S)) {
     return;
   }
   else if (isDeadReturn(B, S)) {
Index: test/Sema/warn-unreachable.c
===================================================================
--- test/Sema/warn-unreachable.c
+++ test/Sema/warn-unreachable.c
@@ -396,3 +396,40 @@
   else
     calledFun();
 }
+
+int pr13910_foo(int x) {
+  if (x == 1)
+    return 0;
+  else
+    return x;
+  __builtin_unreachable(); // expected no warning
+}
+
+int pr13910_bar(int x) {
+  switch (x) {
+  default:
+    return x + 1;
+  }
+  pr13910_foo(x); // expected-warning {{code will never be executed}}
+}
+
+int pr13910_bar2(int x) {
+  if (x == 1)
+    return 0;
+  else
+    return x;
+  pr13910_foo(x);          // expected-warning {{code will never be executed}}
+  __builtin_unreachable(); // expected no warning
+  pr13910_foo(x);          // expected-warning {{code will never be executed}}
+}
+
+void pr13910_noreturn() {
+  raze();
+  __builtin_unreachable(); // expected no warning
+}
+
+void pr13910_assert() {
+  myassert(0 && "unreachable");
+  return;
+  __builtin_unreachable(); // expected no warning
+}