Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -4283,19 +4283,10 @@
     if (!CS.doesNotThrow())
       return false;
 
-    // Non-throwing call sites can loop infinitely, call exit/pthread_exit
-    // etc. and thus not return.  However, LLVM already assumes that
-    //
-    //  - Thread exiting actions are modeled as writes to memory invisible to
-    //    the program.
-    //
-    //  - Loops that don't have side effects (side effects are volatile/atomic
-    //    stores and IO) always terminate (see http://llvm.org/PR965).
-    //    Furthermore IO itself is also modeled as writes to memory invisible to
-    //    the program.
-    //
-    // We rely on those assumptions here, and use the memory effects of the call
-    // target as a proxy for checking that it always returns.
+    // A function which doens't throw and has "willreturn" attribute will
+    // always return.
+    if (CS.hasFnAttr("willreturn"))
+      return true;
 
     // FIXME: This isn't aggressive enough; a call which only writes to a global
     // is guaranteed to return.
Index: llvm/unittests/Analysis/ValueTrackingTest.cpp
===================================================================
--- llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -464,6 +464,7 @@
       "declare void @nounwind_argmemonly(i32*) nounwind argmemonly "
       "declare void @throws_but_readonly(i32*) readonly "
       "declare void @throws_but_argmemonly(i32*) argmemonly "
+      "declare void @nounwind_willreturn(i32*) nounwind \"willreturn\""
       " "
       "declare void @unknown(i32*) "
       " "
@@ -476,6 +477,7 @@
       "  call void @unknown(i32* %p) nounwind argmemonly "
       "  call void @unknown(i32* %p) readonly "
       "  call void @unknown(i32* %p) argmemonly "
+      "  call void @nounwind_willreturn(i32* %p)"
       "  ret void "
       "} ";
 
@@ -497,6 +499,7 @@
       true,  // call void @unknown(i32* %p) nounwind argmemonly
       false, // call void @unknown(i32* %p) readonly
       false, // call void @unknown(i32* %p) argmemonly
+      true,  // call void @nounwind_willreturn(i32* %p)
       false, // ret void
   };