misched: Fix the DAG builder to handle an undef operand at ExitSU.

Assertion failed: (VNI && "No value to read by operand")
rdar://12790267.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169071 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ScheduleDAGInstrs.cpp

@@ -210,6 +210,7 @@ void ScheduleDAGInstrs::addSchedBarrierDeps() {
         Uses[Reg].push_back(PhysRegSUOper(&ExitSU, -1));
       else {
         assert(!IsPostRA && "Virtual register encountered after regalloc.");
+        if (MO.readsReg()) // ignore undef operands
           addVRegUseDeps(&ExitSU, i);
       }
     }

test/CodeGen/X86/misched-new.ll

@@ -51,3 +51,29 @@ if.end:                                           ; preds = %if.then, %entry
 }
 
 declare void @bar(i32,i32)
+
+; Test that the DAG builder can handle an undef vreg on ExitSU.
+; CHECK: hasundef
+; CHECK: call
+
+%t0 = type { i32, i32, i8 }
+%t6 = type { i32 (...)**, %t7* }
+%t7 = type { i32 (...)** }
+
+define void @hasundef() unnamed_addr uwtable ssp align 2 {
+  %1 = alloca %t0, align 8
+  br i1 undef, label %3, label %2
+
+; <label>:2                                       ; preds = %0
+  unreachable
+
+; <label>:3                                       ; preds = %0
+  br i1 undef, label %4, label %5
+
+; <label>:4                                       ; preds = %3
+  call void undef(%t6* undef, %t0* %1)
+  unreachable
+
+; <label>:5                                       ; preds = %3
+  ret void
+}