Tolerate degenerate phi nodes that can occur in the middle of optimization

passes. Fixes PR9112. Patch by Jakub Staszak!

llvm-svn: 125319

lib/Analysis/ValueTracking.cpp

@@ -590,6 +590,10 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
       }
     }
 
+    // Unreachable blocks may have zero-operand PHI nodes.
+    if (P->getNumIncomingValues() == 0)
+      return;
+
     // Otherwise take the unions of the known bit sets of the operands,
     // taking conservative care to avoid excessive recursion.
     if (Depth < MaxDepth - 1 && !KnownZero && !KnownOne) {

test/Transforms/JumpThreading/degenerate-phi.ll

@@ -0,0 +1,24 @@
+; RUN: opt -jump-threading -disable-output %s
+; PR9112
+
+; This is actually a test for value tracking. Jump threading produces
+; "%phi = phi i16" when it removes all edges leading to %unreachable.
+; The .ll parser won't let us write that directly since it's invalid code.
+
+define void @func() nounwind {
+entry:
+  br label %bb
+
+bb:
+  br label %bb
+
+unreachable:
+  %phi = phi i16 [ %add, %unreachable ], [ 0, %next ]
+  %add = add i16 0, %phi
+  %cmp = icmp slt i16 %phi, 0
+  br i1 %cmp, label %unreachable, label %next
+
+next:
+  br label %unreachable
+}
+