Index: llvm/trunk/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp
@@ -2512,26 +2512,41 @@
     return ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
 
   case Instruction::ShuffleVector: {
-    // If the shuffle mask contains any undefined elements, that element of the
-    // result is undefined. Propagating information from a source operand may
-    // not be correct in that case, so just bail out.
-    if (cast<ShuffleVectorInst>(U)->getMask()->containsUndefElement())
-      break;
-
-    // If everything is undef, we can't say anything. This should be simplified.
-    Value *Op0 = U->getOperand(0), *Op1 = U->getOperand(1);
-    if (isa<UndefValue>(Op0) && isa<UndefValue>(Op1))
+    // TODO: This is copied almost directly from the SelectionDAG version of
+    //       ComputeNumSignBits. It would be better if we could share common
+    //       code. If not, make sure that changes are translated to the DAG.
+
+    // Collect the minimum number of sign bits that are shared by every vector
+    // element referenced by the shuffle.
+    auto *Shuf = cast<ShuffleVectorInst>(U);
+    int NumElts = Shuf->getOperand(0)->getType()->getVectorNumElements();
+    int NumMaskElts = Shuf->getMask()->getType()->getVectorNumElements();
+    APInt DemandedLHS(NumElts, 0), DemandedRHS(NumElts, 0);
+    for (int i = 0; i != NumMaskElts; ++i) {
+      int M = Shuf->getMaskValue(i);
+      assert(M < NumElts * 2 && "Invalid shuffle mask constant");
+      // For undef elements, we don't know anything about the common state of
+      // the shuffle result.
+      if (M == -1)
+        return 1;
+      if (M < NumElts)
+        DemandedLHS.setBit(M % NumElts);
+      else
+        DemandedRHS.setBit(M % NumElts);
+    }
+    Tmp = std::numeric_limits<unsigned>::max();
+    if (!!DemandedLHS)
+      Tmp = ComputeNumSignBits(Shuf->getOperand(0), Depth + 1, Q);
+    if (!!DemandedRHS) {
+      Tmp2 = ComputeNumSignBits(Shuf->getOperand(1), Depth + 1, Q);
+      Tmp = std::min(Tmp, Tmp2);
+    }
+    // If we don't know anything, early out and try computeKnownBits fall-back.
+    if (Tmp == 1)
       break;
-
-    // Look through shuffle of 1 source vector.
-    if (isa<UndefValue>(Op0))
-      return ComputeNumSignBits(Op1, Depth + 1, Q);
-    if (isa<UndefValue>(Op1))
-      return ComputeNumSignBits(Op0, Depth + 1, Q);
-
-    // TODO: We can look through shuffles of 2 sources by computing the minimum
-    // sign bits for each operand (similar to what we do for binops).
-    break;
+    assert(Tmp <= V->getType()->getScalarSizeInBits() &&
+           "Failed to determine minimum sign bits");
+    return Tmp;
   }
   }
 
Index: llvm/trunk/test/Transforms/InstCombine/logical-select.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/logical-select.ll
+++ llvm/trunk/test/Transforms/InstCombine/logical-select.ll
@@ -621,11 +621,9 @@
 ; CHECK-NEXT:    [[SEXT1:%.*]] = sext <4 x i1> [[COND1:%.*]] to <4 x i32>
 ; CHECK-NEXT:    [[SEXT2:%.*]] = sext <4 x i1> [[COND2:%.*]] to <4 x i32>
 ; CHECK-NEXT:    [[COND:%.*]] = shufflevector <4 x i32> [[SEXT1]], <4 x i32> [[SEXT2]], <4 x i32> <i32 0, i32 2, i32 4, i32 6>
-; CHECK-NEXT:    [[NOTCOND:%.*]] = xor <4 x i32> [[COND]], <i32 -1, i32 -1, i32 -1, i32 -1>
-; CHECK-NEXT:    [[AND1:%.*]] = and <4 x i32> [[NOTCOND]], [[X:%.*]]
-; CHECK-NEXT:    [[AND2:%.*]] = and <4 x i32> [[COND]], [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = or <4 x i32> [[AND1]], [[AND2]]
-; CHECK-NEXT:    ret <4 x i32> [[SEL]]
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <4 x i32> [[COND]] to <4 x i1>
+; CHECK-NEXT:    [[TMP2:%.*]] = select <4 x i1> [[TMP1]], <4 x i32> [[Y:%.*]], <4 x i32> [[X:%.*]]
+; CHECK-NEXT:    ret <4 x i32> [[TMP2]]
 ;
   %sext1 = sext <4 x i1> %cond1 to <4 x i32>
   %sext2 = sext <4 x i1> %cond2 to <4 x i32>
Index: llvm/trunk/unittests/Analysis/ValueTrackingTest.cpp
===================================================================
--- llvm/trunk/unittests/Analysis/ValueTrackingTest.cpp
+++ llvm/trunk/unittests/Analysis/ValueTrackingTest.cpp
@@ -514,7 +514,6 @@
   EXPECT_EQ(ComputeNumSignBits(RVal, M->getDataLayout()), 1u);
 }
 
-// FIXME:
 // No guarantees for canonical IR in this analysis, so a shuffle element that
 // references an undef value means this can't return any extra information. 
 TEST(ValueTracking, ComputeNumSignBits_Shuffle2) {
@@ -534,7 +533,7 @@
 
   auto *RVal =
       cast<ReturnInst>(F->getEntryBlock().getTerminator())->getOperand(0);
-  EXPECT_EQ(ComputeNumSignBits(RVal, M->getDataLayout()), 32u);
+  EXPECT_EQ(ComputeNumSignBits(RVal, M->getDataLayout()), 1u);
 }
 
 TEST(ValueTracking, ComputeKnownBits) {