Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -1071,12 +1071,16 @@
   // Check whether there's a dominating condition which implies something about
   // this value at the given context.
   if (EnableDomConditions && Depth <= DomConditionsMaxDepth)
+    // FIXME: this call may overwrite the known bits computed from assumes.
     computeKnownBitsFromDominatingCondition(V, KnownZero, KnownOne, DL, Depth,
                                             Q);
 
   Operator *I = dyn_cast<Operator>(V);
   if (!I) return;
 
+  // Save KnownZero and KnownOne so that they won't be overwritten by the
+  // following computeKnownBits calls.
+  APInt SavedKnownZero(KnownZero), SavedKnownOne(KnownOne);
   APInt KnownZero2(KnownZero), KnownOne2(KnownOne);
   switch (I->getOpcode()) {
   default: break;
@@ -1523,6 +1527,8 @@
       }
     }
   }
+  KnownZero |= SavedKnownZero;
+  KnownOne |= SavedKnownOne;
 
   assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
 }
Index: test/Analysis/ValueTracking/assume.ll
===================================================================
--- /dev/null
+++ test/Analysis/ValueTracking/assume.ll
@@ -0,0 +1,14 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+define i32 @assume_add(i32 %a, i32 %b) {
+; CHECK-LABEL: @assume_add(
+  %1 = add i32 %a, %b
+  %last_two_digits = and i32 %1, 3
+  %2 = icmp eq i32 %last_two_digits, 0
+  call void @llvm.assume(i1 %2)
+  %3 = add i32 %1, 3
+; CHECK: %3 = or i32 %1, 3
+  ret i32 %3
+}
+
+declare void @llvm.assume(i1)