diff --git a/include/llvm/Instructions.h b/include/llvm/Instructions.h
index 76190f1..49ec4f5 100644
--- a/include/llvm/Instructions.h
+++ b/include/llvm/Instructions.h
@@ -642,6 +642,18 @@
   /// @brief Return the signed version of the predicate.
   static Predicate getSignedPredicate(Predicate pred);
 
+  /// For example, EQ->EQ, SLE->ULE, UGT->UGT, etc.
+  /// @returns the predicate that would be the result if the operand were
+  /// regarded as unsigned.
+  /// @brief Return the unsigned version of the predicate
+  Predicate getUnsignedPredicate() const {
+    return getUnsignedPredicate(getPredicate());
+  }
+
+  /// This is a static version that you can use without an instruction.
+  /// @brief Return the unsigned version of the predicate.
+  static Predicate getUnsignedPredicate(Predicate pred);
+
   /// isEquality - Return true if this predicate is either EQ or NE.  This also
   /// tests for commutativity.
   static bool isEquality(Predicate P) {
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 5ba4857..e56b559 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -5820,18 +5820,22 @@
     if (RHSCIOp->getType() != LHSCIOp->getType()) 
       return 0;
     
-    // If the signedness of the two compares doesn't agree (i.e. one is a sext
+    // If the signedness of the two casts doesn't agree (i.e. one is a sext
     // and the other is a zext), then we can't handle this.
     if (CI->getOpcode() != LHSCI->getOpcode())
       return 0;
 
-    // Likewise, if the signedness of the [sz]exts and the compare don't match, 
-    // then we can't handle this.
-    if (isSignedExt != isSignedCmp && !ICI.isEquality())
-      return 0;
-    
-    // Okay, just insert a compare of the reduced operands now!
-    return new ICmpInst(ICI.getPredicate(), LHSCIOp, RHSCIOp);
+    // Deal with equality cases early.
+    if (ICI.isEquality())
+      return new ICmpInst(ICI.getPredicate(), LHSCIOp, RHSCIOp);
+
+    // A signed comparison of sign extended values simplifies into a
+    // signed comparison.
+    if (isSignedCmp && isSignedExt)
+      return new ICmpInst(ICI.getPredicate(), LHSCIOp, RHSCIOp);
+
+    // The other three cases all fold into an unsigned comparison.
+    return new ICmpInst(ICI.getUnsignedPredicate(), LHSCIOp, RHSCIOp);
   }
 
   // If we aren't dealing with a constant on the RHS, exit early
diff --git a/lib/VMCore/Instructions.cpp b/lib/VMCore/Instructions.cpp
index 156eff1..b945a5a 100644
--- a/lib/VMCore/Instructions.cpp
+++ b/lib/VMCore/Instructions.cpp
@@ -2429,6 +2429,19 @@
   }
 }
 
+ICmpInst::Predicate ICmpInst::getUnsignedPredicate(Predicate pred) {
+  switch (pred) {
+    default: assert(! "Unknown icmp predicate!");
+    case ICMP_EQ: case ICMP_NE: 
+    case ICMP_UGT: case ICMP_ULT: case ICMP_UGE: case ICMP_ULE: 
+       return pred;
+    case ICMP_SGT: return ICMP_UGT;
+    case ICMP_SLT: return ICMP_ULT;
+    case ICMP_SGE: return ICMP_UGE;
+    case ICMP_SLE: return ICMP_ULE;
+  }
+}
+
 bool ICmpInst::isSignedPredicate(Predicate pred) {
   switch (pred) {
     default: assert(! "Unknown icmp predicate!");
diff --git a/test/Transforms/InstCombine/2008-01-21-MismatchedCastAndCompare.ll b/test/Transforms/InstCombine/2008-01-21-MismatchedCastAndCompare.ll
new file mode 100644
index 0000000..12bfb80
--- /dev/null
+++ b/test/Transforms/InstCombine/2008-01-21-MismatchedCastAndCompare.ll
@@ -0,0 +1,17 @@
+; RUN: llvm-as < %s | opt -instcombine | notcast
+; RUN: llvm-as < %s | opt -instcombine | not grep {icmp s}
+; PR1940
+
+define i1 @test1(i8 %A, i8 %B) {
+        %a = zext i8 %A to i32
+        %b = zext i8 %B to i32
+        %c = icmp sgt i32 %a, %b
+        ret i1 %c
+}
+
+define i1 @test2(i8 %A, i8 %B) {
+        %a = sext i8 %A to i32
+        %b = sext i8 %B to i32
+        %c = icmp ugt i32 %a, %b
+        ret i1 %c
+}