Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2169,6 +2169,75 @@
                             ConstantExpr::getNeg(LHSC));
     }
     break;
+  case Instruction::Select:
+    // match a select chain which produces one of three values based on whether
+    // the LHS is less than, equal to, or greater than RHS respectively.
+    auto matchThreeWaySignedCompare = [](SelectInst *SI, Value *&LHS,
+                                         Value*& RHS, ConstantInt *&Less,
+                                         ConstantInt *&Equal,
+                                         ConstantInt *&Greater) {
+      // TODO: Generalize this to work with other comparison idioms or ensure
+      // they get canonicalized into this form.
+
+      // select i1 (a == b), i32 0, i32 (select i1 (a < b), i32 -1, i32 1),
+      // except -1, 0, 1 are placeholders for any three constants
+      ICmpInst::Predicate PredA, PredB;
+      if (match(SI->getTrueValue(), m_ConstantInt(Equal)) &&
+          match(SI->getCondition(), m_ICmp(PredA, m_Value(LHS),
+                                           m_Value(RHS))) &&
+          match(SI->getFalseValue(),
+                m_Select(m_ICmp(PredB, m_Specific(LHS), m_Specific(RHS)),
+                         m_ConstantInt(Less), m_ConstantInt(Greater)))) {
+        if (PredA == ICmpInst::ICMP_EQ &&
+            PredB == ICmpInst::ICMP_SLT)
+          return true;
+      }
+      return false;
+    };
+
+    // If we're testing a constant value against the result of a three way
+    // comparison, the result can be expressed directly in terms of the
+    // original values being compared.  Note: We could possibly be more
+    // aggressive here and remove the hasOneUse test.  The original select is
+    // really likely to simplify or sink when we remove a test of the result.
+    Value *OrigLHS, *OrigRHS;
+    ConstantInt *C1, *C2, *C3;
+    if (ICI.hasOneUse() &&
+        matchThreeWaySignedCompare(cast<SelectInst>(LHSI), OrigLHS, OrigRHS,
+                                   C1, C2, C3)) {
+      assert(C1 && C2 && C3 && RHS);
+
+      bool TrueWhenLessThan =
+        ConstantExpr::getCompare(ICI.getPredicate(), C1, RHS)->isAllOnesValue();
+      bool TrueWhenEqual =
+        ConstantExpr::getCompare(ICI.getPredicate(), C2, RHS)->isAllOnesValue();
+      bool TrueWhenGreaterThan =
+        ConstantExpr::getCompare(ICI.getPredicate(), C3, RHS)->isAllOnesValue();
+
+      // Eight combinations ...
+      if (TrueWhenLessThan && TrueWhenEqual && TrueWhenGreaterThan)
+        return replaceInstUsesWith(ICI, Builder->getTrue());
+      else if (TrueWhenLessThan && TrueWhenEqual && !TrueWhenGreaterThan)
+        return new ICmpInst(ICmpInst::ICMP_SLE, OrigLHS, OrigRHS);
+      else if (TrueWhenLessThan && !TrueWhenEqual && TrueWhenGreaterThan)
+        return new ICmpInst(ICmpInst::ICMP_NE, OrigLHS, OrigRHS);
+      else if (TrueWhenLessThan && !TrueWhenEqual && !TrueWhenGreaterThan)
+        return new ICmpInst(ICmpInst::ICMP_SLT, OrigLHS, OrigRHS);
+      else if (!TrueWhenLessThan && TrueWhenEqual && TrueWhenGreaterThan)
+        return new ICmpInst(ICmpInst::ICMP_SGE, OrigLHS, OrigRHS);
+      else if (!TrueWhenLessThan && TrueWhenEqual && !TrueWhenGreaterThan)
+        return new ICmpInst(ICmpInst::ICMP_EQ, OrigLHS, OrigRHS);
+      else if (!TrueWhenLessThan && !TrueWhenEqual && TrueWhenGreaterThan)
+        return new ICmpInst(ICmpInst::ICMP_SGT, OrigLHS, OrigRHS);
+      else if (!TrueWhenLessThan && !TrueWhenEqual && !TrueWhenGreaterThan)
+        return replaceInstUsesWith(ICI, Builder->getFalse());
+      else
+        llvm_unreachable("missed a combination");
+    }
+
+    // TODO: This could obvious be extended to handle three way unsigned
+    // compare idioms as well.  Also, fcmp idioms?
+    break;
   }
 
   // Simplify icmp_eq and icmp_ne instructions with integer constant RHS.
Index: test/Transforms/InstCombine/compare-3way.ll
===================================================================
--- test/Transforms/InstCombine/compare-3way.ll
+++ test/Transforms/InstCombine/compare-3way.ll
@@ -0,0 +1,395 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+declare void @use(i32)
+
+; These 18 excersice all combinations of signed comparison
+; for each of the three values produced by your typical 
+; 3way compare function (-1, 0, 1)
+
+define void @test_low_sgt(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_low_sgt
+; CHECK:  %cmp = icmp slt i64 %a, %b
+; CHECK: br i1 %cmp, label %normal, label %unreached
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sgt i32 %result, -1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_low_slt(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_low_slt
+; CHECK: br i1 false, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp slt i32 %result, -1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_low_sge(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_low_sge
+; CHECK: br i1 true, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sge i32 %result, -1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_low_sle(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_low_sle
+; CHECK: %cmp = icmp slt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sle i32 %result, -1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_low_ne(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_low_ne
+; CHECK:  %cmp = icmp slt i64 %a, %b
+; CHECK: br i1 %cmp, label %normal, label %unreached
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp ne i32 %result, -1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_low_eq(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_low_eq
+; CHECK:  %cmp = icmp slt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp eq i32 %result, -1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_mid_sgt(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_mid_sgt
+; CHECK:  %cmp = icmp sgt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sgt i32 %result, 0
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_mid_slt(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_mid_slt
+; CHECK:  %cmp = icmp slt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp slt i32 %result, 0
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_mid_sge(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_mid_sge
+; CHECK: %cmp = icmp slt i64 %a, %b
+; CHECK: br i1 %cmp, label %normal, label %unreached
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sge i32 %result, 0
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_mid_sle(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_mid_sle
+; CHECK: %cmp = icmp sgt i64 %a, %b
+; CHECK: br i1 %cmp, label %normal, label %unreached
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sle i32 %result, 0
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_mid_ne(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_mid_ne
+; CHECK:  %cmp = icmp eq i64 %a, %b
+; CHECK: br i1 %cmp, label %normal, label %unreached
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp ne i32 %result, 0
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_mid_eq(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_mid_eq
+; CHECK:  icmp eq i64 %a, %b
+; CHECK: br i1 %eq, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp eq i32 %result, 0
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_high_sgt(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_high_sgt
+; CHECK: br i1 false, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sgt i32 %result, 1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_high_slt(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_high_slt
+; CHECK:  %cmp = icmp sgt i64 %a, %b
+; CHECK: br i1 %cmp, label %normal, label %unreached
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp slt i32 %result, 1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_high_sge(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_high_sge
+; CHECK: %cmp = icmp sgt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sge i32 %result, 1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_high_sle(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_high_sle
+; CHECK: br i1 true, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp sle i32 %result, 1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_high_ne(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_high_ne
+; CHECK:  %cmp = icmp sgt i64 %a, %b
+; CHECK: br i1 %cmp, label %normal, label %unreached
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp ne i32 %result, 1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @test_high_eq(i64 %a, i64 %b) {
+; CHECK-LABEL: @test_high_eq
+; CHECK:  %cmp = icmp sgt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -1, i32 1
+  %result = select i1 %eq, i32 0, i32 %.
+  %cmp = icmp eq i32 %result, 1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+; These five make sure we didn't accidentally hard code one of the
+; produced values
+
+define void @non_standard_low(i64 %a, i64 %b) {
+; CHECK-LABEL: @non_standard_low
+; CHECK:  %cmp = icmp slt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -3, i32 -1
+  %result = select i1 %eq, i32 -2, i32 %.
+  %cmp = icmp eq i32 %result, -3
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @non_standard_mid(i64 %a, i64 %b) {
+; CHECK-LABEL: @non_standard_mid
+; CHECK:  icmp eq i64 %a, %b
+; CHECK: br i1 %eq, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -3, i32 -1
+  %result = select i1 %eq, i32 -2, i32 %.
+  %cmp = icmp eq i32 %result, -2
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @non_standard_high(i64 %a, i64 %b) {
+; CHECK-LABEL: @non_standard_high
+; CHECK:  %cmp = icmp sgt i64 %a, %b
+; CHECK: br i1 %cmp, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -3, i32 -1
+  %result = select i1 %eq, i32 -2, i32 %.
+  %cmp = icmp eq i32 %result, -1
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @non_standard_bound1(i64 %a, i64 %b) {
+; CHECK-LABEL: @non_standard_bound1
+; CHECK:  br i1 false, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -3, i32 -1
+  %result = select i1 %eq, i32 -2, i32 %.
+  %cmp = icmp eq i32 %result, -20
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}
+
+define void @non_standard_bound2(i64 %a, i64 %b) {
+; CHECK-LABEL: @non_standard_bound2
+; CHECK:  br i1 false, label %unreached, label %normal
+  %eq = icmp eq i64 %a, %b
+  %slt = icmp slt i64 %a, %b
+  %. = select i1 %slt, i32 -3, i32 -1
+  %result = select i1 %eq, i32 -2, i32 %.
+  %cmp = icmp eq i32 %result, 0
+  br i1 %cmp, label %unreached, label %normal
+normal:
+  ret void
+unreached:
+  call void @use(i32 %result)
+  ret void
+}