Index: llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -4879,12 +4879,15 @@
       // induction variable. Notice that we can only optimize the 'trunc' case
       // because (a) FP conversions lose precision, (b) sext/zext may wrap, and
       // (c) other casts depend on pointer size.
-      auto ID = Legal->getInductionVars()->lookup(OldInduction);
-      if (isa<TruncInst>(CI) && CI->getOperand(0) == OldInduction &&
-          ID.getConstIntStepValue()) {
-        widenIntInduction(OldInduction, cast<TruncInst>(CI));
-        break;
-      }
+      if (auto *Trunc = dyn_cast<TruncInst>(CI))
+        if (auto *Phi = dyn_cast<PHINode>(Trunc->getOperand(0))) {
+          auto II = Legal->getInductionVars()->find(Phi);
+          if (II != Legal->getInductionVars()->end())
+            if (II->second.getConstIntStepValue()) {
+              widenIntInduction(Phi, Trunc);
+              break;
+            }
+        }
 
       /// Vectorize casts.
       Type *DestTy =
@@ -7224,12 +7227,17 @@
   case Instruction::Trunc:
   case Instruction::FPTrunc:
   case Instruction::BitCast: {
-    // We optimize the truncation of induction variable.
-    // The cost of these is the same as the scalar operation.
-    if (I->getOpcode() == Instruction::Trunc &&
-        Legal->isInductionVariable(I->getOperand(0)))
-      return TTI.getCastInstrCost(I->getOpcode(), I->getType(),
-                                  I->getOperand(0)->getType());
+    // We optimize the truncation of induction variables having constant
+    // integer steps. The cost of these truncations is the same as the scalar
+    // operation.
+    if (auto *Trunc = dyn_cast<TruncInst>(I))
+      if (auto *Phi = dyn_cast<PHINode>(Trunc->getOperand(0))) {
+        auto II = Legal->getInductionVars()->find(Phi);
+        if (II != Legal->getInductionVars()->end())
+          if (II->second.getConstIntStepValue())
+            return TTI.getCastInstrCost(Instruction::Trunc, Trunc->getDestTy(),
+                                        Trunc->getSrcTy());
+      }
 
     Type *SrcScalarTy = I->getOperand(0)->getType();
     Type *SrcVecTy = ToVectorTy(SrcScalarTy, VF);
Index: llvm/trunk/test/Transforms/LoopVectorize/induction.ll
===================================================================
--- llvm/trunk/test/Transforms/LoopVectorize/induction.ll
+++ llvm/trunk/test/Transforms/LoopVectorize/induction.ll
@@ -773,3 +773,34 @@
 exit:
   ret void
 }
+
+; CHECK-LABEL: @non_primary_iv_trunc(
+; CHECK:       vector.body:
+; CHECK-NEXT:    %index = phi i64 [ 0, %vector.ph ], [ %index.next, %vector.body ]
+; CHECK:         [[VEC_IND:%.*]] = phi <2 x i32> [ <i32 0, i32 2>, %vector.ph ], [ [[VEC_IND_NEXT:%.*]], %vector.body ]
+; CHECK:         [[TMP3:%.*]] = add i64 %index, 0
+; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr inbounds i32, i32* %a, i64 [[TMP3]]
+; CHECK-NEXT:    [[TMP5:%.*]] = getelementptr i32, i32* [[TMP4]], i32 0
+; CHECK-NEXT:    [[TMP6:%.*]] = bitcast i32* [[TMP5]] to <2 x i32>*
+; CHECK-NEXT:    store <2 x i32> [[VEC_IND]], <2 x i32>* [[TMP6]], align 4
+; CHECK-NEXT:    %index.next = add i64 %index, 2
+; CHECK:         [[VEC_IND_NEXT]] = add <2 x i32> [[VEC_IND]], <i32 4, i32 4>
+; CHECK:         br i1 {{.*}}, label %middle.block, label %vector.body
+define void @non_primary_iv_trunc(i32* %a, i64 %n) {
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i64 [ %i.next, %for.body ], [ 0, %entry ]
+  %j = phi i64 [ %j.next, %for.body ], [ 0, %entry ]
+  %tmp0 = getelementptr inbounds i32, i32* %a, i64 %i
+  %tmp1 = trunc i64 %j to i32
+  store i32 %tmp1, i32* %tmp0, align 4
+  %i.next = add nuw nsw i64 %i, 1
+  %j.next = add nuw nsw i64 %j, 2
+  %cond = icmp slt i64 %i.next, %n
+  br i1 %cond, label %for.body, label %for.end
+
+for.end:
+  ret void
+}
Index: llvm/trunk/test/Transforms/LoopVectorize/reverse_iter.ll
===================================================================
--- llvm/trunk/test/Transforms/LoopVectorize/reverse_iter.ll
+++ llvm/trunk/test/Transforms/LoopVectorize/reverse_iter.ll
@@ -2,7 +2,8 @@
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 
-; Make sure that the reverse iterators are calculated using 64bit arithmetic, not 32.
+; PR15882: This test ensures that we do not produce wrapping arithmetic when
+; creating constant reverse step vectors.
 ;
 ; int foo(int n, int *A) {
 ;   int sum;
@@ -13,7 +14,7 @@
 ;
 
 ;CHECK-LABEL: @foo(
-;CHECK:  <i64 0, i64 -1, i64 -2, i64 -3>
+;CHECK:  <i32 0, i32 -1, i32 -2, i32 -3>
 ;CHECK: ret
 define i32 @foo(i32 %n, i32* nocapture %A) {
   %1 = icmp sgt i32 %n, 0