Index: lib/Analysis/VectorUtils.cpp
===================================================================
--- lib/Analysis/VectorUtils.cpp
+++ lib/Analysis/VectorUtils.cpp
@@ -499,6 +499,7 @@
 
     uint64_t V = DB.getDemandedBits(I).getZExtValue();
     DBits[Leader] |= V;
+    DBits[I] = V;
 
     // Casts, loads and instructions outside of our range terminate a chain
     // successfully.
@@ -549,6 +550,20 @@
     // Round up to a power of 2
     if (!isPowerOf2_64((uint64_t)MinBW))
       MinBW = NextPowerOf2(MinBW);
+
+    // We don't modify the types of PHIs. Reductions will already have been
+    // truncated if possible, and inductions' sizes will have been chosen by
+    // indvars.
+    // If we are required to shrink a PHI, abandon this entire equivalence class.
+    bool Abort = false;
+    for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI)
+      if (isa<PHINode>(*MI) && MinBW < (*MI)->getType()->getScalarSizeInBits()) {
+        Abort = true;
+        break;
+      }
+    if (Abort)
+      continue;
+
     for (auto MI = ECs.member_begin(I), ME = ECs.member_end(); MI != ME; ++MI) {
       if (!isa<Instruction>(*MI))
         continue;
Index: test/Transforms/LoopVectorize/AArch64/loop-vectorization-factors.ll
===================================================================
--- test/Transforms/LoopVectorize/AArch64/loop-vectorization-factors.ll
+++ test/Transforms/LoopVectorize/AArch64/loop-vectorization-factors.ll
@@ -205,5 +205,63 @@
   br i1 %exitcond, label %for.cond.cleanup, label %for.body
 }
 
+; CHECK-LABEL: @add_phifail(
+; CHECK: load <16 x i8>, <16 x i8>*
+; CHECK: add nuw nsw <16 x i32>
+; CHECK: store <16 x i8>
+; Function Attrs: nounwind
+define void @add_phifail(i8* noalias nocapture readonly %p, i8* noalias nocapture %q, i32 %len) #0 {
+entry:
+  %cmp8 = icmp sgt i32 %len, 0
+  br i1 %cmp8, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.body, %entry
+  ret void
+
+for.body:                                         ; preds = %entry, %for.body
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %a_phi = phi i32 [ %conv, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i8, i8* %p, i64 %indvars.iv
+  %0 = load i8, i8* %arrayidx
+  %conv = zext i8 %0 to i32
+  %add = add nuw nsw i32 %conv, 2
+  %conv1 = trunc i32 %add to i8
+  %arrayidx3 = getelementptr inbounds i8, i8* %q, i64 %indvars.iv
+  store i8 %conv1, i8* %arrayidx3
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %len
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+}
+
+; CHECK-LABEL: @add_phifail2(
+; CHECK: load <16 x i8>, <16 x i8>*
+; CHECK: add nuw nsw <16 x i32>
+; CHECK: store <16 x i8>
+; Function Attrs: nounwind
+define i8 @add_phifail2(i8* noalias nocapture readonly %p, i8* noalias nocapture %q, i32 %len) #0 {
+entry:
+  br label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.body, %entry
+  %ret = trunc i32 %a_phi to i8
+  ret i8 %ret
+
+for.body:                                         ; preds = %entry, %for.body
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %a_phi = phi i32 [ %conv, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i8, i8* %p, i64 %indvars.iv
+  %0 = load i8, i8* %arrayidx
+  %conv = zext i8 %0 to i32
+  %add = add nuw nsw i32 %conv, 2
+  %conv1 = trunc i32 %add to i8
+  %arrayidx3 = getelementptr inbounds i8, i8* %q, i64 %indvars.iv
+  store i8 %conv1, i8* %arrayidx3
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %len
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+}
 
 attributes #0 = { nounwind }
+