Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -54,12 +54,6 @@
 static cl::opt<unsigned> DomConditionsMaxUses("dom-conditions-max-uses",
                                               cl::Hidden, cl::init(20));
 
-// This optimization is known to cause performance regressions is some cases,
-// keep it under a temporary flag for now.
-static cl::opt<bool>
-DontImproveNonNegativePhiBits("dont-improve-non-negative-phi-bits",
-                              cl::Hidden, cl::init(true));
-
 /// Returns the bitwidth of the given scalar or pointer type. For vector types,
 /// returns the element type's bitwidth.
 static unsigned getBitWidth(Type *Ty, const DataLayout &DL) {
@@ -1300,9 +1294,6 @@
           Known.Zero.setLowBits(std::min(Known2.countMinTrailingZeros(),
                                          Known3.countMinTrailingZeros()));
 
-          if (DontImproveNonNegativePhiBits)
-            break;
-
           auto *OverflowOp = dyn_cast<OverflowingBinaryOperator>(LU);
           if (OverflowOp && OverflowOp->hasNoSignedWrap()) {
             // If initial value of recurrence is nonnegative, and we are adding
Index: test/Transforms/BBVectorize/loop1.ll
===================================================================
--- test/Transforms/BBVectorize/loop1.ll
+++ test/Transforms/BBVectorize/loop1.ll
@@ -1,7 +1,7 @@
 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"
 target triple = "x86_64-unknown-linux-gnu"
 ; RUN: opt < %s -bb-vectorize -bb-vectorize-req-chain-depth=3 -bb-vectorize-ignore-target-info -instcombine -gvn -S | FileCheck %s
-; RUN: opt < %s -dont-improve-non-negative-phi-bits=false -basicaa -loop-unroll -unroll-threshold=45 -unroll-partial-threshold=45 -unroll-allow-partial -bb-vectorize -bb-vectorize-req-chain-depth=3 -bb-vectorize-ignore-target-info -instcombine -gvn -S | FileCheck %s -check-prefix=CHECK-UNRL
+; RUN: opt < %s -basicaa -loop-unroll -unroll-threshold=45 -unroll-partial-threshold=45 -unroll-allow-partial -bb-vectorize -bb-vectorize-req-chain-depth=3 -bb-vectorize-ignore-target-info -instcombine -gvn -S | FileCheck %s -check-prefix=CHECK-UNRL
 ; The second check covers the use of alias analysis (with loop unrolling).
 
 define void @test1(double* noalias %out, double* noalias %in1, double* noalias %in2) nounwind uwtable {