Index: lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- lib/Transforms/Vectorize/LoopVectorize.cpp
+++ lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -281,7 +281,7 @@
     "interleave-loops", cl::init(true), cl::Hidden,
     cl::desc("Enable loop interleaving in Loop vectorization passes"));
 cl::opt<bool> llvm::EnableLoopVectorization(
-    "vectorize-loops", cl::init(false), cl::Hidden,
+    "vectorize-loops", cl::init(true), cl::Hidden,
     cl::desc("Run the Loop vectorization passes"));
 
 /// A helper function for converting Scalar types to vector types.
Index: test/CodeGen/Hexagon/bug15515-shuffle.ll
===================================================================
--- test/CodeGen/Hexagon/bug15515-shuffle.ll
+++ test/CodeGen/Hexagon/bug15515-shuffle.ll
@@ -1,4 +1,4 @@
-; RUN: opt -march=hexagon -O2 -vectorize-loops -S < %s
+; RUN: opt -march=hexagon -O2 -S < %s
 ; REQUIRES: asserts
 ;
 ; -fvectorize-loops infinite compile/memory
Index: test/Transforms/LoopVectorize/X86/metadata-enable.ll
===================================================================
--- test/Transforms/LoopVectorize/X86/metadata-enable.ll
+++ test/Transforms/LoopVectorize/X86/metadata-enable.ll
@@ -5,11 +5,11 @@
 ; RUN: opt < %s -mcpu=corei7 -O3 -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=O3DEFAULT
 ; RUN: opt < %s -mcpu=corei7 -Os -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=Os
 ; RUN: opt < %s -mcpu=corei7 -Oz -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=Oz
-; RUN: opt < %s -mcpu=corei7 -O1 -vectorize-loops -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=O1VEC
-; RUN: opt < %s -mcpu=corei7 -Oz -vectorize-loops -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=OzVEC
+; RUN: opt < %s -mcpu=corei7 -O1 -disable-loop-vectorization=false -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=O1VEC
+; RUN: opt < %s -mcpu=corei7 -Oz -disable-loop-vectorization=false -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=OzVEC
 ; RUN: opt < %s -mcpu=corei7 -O1 -loop-vectorize -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=O1VEC2
 ; RUN: opt < %s -mcpu=corei7 -Oz -loop-vectorize -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=OzVEC2
-; RUN: opt < %s -mcpu=corei7 -O3 -unroll-threshold=150 -disable-loop-vectorization -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=O3DIS
+; RUN: opt < %s -mcpu=corei7 -O3 -unroll-threshold=150 -vectorize-loops=false -S -unroll-allow-partial=0 | FileCheck %s --check-prefix=O3DIS
 
 ; This file tests the llvm.loop.vectorize.enable metadata forcing
 ; vectorization even when optimization levels are too low, or when
Index: test/Transforms/LoopVectorize/opt.ll
===================================================================
--- test/Transforms/LoopVectorize/opt.ll
+++ test/Transforms/LoopVectorize/opt.ll
@@ -1,5 +1,5 @@
 ; RUN: opt -S -O3 -force-vector-width=2 -force-vector-interleave=1 < %s | FileCheck --check-prefix=LOOPVEC %s
-; RUN: opt -S -O3 -disable-loop-vectorization -force-vector-width=2 -force-vector-interleave=1 < %s | FileCheck --check-prefix=NOLOOPVEC %s
+; RUN: opt -S -O3 -vectorize-loops=false -force-vector-width=2 -force-vector-interleave=1 < %s | FileCheck --check-prefix=NOLOOPVEC %s
 
 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"
 
Index: tools/opt/opt.cpp
===================================================================
--- tools/opt/opt.cpp
+++ tools/opt/opt.cpp
@@ -56,6 +56,7 @@
 #include "llvm/Transforms/IPO/AlwaysInliner.h"
 #include "llvm/Transforms/IPO/PassManagerBuilder.h"
 #include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Vectorize/LoopVectorize.h"
 #include <algorithm>
 #include <memory>
 using namespace llvm;
@@ -179,7 +180,7 @@
 static cl::opt<bool>
 DisableLoopVectorization("disable-loop-vectorization",
                      cl::desc("Disable the loop vectorization pass"),
-                     cl::init(false));
+                     cl::init(true));
 
 static cl::opt<bool>
 DisableSLPVectorization("disable-slp-vectorization",
@@ -381,11 +382,21 @@
   Builder.DisableUnrollLoops = (DisableLoopUnrolling.getNumOccurrences() > 0) ?
                                DisableLoopUnrolling : OptLevel == 0;
 
-  // This is final, unless there is a #pragma vectorize enable
-  if (DisableLoopVectorization)
+  // Check if option **explicitly** enabled or disabled via
+  // -disable-loop-vectorization=false or -vectorize-loops=false
+  // Both are true by default. The first (-disable-loop-vectorization) is only
+  // used to detect a specific user, because opt disables it unless:
+  // "OptLevel > 1 && SizeLevel < 2"
+  // The second (-vectorize-loops) enables vectorization in the LoopVectorize
+  // pass (on by default), and is used here to *disable* vectorization.
+  // The third flag that exists: -loop-vectorize, control adding the pass to the
+  // pass manager. If set, the pass is added, and there is no additional check
+  // here for it.
+  if (!EnableLoopVectorization)
     Builder.LoopVectorize = false;
-  // If option wasn't forced via cmd line (-vectorize-loops, -loop-vectorize)
-  else if (!Builder.LoopVectorize)
+  else if (!DisableLoopVectorization)
+    Builder.LoopVectorize = true;
+  else
     Builder.LoopVectorize = OptLevel > 1 && SizeLevel < 2;
 
   // When #pragma vectorize is on for SLP, do the same as above