Index: llvm/trunk/include/llvm/IR/GlobalValue.h
===================================================================
--- llvm/trunk/include/llvm/IR/GlobalValue.h
+++ llvm/trunk/include/llvm/IR/GlobalValue.h
@@ -346,6 +346,10 @@
     return !(isDeclarationForLinker() || isWeakForLinker());
   }
 
+  // Returns true if the alignment of the value can be unilaterally
+  // increased.
+  bool canIncreaseAlignment() const;
+
   /// This method unlinks 'this' from the containing module, but does not delete
   /// it.
   virtual void removeFromParent() = 0;
Index: llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
+++ llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
@@ -1742,8 +1742,8 @@
       // over-aligning global variables that have an explicit section is
       // forbidden.
       GlobalVariable *GV;
-      if ((GV = dyn_cast<GlobalVariable>(Val)) && GV->hasUniqueInitializer() &&
-          !GV->hasSection() && GV->getAlignment() < PrefAlign &&
+      if ((GV = dyn_cast<GlobalVariable>(Val)) && GV->canIncreaseAlignment() &&
+          GV->getAlignment() < PrefAlign &&
           DL->getTypeAllocSize(GV->getType()->getElementType()) >=
               MinSize + Offset2)
         GV->setAlignment(PrefAlign);
Index: llvm/trunk/lib/IR/Globals.cpp
===================================================================
--- llvm/trunk/lib/IR/Globals.cpp
+++ llvm/trunk/lib/IR/Globals.cpp
@@ -12,11 +12,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/IR/GlobalValue.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Triple.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/GlobalAlias.h"
+#include "llvm/IR/GlobalValue.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
@@ -134,6 +135,47 @@
   return false;
 }
 
+bool GlobalValue::canIncreaseAlignment() const {
+  // Firstly, can only increase the alignment of a global if it
+  // is a strong definition.
+  if (!isStrongDefinitionForLinker())
+    return false;
+
+  // It also has to either not have a section defined, or, not have
+  // alignment specified. (If it is assigned a section, the global
+  // could be densely packed with other objects in the section, and
+  // increasing the alignment could cause padding issues.)
+  if (hasSection() && getAlignment() > 0)
+    return false;
+
+  // On ELF platforms, we're further restricted in that we can't
+  // increase the alignment of any variable which might be emitted
+  // into a shared library, and which is exported. If the main
+  // executable accesses a variable found in a shared-lib, the main
+  // exe actually allocates memory for and exports the symbol ITSELF,
+  // overriding the symbol found in the library. That is, at link
+  // time, the observed alignment of the variable is copied into the
+  // executable binary. (A COPY relocation is also generated, to copy
+  // the initial data from the shadowed variable in the shared-lib
+  // into the location in the main binary, before running code.)
+  //
+  // And thus, even though you might think you are defining the
+  // global, and allocating the memory for the global in your object
+  // file, and thus should be able to set the alignment arbitrarily,
+  // that's not actually true. Doing so can cause an ABI breakage; an
+  // executable might have already been built with the previous
+  // alignment of the variable, and then assuming an increased
+  // alignment will be incorrect.
+
+  // Conservatively assume ELF if there's no parent pointer.
+  bool isELF =
+      (!Parent || Triple(Parent->getTargetTriple()).isOSBinFormatELF());
+  if (isELF && hasDefaultVisibility() && !hasLocalLinkage())
+    return false;
+
+  return true;
+}
+
 //===----------------------------------------------------------------------===//
 // GlobalVariable Implementation
 //===----------------------------------------------------------------------===//
Index: llvm/trunk/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/Local.cpp
+++ llvm/trunk/lib/Transforms/Utils/Local.cpp
@@ -944,37 +944,44 @@
 static unsigned enforceKnownAlignment(Value *V, unsigned Align,
                                       unsigned PrefAlign,
                                       const DataLayout &DL) {
+  assert(PrefAlign > Align);
+
   V = V->stripPointerCasts();
 
   if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
+    // TODO: ideally, computeKnownBits ought to have used
+    // AllocaInst::getAlignment() in its computation already, making
+    // the below max redundant. But, as it turns out,
+    // stripPointerCasts recurses through infinite layers of bitcasts,
+    // while computeKnownBits is not allowed to traverse more than 6
+    // levels.
+    Align = std::max(AI->getAlignment(), Align);
+    if (PrefAlign <= Align)
+      return Align;
+
     // If the preferred alignment is greater than the natural stack alignment
     // then don't round up. This avoids dynamic stack realignment.
     if (DL.exceedsNaturalStackAlignment(PrefAlign))
       return Align;
-    // If there is a requested alignment and if this is an alloca, round up.
-    if (AI->getAlignment() >= PrefAlign)
-      return AI->getAlignment();
     AI->setAlignment(PrefAlign);
     return PrefAlign;
   }
 
   if (auto *GO = dyn_cast<GlobalObject>(V)) {
+    // TODO: as above, this shouldn't be necessary.
+    Align = std::max(GO->getAlignment(), Align);
+    if (PrefAlign <= Align)
+      return Align;
+
     // If there is a large requested alignment and we can, bump up the alignment
     // of the global.  If the memory we set aside for the global may not be the
     // memory used by the final program then it is impossible for us to reliably
     // enforce the preferred alignment.
-    if (!GO->isStrongDefinitionForLinker())
+    if (!GO->canIncreaseAlignment())
       return Align;
 
-    if (GO->getAlignment() >= PrefAlign)
-      return GO->getAlignment();
-    // We can only increase the alignment of the global if it has no alignment
-    // specified or if it is not assigned a section.  If it is assigned a
-    // section, the global could be densely packed with other objects in the
-    // section, increasing the alignment could cause padding issues.
-    if (!GO->hasSection() || GO->getAlignment() == 0)
-      GO->setAlignment(PrefAlign);
-    return GO->getAlignment();
+    GO->setAlignment(PrefAlign);
+    return PrefAlign;
   }
 
   return Align;
Index: llvm/trunk/test/CodeGen/ARM/memfunc.ll
===================================================================
--- llvm/trunk/test/CodeGen/ARM/memfunc.ll
+++ llvm/trunk/test/CodeGen/ARM/memfunc.ll
@@ -1,10 +1,10 @@
-; RUN: llc < %s -mtriple=armv7-apple-ios -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-IOS
-; RUN: llc < %s -mtriple=thumbv7m-none-macho -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-DARWIN
-; RUN: llc < %s -mtriple=arm-none-eabi -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-EABI
-; RUN: llc < %s -mtriple=arm-none-eabihf -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-EABI
-; RUN: llc < %s -mtriple=arm-none-androideabi -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-EABI
-; RUN: llc < %s -mtriple=arm-none-gnueabi -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-GNUEABI
-; RUN: llc < %s -mtriple=arm-none-gnueabihf -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-GNUEABI
+; RUN: llc < %s -mtriple=armv7-apple-ios -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-IOS --check-prefix=CHECK
+; RUN: llc < %s -mtriple=thumbv7m-none-macho -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-DARWIN --check-prefix=CHECK
+; RUN: llc < %s -mtriple=arm-none-eabi -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-EABI --check-prefix=CHECK
+; RUN: llc < %s -mtriple=arm-none-eabihf -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-EABI --check-prefix=CHECK
+; RUN: llc < %s -mtriple=arm-none-androideabi -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-EABI --check-prefix=CHECK
+; RUN: llc < %s -mtriple=arm-none-gnueabi -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-GNUEABI --check-prefix=CHECK
+; RUN: llc < %s -mtriple=arm-none-gnueabihf -disable-post-ra -o - | FileCheck %s --check-prefix=CHECK-GNUEABI --check-prefix=CHECK
 
 define void @f1(i8* %dest, i8* %src) {
 entry:
@@ -402,8 +402,8 @@
 ; CHECK: arr1:
 ; CHECK-IOS: .align 3
 ; CHECK-DARWIN: .align 2
-; CHECK-EABI: .align 2
-; CHECK-GNUEABI: .align 2
+; CHECK-EABI-NOT: .align
+; CHECK-GNUEABI-NOT: .align
 ; CHECK: arr2:
 ; CHECK: {{\.section.+foo,bar}}
 ; CHECK-NOT: .align