[llvm] r257902 - Stop increasing alignment of externally-visible globals on ELF
James Y Knight via llvm-commits
llvm-commits at lists.llvm.org
Fri Jan 15 08:33:07 PST 2016
Author: jyknight
Date: Fri Jan 15 10:33:06 2016
New Revision: 257902
URL: http://llvm.org/viewvc/llvm-project?rev=257902&view=rev
Log:
Stop increasing alignment of externally-visible globals on ELF
platforms.
With ELF, the alignment of a global variable in a shared library will
get copied into an executables linked against it, if the executable even
accesss the variable. So, it's not possible to implicitly increase
alignment based on access patterns, or you'll break existing binaries.
This happened to affect libc++'s std::cout symbol, for example. See
thread: http://thread.gmane.org/gmane.comp.compilers.clang.devel/45311
(This is a re-commit of r257719, without the bug reported in
PR26144. I've tweaked the code to not assert-fail in
enforceKnownAlignment when computeKnownBits doesn't recurse far enough
to find the underlying Alloca/GlobalObject value.)
Differential Revision: http://reviews.llvm.org/D16145
Modified:
llvm/trunk/include/llvm/IR/GlobalValue.h
llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
llvm/trunk/lib/IR/Globals.cpp
llvm/trunk/lib/Transforms/Utils/Local.cpp
llvm/trunk/test/CodeGen/ARM/memfunc.ll
Modified: llvm/trunk/include/llvm/IR/GlobalValue.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/GlobalValue.h?rev=257902&r1=257901&r2=257902&view=diff
==============================================================================
--- llvm/trunk/include/llvm/IR/GlobalValue.h (original)
+++ llvm/trunk/include/llvm/IR/GlobalValue.h Fri Jan 15 10:33:06 2016
@@ -346,6 +346,10 @@ public:
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;
Modified: llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp?rev=257902&r1=257901&r2=257902&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp (original)
+++ llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp Fri Jan 15 10:33:06 2016
@@ -1742,8 +1742,8 @@ bool CodeGenPrepare::optimizeCallInst(Ca
// 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);
Modified: llvm/trunk/lib/IR/Globals.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Globals.cpp?rev=257902&r1=257901&r2=257902&view=diff
==============================================================================
--- llvm/trunk/lib/IR/Globals.cpp (original)
+++ llvm/trunk/lib/IR/Globals.cpp Fri Jan 15 10:33:06 2016
@@ -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 @@ bool GlobalValue::isDeclaration() const
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
//===----------------------------------------------------------------------===//
Modified: llvm/trunk/lib/Transforms/Utils/Local.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/Local.cpp?rev=257902&r1=257901&r2=257902&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/Local.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/Local.cpp Fri Jan 15 10:33:06 2016
@@ -944,37 +944,44 @@ bool llvm::EliminateDuplicatePHINodes(Ba
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;
Modified: llvm/trunk/test/CodeGen/ARM/memfunc.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/ARM/memfunc.ll?rev=257902&r1=257901&r2=257902&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/ARM/memfunc.ll (original)
+++ llvm/trunk/test/CodeGen/ARM/memfunc.ll Fri Jan 15 10:33:06 2016
@@ -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 @@ entry:
; 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
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