Merging r257902 (and r257775)

[oota-llvm.git] / lib / IR / Globals.cpp

diff --git a/lib/IR/Globals.cpp b/lib/IR/Globals.cpp
index 6159f93faf89bdb368cf31f5b81b556f1456883c..a61b62bd9687e851a2afd24f714db6891d977b6a 100644 (file)
--- a/lib/IR/Globals.cpp
+++ b/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 @@ 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
 //===----------------------------------------------------------------------===//