PPC64: Constant initializers with dynamic relocations go in .data.rel.ro.

This fixes warning messages observed in the oggenc application test in
projects/test-suite.  Special handling is needed for the 64-bit
PowerPC SVR4 ABI when a constant is initialized with a pointer to a
function in a shared library.  Because a function address is
implemented as the address of a function descriptor, the use of copy
relocations can lead to problems with initialization.  GNU ld
therefore replaces copy relocations with dynamic relocations to be
resolved by the dynamic linker.  This means the constant cannot reside
in the read-only data section, but instead belongs in .data.rel.ro,
which is designed for constants containing dynamic relocations.

The implementation creates a class PPC64LinuxTargetObjectFile
inheriting from TargetLoweringObjectFileELF, which behaves like its
parent except to place constants of this sort into .data.rel.ro.

The test case is reduced from the oggenc application.

llvm-svn: 181723

1 files changed, 57 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCTargetObjectFile.cpp b/llvm/lib/Target/PowerPC/PPCTargetObjectFile.cpp
new file mode 100644
index 0000000..b374b59
--- /dev/null
+++ b/llvm/lib/Target/PowerPC/PPCTargetObjectFile.cpp
@@ -0,0 +1,57 @@
+//===-- PPCTargetObjectFile.cpp - PPC Object Info -------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PPCTargetObjectFile.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCSectionELF.h"
+#include "llvm/Target/Mangler.h"
+
+using namespace llvm;
+
+void
+PPC64LinuxTargetObjectFile::
+Initialize(MCContext &Ctx, const TargetMachine &TM) {
+  TargetLoweringObjectFileELF::Initialize(Ctx, TM);
+  InitializeELF(TM.Options.UseInitArray);
+}
+
+const MCSection * PPC64LinuxTargetObjectFile::
+SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
+                       Mangler *Mang, const TargetMachine &TM) const {
+
+  const MCSection *DefaultSection = 
+    TargetLoweringObjectFileELF::SelectSectionForGlobal(GV, Kind, Mang, TM);
+
+  if (DefaultSection != ReadOnlySection)
+    return DefaultSection;
+
+  // Here override isReadOnly() to isReadOnlyWithRel() for PPC64 SVR4 ABI
+  // when we have a constant that contains global relocations.  This is
+  // necessary because of this ABI's handling of pointers to functions in
+  // a shared library.  The address of a function is actually the address
+  // of a function descriptor, which resides in the .opd section.  Generated
+  // code uses the descriptor directly rather than going via the GOT as some
+  // other ABIs do, which means that initialized function pointers must
+  // reference the descriptor.  The linker must convert copy relocs of
+  // pointers to functions in shared libraries into dynamic relocations,
+  // because of an ordering problem with initialization of copy relocs and
+  // PLT entries.  The dynamic relocation will be initialized by the dynamic
+  // linker, so we must use the DataRelRO section instead of ReadOnlySection.
+  // For more information, see the description of ELIMINATE_COPY_RELOCS in
+  // GNU ld.
+  const GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV);
+
+  if (GVar && GVar->isConstant() &&
+      (GVar->getInitializer()->getRelocationInfo() ==
+       Constant::GlobalRelocations))
+    return DataRelROSection;
+
+  return DefaultSection;
+}