From 699f5d6bbc43fbd62e60f913379565bd86c6df8c Mon Sep 17 00:00:00 2001
From: Rafael Espindola <rafael.espindola@gmail.com>
Date: Wed, 7 Feb 2018 22:15:33 +0000
Subject: Recommit r324107 again.

The difference from the previous try is that we no longer directly
access function declarations from position independent executables. It
should work, but currently doesn't with some linkers.

It now includes a fix to not mark available_externally definitions as
dso_local.

Original message:

Start setting dso_local in clang.

This starts adding dso_local to clang.

The hope is to eventually have TargetMachine::shouldAssumeDsoLocal go
away. My objective for now is to move enough of it to clang to remove
the need for the TargetMachine one to handle PIE copy relocations and
-fno-plt. With that it should then be easy to implement a
-fno-copy-reloc in clang.

This patch just adds the cases where we assume a symbol to be local
based on the file being compiled for an executable or a shared
library.

llvm-svn: 324535
---
 clang/lib/CodeGen/CodeGenModule.cpp | 67 ++++++++++++++++++++++++++++++++++---
 1 file changed, 62 insertions(+), 5 deletions(-)

(limited to 'clang/lib/CodeGen/CodeGenModule.cpp')

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index b3974b6de4b9..7fa4fddc79e1 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -715,6 +715,63 @@ void CodeGenModule::setGlobalVisibility(llvm::GlobalValue *GV,
     GV->setVisibility(GetLLVMVisibility(LV.getVisibility()));
 }
 
+static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
+                                 llvm::GlobalValue *GV, const NamedDecl *D) {
+  const llvm::Triple &TT = CGM.getTriple();
+  // Only handle ELF for now.
+  if (!TT.isOSBinFormatELF())
+    return false;
+
+  // If this is not an executable, don't assume anything is local.
+  const auto &CGOpts = CGM.getCodeGenOpts();
+  llvm::Reloc::Model RM = CGOpts.RelocationModel;
+  const auto &LOpts = CGM.getLangOpts();
+  if (RM != llvm::Reloc::Static && !LOpts.PIE)
+    return false;
+
+  // A definition cannot be preempted from an executable.
+  if (!GV->isDeclarationForLinker())
+    return true;
+
+  // Most PIC code sequences that assume that a symbol is local cannot produce a
+  // 0 if it turns out the symbol is undefined. While this is ABI and relocation
+  // depended, it seems worth it to handle it here.
+  if (RM == llvm::Reloc::PIC_ && GV->hasExternalWeakLinkage())
+    return false;
+
+  // PPC has no copy relocations and cannot use a plt entry as a symbol address.
+  llvm::Triple::ArchType Arch = TT.getArch();
+  if (Arch == llvm::Triple::ppc || Arch == llvm::Triple::ppc64 ||
+      Arch == llvm::Triple::ppc64le)
+    return false;
+
+  // If we can use copy relocations we can assume it is local.
+  if (isa<VarDecl>(D) &&
+      (RM == llvm::Reloc::Static || CGOpts.PIECopyRelocations))
+    return true;
+
+  // If we can use a plt entry as the symbol address we can assume it
+  // is local.
+  // FIXME: This should work for PIE, but the gold linker doesn't support it.
+  if (isa<FunctionDecl>(D) && !CGOpts.NoPLT && RM == llvm::Reloc::Static)
+    return true;
+
+  // Otherwise don't assue it is local.
+  return false;
+}
+
+void CodeGenModule::setDSOLocal(llvm::GlobalValue *GV,
+                                const NamedDecl *D) const {
+  if (shouldAssumeDSOLocal(*this, GV, D))
+    GV->setDSOLocal(true);
+}
+
+void CodeGenModule::setGVProperties(llvm::GlobalValue *GV,
+                                    const NamedDecl *D) const {
+  setGlobalVisibility(GV, D);
+  setDSOLocal(GV, D);
+}
+
 static llvm::GlobalVariable::ThreadLocalMode GetLLVMTLSModel(StringRef S) {
   return llvm::StringSwitch<llvm::GlobalVariable::ThreadLocalMode>(S)
       .Case("global-dynamic", llvm::GlobalVariable::GeneralDynamicTLSModel)
@@ -1172,7 +1229,7 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
 void CodeGenModule::SetCommonAttributes(const Decl *D,
                                         llvm::GlobalValue *GV) {
   if (const auto *ND = dyn_cast_or_null<NamedDecl>(D))
-    setGlobalVisibility(GV, ND);
+    setGVProperties(GV, ND);
   else
     GV->setVisibility(llvm::GlobalValue::DefaultVisibility);
 
@@ -1312,7 +1369,7 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
   // overridden by a definition.
 
   setLinkageForGV(F, FD);
-  setGlobalVisibility(F, FD);
+  setGVProperties(F, FD);
 
   if (FD->getAttr<PragmaClangTextSectionAttr>()) {
     F->addFnAttr("implicit-section-name");
@@ -2639,7 +2696,7 @@ CodeGenModule::GetOrCreateLLVMGlobal(StringRef MangledName,
     GV->setAlignment(getContext().getDeclAlign(D).getQuantity());
 
     setLinkageForGV(GV, D);
-    setGlobalVisibility(GV, D);
+    setGVProperties(GV, D);
 
     if (D->getTLSKind()) {
       if (D->getTLSKind() == VarDecl::TLS_Dynamic)
@@ -3458,7 +3515,7 @@ void CodeGenModule::EmitGlobalFunctionDefinition(GlobalDecl GD,
   setFunctionDLLStorageClass(GD, Fn);
 
   // FIXME: this is redundant with part of setFunctionDefinitionAttributes
-  setGlobalVisibility(Fn, D);
+  setGVProperties(Fn, D);
 
   MaybeHandleStaticInExternC(D, Fn);
 
@@ -4054,7 +4111,7 @@ ConstantAddress CodeGenModule::GetAddrOfGlobalTemporary(
       getModule(), Type, Constant, Linkage, InitialValue, Name.c_str(),
       /*InsertBefore=*/nullptr, llvm::GlobalVariable::NotThreadLocal, TargetAS);
   if (emitter) emitter->finalize(GV);
-  setGlobalVisibility(GV, VD);
+  setGVProperties(GV, VD);
   GV->setAlignment(Align.getQuantity());
   if (supportsCOMDAT() && GV->isWeakForLinker())
     GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
-- 
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