Implement Attribute Target MultiVersioning

GCC's attribute 'target', in addition to being an optimization hint,
also allows function multiversioning. We currently have the former
implemented, this is the latter's implementation.

This works by enabling functions with the same name/signature to coexist,
so that they can all be emitted. Multiversion state is stored in the
FunctionDecl itself, and SemaDecl manages the definitions.
Note that it ends up having to permit redefinition of functions so
that they can all be emitted. Additionally, all versions of the function
must be emitted, so this also manages that.

Note that this includes some additional rules that GCC does not, since
defining something as a MultiVersion function after a usage has been made illegal.

The only 'history rewriting' that happens is if a function is emitted before
it has been converted to a multiversion'ed function, at which point its name
needs to be changed.

Function templates and virtual functions are NOT yet supported (not supported
in GCC either).

Additionally, constructors/destructors are disallowed, but the former is 
planned.

llvm-svn: 322028

1 files changed, 175 insertions, 24 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index df33fc3e820..85bd53741e1 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -395,6 +395,7 @@ void CodeGenModule::Release() {
   applyGlobalValReplacements();
   applyReplacements();
   checkAliases();
+  emitMultiVersionFunctions();
   EmitCXXGlobalInitFunc();
   EmitCXXGlobalDtorFunc();
   EmitCXXThreadLocalInitFunc();
@@ -725,36 +726,48 @@ void CodeGenModule::setTLSMode(llvm::GlobalValue *GV, const VarDecl &D) const {
   GV->setThreadLocalMode(TLM);
 }
 
-StringRef CodeGenModule::getMangledName(GlobalDecl GD) {
-  GlobalDecl CanonicalGD = GD.getCanonicalDecl();
+static void AppendTargetMangling(const CodeGenModule &CGM,
+                                 const TargetAttr *Attr, raw_ostream &Out) {
+  if (Attr->isDefaultVersion())
+    return;
 
-  // Some ABIs don't have constructor variants.  Make sure that base and
-  // complete constructors get mangled the same.
-  if (const auto *CD = dyn_cast<CXXConstructorDecl>(CanonicalGD.getDecl())) {
-    if (!getTarget().getCXXABI().hasConstructorVariants()) {
-      CXXCtorType OrigCtorType = GD.getCtorType();
-      assert(OrigCtorType == Ctor_Base || OrigCtorType == Ctor_Complete);
-      if (OrigCtorType == Ctor_Base)
-        CanonicalGD = GlobalDecl(CD, Ctor_Complete);
-    }
+  Out << '.';
+  const auto &Target = CGM.getTarget();
+  TargetAttr::ParsedTargetAttr Info =
+      Attr->parse([&Target](StringRef LHS, StringRef RHS) {
+                    return Target.multiVersionSortPriority(LHS) >
+                           Target.multiVersionSortPriority(RHS);
+                  });
+
+  bool IsFirst = true;
+
+  if (!Info.Architecture.empty()) {
+    IsFirst = false;
+    Out << "arch_" << Info.Architecture;
   }
 
-  auto FoundName = MangledDeclNames.find(CanonicalGD);
-  if (FoundName != MangledDeclNames.end())
-    return FoundName->second;
+  for (StringRef Feat : Info.Features) {
+    if (!IsFirst)
+      Out << '_';
+    IsFirst = false;
+    Out << Feat.substr(1);
+  }
+}
 
-  const auto *ND = cast<NamedDecl>(GD.getDecl());
+static std::string getMangledNameImpl(const CodeGenModule &CGM, GlobalDecl GD,
+                                      const NamedDecl *ND,
+                                      bool OmitTargetMangling = false) {
   SmallString<256> Buffer;
-  StringRef Str;
-  if (getCXXABI().getMangleContext().shouldMangleDeclName(ND)) {
+  llvm::raw_svector_ostream Out(Buffer);
+  MangleContext &MC = CGM.getCXXABI().getMangleContext();
+  if (MC.shouldMangleDeclName(ND)) {
     llvm::raw_svector_ostream Out(Buffer);
     if (const auto *D = dyn_cast<CXXConstructorDecl>(ND))
-      getCXXABI().getMangleContext().mangleCXXCtor(D, GD.getCtorType(), Out);
+      MC.mangleCXXCtor(D, GD.getCtorType(), Out);
     else if (const auto *D = dyn_cast<CXXDestructorDecl>(ND))
-      getCXXABI().getMangleContext().mangleCXXDtor(D, GD.getDtorType(), Out);
+      MC.mangleCXXDtor(D, GD.getDtorType(), Out);
     else
-      getCXXABI().getMangleContext().mangleName(ND, Out);
-    Str = Out.str();
+      MC.mangleName(ND, Out);
   } else {
     IdentifierInfo *II = ND->getIdentifier();
     assert(II && "Attempt to mangle unnamed decl.");
@@ -764,14 +777,74 @@ StringRef CodeGenModule::getMangledName(GlobalDecl GD) {
         FD->getType()->castAs<FunctionType>()->getCallConv() == CC_X86RegCall) {
       llvm::raw_svector_ostream Out(Buffer);
       Out << "__regcall3__" << II->getName();
-      Str = Out.str();
     } else {
-      Str = II->getName();
+      Out << II->getName();
+    }
+  }
+
+  if (const auto *FD = dyn_cast<FunctionDecl>(ND))
+    if (FD->isMultiVersion() && !OmitTargetMangling)
+      AppendTargetMangling(CGM, FD->getAttr<TargetAttr>(), Out);
+  return Out.str();
+}
+
+void CodeGenModule::UpdateMultiVersionNames(GlobalDecl GD,
+                                            const FunctionDecl *FD) {
+  if (!FD->isMultiVersion())
+    return;
+
+  // Get the name of what this would be without the 'target' attribute.  This
+  // allows us to lookup the version that was emitted when this wasn't a
+  // multiversion function.
+  std::string NonTargetName =
+      getMangledNameImpl(*this, GD, FD, /*OmitTargetMangling=*/true);
+  GlobalDecl OtherGD;
+  if (lookupRepresentativeDecl(NonTargetName, OtherGD)) {
+    assert(OtherGD.getCanonicalDecl()
+               .getDecl()
+               ->getAsFunction()
+               ->isMultiVersion() &&
+           "Other GD should now be a multiversioned function");
+    // OtherFD is the version of this function that was mangled BEFORE
+    // becoming a MultiVersion function.  It potentially needs to be updated.
+    const FunctionDecl *OtherFD =
+        OtherGD.getCanonicalDecl().getDecl()->getAsFunction();
+    std::string OtherName = getMangledNameImpl(*this, OtherGD, OtherFD);
+    // This is so that if the initial version was already the 'default'
+    // version, we don't try to update it.
+    if (OtherName != NonTargetName) {
+      Manglings.erase(NonTargetName);
+      auto Result = Manglings.insert(std::make_pair(OtherName, OtherGD));
+      MangledDeclNames[OtherGD.getCanonicalDecl()] = Result.first->first();
+      if (llvm::GlobalValue *Entry = GetGlobalValue(NonTargetName))
+        Entry->setName(OtherName);
     }
   }
+}
+
+StringRef CodeGenModule::getMangledName(GlobalDecl GD) {
+  GlobalDecl CanonicalGD = GD.getCanonicalDecl();
+
+  // Some ABIs don't have constructor variants.  Make sure that base and
+  // complete constructors get mangled the same.
+  if (const auto *CD = dyn_cast<CXXConstructorDecl>(CanonicalGD.getDecl())) {
+    if (!getTarget().getCXXABI().hasConstructorVariants()) {
+      CXXCtorType OrigCtorType = GD.getCtorType();
+      assert(OrigCtorType == Ctor_Base || OrigCtorType == Ctor_Complete);
+      if (OrigCtorType == Ctor_Base)
+        CanonicalGD = GlobalDecl(CD, Ctor_Complete);
+    }
+  }
+
+  auto FoundName = MangledDeclNames.find(CanonicalGD);
+  if (FoundName != MangledDeclNames.end())
+    return FoundName->second;
+
 
   // Keep the first result in the case of a mangling collision.
-  auto Result = Manglings.insert(std::make_pair(Str, GD));
+  const auto *ND = cast<NamedDecl>(GD.getDecl());
+  auto Result =
+      Manglings.insert(std::make_pair(getMangledNameImpl(*this, GD, ND), GD));
   return MangledDeclNames[CanonicalGD] = Result.first->first();
 }
 
@@ -2061,6 +2134,74 @@ void CodeGenModule::EmitGlobalDefinition(GlobalDecl GD, llvm::GlobalValue *GV) {
 static void ReplaceUsesOfNonProtoTypeWithRealFunction(llvm::GlobalValue *Old,
                                                       llvm::Function *NewFn);
 
+void CodeGenModule::emitMultiVersionFunctions() {
+  for (GlobalDecl GD : MultiVersionFuncs) {
+    SmallVector<CodeGenFunction::MultiVersionResolverOption, 10> Options;
+    const FunctionDecl *FD = cast<FunctionDecl>(GD.getDecl());
+    getContext().forEachMultiversionedFunctionVersion(
+        FD, [this, &GD, &Options](const FunctionDecl *CurFD) {
+          GlobalDecl CurGD{
+              (CurFD->isDefined() ? CurFD->getDefinition() : CurFD)};
+          StringRef MangledName = getMangledName(CurGD);
+          llvm::Constant *Func = GetGlobalValue(MangledName);
+          if (!Func) {
+            if (CurFD->isDefined()) {
+              EmitGlobalFunctionDefinition(CurGD, nullptr);
+              Func = GetGlobalValue(MangledName);
+            } else {
+              const CGFunctionInfo &FI =
+                  getTypes().arrangeGlobalDeclaration(GD);
+              llvm::FunctionType *Ty = getTypes().GetFunctionType(FI);
+              Func = GetAddrOfFunction(CurGD, Ty, /*ForVTable=*/false,
+                                       /*DontDefer=*/false, ForDefinition);
+            }
+            assert(Func && "This should have just been created");
+          }
+          Options.emplace_back(getTarget(), cast<llvm::Function>(Func),
+                               CurFD->getAttr<TargetAttr>()->parse());
+        });
+
+    llvm::Function *ResolverFunc = cast<llvm::Function>(
+        GetGlobalValue((getMangledName(GD) + ".resolver").str()));
+    std::stable_sort(
+        Options.begin(), Options.end(),
+        std::greater<CodeGenFunction::MultiVersionResolverOption>());
+    CodeGenFunction CGF(*this);
+    CGF.EmitMultiVersionResolver(ResolverFunc, Options);
+  }
+}
+
+/// If an ifunc for the specified mangled name is not in the module, create and
+/// return an llvm IFunc Function with the specified type.
+llvm::Constant *
+CodeGenModule::GetOrCreateMultiVersionIFunc(GlobalDecl GD, llvm::Type *DeclTy,
+                                            StringRef MangledName,
+                                            const FunctionDecl *FD) {
+  std::string IFuncName = (MangledName + ".ifunc").str();
+  if (llvm::GlobalValue *IFuncGV = GetGlobalValue(IFuncName))
+    return IFuncGV;
+
+  // Since this is the first time we've created this IFunc, make sure
+  // that we put this multiversioned function into the list to be
+  // replaced later.
+  MultiVersionFuncs.push_back(GD);
+
+  std::string ResolverName = (MangledName + ".resolver").str();
+  llvm::Type *ResolverType = llvm::FunctionType::get(
+      llvm::PointerType::get(DeclTy,
+                             Context.getTargetAddressSpace(FD->getType())),
+      false);
+  llvm::Constant *Resolver =
+      GetOrCreateLLVMFunction(ResolverName, ResolverType, GlobalDecl{},
+                              /*ForVTable=*/false);
+  llvm::GlobalIFunc *GIF = llvm::GlobalIFunc::create(
+      DeclTy, 0, llvm::Function::ExternalLinkage, "", Resolver, &getModule());
+  GIF->setName(IFuncName);
+  SetCommonAttributes(FD, GIF);
+
+  return GIF;
+}
+
 /// GetOrCreateLLVMFunction - If the specified mangled name is not in the
 /// module, create and return an llvm Function with the specified type. If there
 /// is something in the module with the specified name, return it potentially
@@ -2074,6 +2215,16 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
     ForDefinition_t IsForDefinition) {
   const Decl *D = GD.getDecl();
 
+  // Any attempts to use a MultiVersion function should result in retrieving
+  // the iFunc instead. Name Mangling will handle the rest of the changes.
+  if (const FunctionDecl *FD = cast_or_null<FunctionDecl>(D)) {
+    if (FD->isMultiVersion() && FD->getAttr<TargetAttr>()->isDefaultVersion()) {
+      UpdateMultiVersionNames(GD, FD);
+      if (!IsForDefinition)
+        return GetOrCreateMultiVersionIFunc(GD, Ty, MangledName, FD);
+    }
+  }
+
   // Lookup the entry, lazily creating it if necessary.
   llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
   if (Entry) {