Implement Function Multiversioning for Non-ELF Systems.

Similar to how ICC handles CPU-Dispatch on Windows, this patch uses the
resolver function directly to forward the call to the proper function.
This is not nearly as efficient as IFuncs of course, but is still quite
useful for large functions specifically developed for certain
processors.

This is unfortunately still limited to x86, since it depends on
__builtin_cpu_supports and __builtin_cpu_is, which are x86 builtins.

The naming for the resolver/forwarding function for cpu-dispatch was
taken from ICC's implementation, which uses the unmodified name for this
(no mangling additions).  This is possible, since cpu-dispatch uses '.A'
for the 'default' version.

In 'target' multiversioning, this function keeps the '.resolver'
extension in order to keep the default function keeping the default
mangling.

Change-Id: I4731555a39be26c7ad59a2d8fda6fa1a50f73284

Differential Revision: https://reviews.llvm.org/D53586

llvm-svn: 345298

1 files changed, 65 insertions, 34 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 9d3187bcc90..129a7703e6a 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -892,10 +892,11 @@ static std::string getCPUSpecificMangling(const CodeGenModule &CGM,
 static void AppendCPUSpecificCPUDispatchMangling(const CodeGenModule &CGM,
                                                  const CPUSpecificAttr *Attr,
                                                  raw_ostream &Out) {
-  // cpu_specific gets the current name, dispatch gets the resolver.
+  // cpu_specific gets the current name, dispatch gets the resolver if IFunc is
+  // supported.
   if (Attr)
     Out << getCPUSpecificMangling(CGM, Attr->getCurCPUName()->getName());
-  else
+  else if (CGM.getTarget().supportsIFunc())
     Out << ".resolver";
 }
 
@@ -2507,13 +2508,19 @@ void CodeGenModule::emitMultiVersionFunctions() {
                                TA->getArchitecture(), Feats);
         });
 
-    llvm::Function *ResolverFunc = cast<llvm::Function>(
-        GetGlobalValue((getMangledName(GD) + ".resolver").str()));
+    llvm::Function *ResolverFunc;
+    const TargetInfo &TI = getTarget();
+
+    if (TI.supportsIFunc() || FD->isTargetMultiVersion())
+      ResolverFunc = cast<llvm::Function>(
+          GetGlobalValue((getMangledName(GD) + ".resolver").str()));
+    else
+      ResolverFunc = cast<llvm::Function>(GetGlobalValue(getMangledName(GD)));
+
     if (supportsCOMDAT())
       ResolverFunc->setComdat(
           getModule().getOrInsertComdat(ResolverFunc->getName()));
 
-    const TargetInfo &TI = getTarget();
     std::stable_sort(
         Options.begin(), Options.end(),
         [&TI](const CodeGenFunction::MultiVersionResolverOption &LHS,
@@ -2533,13 +2540,21 @@ void CodeGenModule::emitCPUDispatchDefinition(GlobalDecl GD) {
   llvm::Type *DeclTy = getTypes().ConvertTypeForMem(FD->getType());
 
   StringRef ResolverName = getMangledName(GD);
-  llvm::Type *ResolverType = llvm::FunctionType::get(
-      llvm::PointerType::get(DeclTy,
-                             Context.getTargetAddressSpace(FD->getType())),
-      false);
-  auto *ResolverFunc = cast<llvm::Function>(
-      GetOrCreateLLVMFunction(ResolverName, ResolverType, GlobalDecl{},
-                              /*ForVTable=*/false));
+
+  llvm::Type *ResolverType;
+  GlobalDecl ResolverGD;
+  if (getTarget().supportsIFunc())
+    ResolverType = llvm::FunctionType::get(
+        llvm::PointerType::get(DeclTy,
+                               Context.getTargetAddressSpace(FD->getType())),
+        false);
+  else {
+    ResolverType = DeclTy;
+    ResolverGD = GD;
+  }
+
+  auto *ResolverFunc = cast<llvm::Function>(GetOrCreateLLVMFunction(
+      ResolverName, ResolverType, ResolverGD, /*ForVTable=*/false));
 
   SmallVector<CodeGenFunction::MultiVersionResolverOption, 10> Options;
   const TargetInfo &Target = getTarget();
@@ -2571,16 +2586,24 @@ void CodeGenModule::emitCPUDispatchDefinition(GlobalDecl GD) {
   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,
-                                            const FunctionDecl *FD) {
+/// If a dispatcher for the specified mangled name is not in the module, create
+/// and return an llvm Function with the specified type.
+llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(
+    GlobalDecl GD, llvm::Type *DeclTy, const FunctionDecl *FD) {
   std::string MangledName =
       getMangledNameImpl(*this, GD, FD, /*OmitMultiVersionMangling=*/true);
-  std::string IFuncName = MangledName + ".ifunc";
-  if (llvm::GlobalValue *IFuncGV = GetGlobalValue(IFuncName))
-    return IFuncGV;
+
+  // Holds the name of the resolver, in ifunc mode this is the ifunc (which has
+  // a separate resolver).
+  std::string ResolverName = MangledName;
+  if (getTarget().supportsIFunc())
+    ResolverName += ".ifunc";
+  else if (FD->isTargetMultiVersion())
+    ResolverName += ".resolver";
+
+  // If this already exists, just return that one.
+  if (llvm::GlobalValue *ResolverGV = GetGlobalValue(ResolverName))
+    return ResolverGV;
 
   // Since this is the first time we've created this IFunc, make sure
   // that we put this multiversioned function into the list to be
@@ -2588,20 +2611,28 @@ CodeGenModule::GetOrCreateMultiVersionIFunc(GlobalDecl GD, llvm::Type *DeclTy,
   if (!FD->isCPUDispatchMultiVersion() && !FD->isCPUSpecificMultiVersion())
     MultiVersionFuncs.push_back(GD);
 
-  std::string ResolverName = MangledName + ".resolver";
-  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);
+  if (getTarget().supportsIFunc()) {
+    llvm::Type *ResolverType = llvm::FunctionType::get(
+        llvm::PointerType::get(
+            DeclTy, getContext().getTargetAddressSpace(FD->getType())),
+        false);
+    llvm::Constant *Resolver = GetOrCreateLLVMFunction(
+        MangledName + ".resolver", ResolverType, GlobalDecl{},
+        /*ForVTable=*/false);
+    llvm::GlobalIFunc *GIF = llvm::GlobalIFunc::create(
+        DeclTy, 0, llvm::Function::ExternalLinkage, "", Resolver, &getModule());
+    GIF->setName(ResolverName);
+    SetCommonAttributes(FD, GIF);
+
+    return GIF;
+  }
 
-  return GIF;
+  llvm::Constant *Resolver = GetOrCreateLLVMFunction(
+      ResolverName, DeclTy, GlobalDecl{}, /*ForVTable=*/false);
+  assert(isa<llvm::GlobalValue>(Resolver) &&
+         "Resolver should be created for the first time");
+  SetCommonAttributes(FD, cast<llvm::GlobalValue>(Resolver));
+  return Resolver;
 }
 
 /// GetOrCreateLLVMFunction - If the specified mangled name is not in the
@@ -2641,7 +2672,7 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
       if (TA && TA->isDefaultVersion())
         UpdateMultiVersionNames(GD, FD);
       if (!IsForDefinition)
-        return GetOrCreateMultiVersionIFunc(GD, Ty, FD);
+        return GetOrCreateMultiVersionResolver(GD, Ty, FD);
     }
   }