[FMV][AArch64] Do not emit ifunc resolver on use. (llvm#97761)

It was raised in llvm#81494 that
we are not generating correct code when there is no TU-local caller.

The suggestion was to emit a resolver:
* Whenever there is a use in the TU.
* When the TU has a definition of the default version.

See the comment for more details:

llvm#81494 (comment)

This got addressed with llvm#84405.

Generating a resolver on use means that we may end up with multiple
resolvers across different translation units. Those resolvers may not be
the same because each translation unit may contain different version
declarations (user's fault). Therefore the order of linking the final
image determines which of these weak symbols gets selected, resulting in
non consisted behavior. I am proposing to stop emitting a resolver on
use and only do so in the translation unit which contains the default
definition. This way we guarantee the existence of a single resolver.
Now, when a versioned function is used we want to emit a declaration of
the function symbol omitting the multiversion mangling.

I have added a requirement to ACLE mandating that all the function
versions are declared in the translation unit which contains the default
definition: ARM-software/acle#328

Author: labrinea

clang/lib/CodeGen/CodeGenModule.cpp

@@ -3796,8 +3796,7 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
     // Forward declarations are emitted lazily on first use.
     if (!FD->doesThisDeclarationHaveABody()) {
       if (!FD->doesDeclarationForceExternallyVisibleDefinition() &&
-          (!FD->isMultiVersion() ||
-           !FD->getASTContext().getTargetInfo().getTriple().isAArch64()))
+          (!FD->isMultiVersion() || !getTarget().getTriple().isAArch64()))
         return;
 
       StringRef MangledName = getMangledName(GD);
@@ -4191,23 +4190,6 @@ llvm::GlobalValue::LinkageTypes getMultiversionLinkage(CodeGenModule &CGM,
   return llvm::GlobalValue::WeakODRLinkage;
 }
 
-static FunctionDecl *createDefaultTargetVersionFrom(const FunctionDecl *FD) {
-  auto *DeclCtx = const_cast<DeclContext *>(FD->getDeclContext());
-  TypeSourceInfo *TInfo = FD->getTypeSourceInfo();
-  StorageClass SC = FD->getStorageClass();
-  DeclarationName Name = FD->getNameInfo().getName();
-
-  FunctionDecl *NewDecl =
-      FunctionDecl::Create(FD->getASTContext(), DeclCtx, FD->getBeginLoc(),
-                           FD->getEndLoc(), Name, TInfo->getType(), TInfo, SC);
-
-  NewDecl->setIsMultiVersion();
-  NewDecl->addAttr(TargetVersionAttr::CreateImplicit(
-      NewDecl->getASTContext(), "default", NewDecl->getSourceRange()));
-
-  return NewDecl;
-}
-
 void CodeGenModule::emitMultiVersionFunctions() {
   std::vector<GlobalDecl> MVFuncsToEmit;
   MultiVersionFuncs.swap(MVFuncsToEmit);
@@ -4234,29 +4216,30 @@ void CodeGenModule::emitMultiVersionFunctions() {
       return cast<llvm::Function>(Func);
     };
 
-    bool HasDefaultDecl = !FD->isTargetVersionMultiVersion();
-    bool ShouldEmitResolver =
-        !getContext().getTargetInfo().getTriple().isAArch64();
+    // For AArch64, a resolver is only emitted if a function marked with
+    // target_version("default")) or target_clones() is present and defined
+    // in this TU. For other architectures it is always emitted.
+    bool ShouldEmitResolver = !getTarget().getTriple().isAArch64();
     SmallVector<CodeGenFunction::MultiVersionResolverOption, 10> Options;
 
     getContext().forEachMultiversionedFunctionVersion(
         FD, [&](const FunctionDecl *CurFD) {
           llvm::SmallVector<StringRef, 8> Feats;
+          bool IsDefined = CurFD->doesThisDeclarationHaveABody();
 
           if (const auto *TA = CurFD->getAttr<TargetAttr>()) {
             TA->getAddedFeatures(Feats);
             llvm::Function *Func = createFunction(CurFD);
             Options.emplace_back(Func, TA->getArchitecture(), Feats);
           } else if (const auto *TVA = CurFD->getAttr<TargetVersionAttr>()) {
-            bool HasDefaultDef = TVA->isDefaultVersion() &&
-                                 CurFD->doesThisDeclarationHaveABody();
-            HasDefaultDecl |= TVA->isDefaultVersion();
-            ShouldEmitResolver |= (CurFD->isUsed() || HasDefaultDef);
+            if (TVA->isDefaultVersion() && IsDefined)
+              ShouldEmitResolver = true;
             TVA->getFeatures(Feats);
             llvm::Function *Func = createFunction(CurFD);
             Options.emplace_back(Func, /*Architecture*/ "", Feats);
           } else if (const auto *TC = CurFD->getAttr<TargetClonesAttr>()) {
-            ShouldEmitResolver |= CurFD->doesThisDeclarationHaveABody();
+            if (IsDefined)
+              ShouldEmitResolver = true;
             for (unsigned I = 0; I < TC->featuresStrs_size(); ++I) {
               if (!TC->isFirstOfVersion(I))
                 continue;
@@ -4282,13 +4265,6 @@ void CodeGenModule::emitMultiVersionFunctions() {
     if (!ShouldEmitResolver)
       continue;
 
-    if (!HasDefaultDecl) {
-      FunctionDecl *NewFD = createDefaultTargetVersionFrom(FD);
-      llvm::Function *Func = createFunction(NewFD);
-      llvm::SmallVector<StringRef, 1> Feats;
-      Options.emplace_back(Func, /*Architecture*/ "", Feats);
-    }
-
     llvm::Constant *ResolverConstant = GetOrCreateMultiVersionResolver(GD);
     if (auto *IFunc = dyn_cast<llvm::GlobalIFunc>(ResolverConstant)) {
       ResolverConstant = IFunc->getResolver();
@@ -4339,6 +4315,14 @@ void CodeGenModule::emitMultiVersionFunctions() {
     emitMultiVersionFunctions();
 }
 
+static void replaceDeclarationWith(llvm::GlobalValue *Old,
+                                   llvm::Constant *New) {
+  assert(cast<llvm::Function>(Old)->isDeclaration() && "Not a declaration");
+  New->takeName(Old);
+  Old->replaceAllUsesWith(New);
+  Old->eraseFromParent();
+}
+
 void CodeGenModule::emitCPUDispatchDefinition(GlobalDecl GD) {
   const auto *FD = cast<FunctionDecl>(GD.getDecl());
   assert(FD && "Not a FunctionDecl?");
@@ -4443,12 +4427,9 @@ void CodeGenModule::emitCPUDispatchDefinition(GlobalDecl GD) {
     // Fix up function declarations that were created for cpu_specific before
     // cpu_dispatch was known
     if (!isa<llvm::GlobalIFunc>(IFunc)) {
-      assert(cast<llvm::Function>(IFunc)->isDeclaration());
       auto *GI = llvm::GlobalIFunc::create(DeclTy, 0, Linkage, "", ResolverFunc,
                                            &getModule());
-      GI->takeName(IFunc);
-      IFunc->replaceAllUsesWith(GI);
-      IFunc->eraseFromParent();
+      replaceDeclarationWith(IFunc, GI);
       IFunc = GI;
     }
 
@@ -4478,7 +4459,8 @@ void CodeGenModule::AddDeferredMultiVersionResolverToEmit(GlobalDecl GD) {
 }
 
 /// If a dispatcher for the specified mangled name is not in the module, create
-/// and return an llvm Function with the specified type.
+/// and return it. The dispatcher is either an llvm Function with the specified
+/// type, or a global ifunc.
 llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
   const auto *FD = cast<FunctionDecl>(GD.getDecl());
   assert(FD && "Not a FunctionDecl?");
@@ -4506,8 +4488,15 @@ llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
     ResolverName += ".resolver";
   }
 
-  // If the resolver has already been created, just return it.
-  if (llvm::GlobalValue *ResolverGV = GetGlobalValue(ResolverName))
+  // If the resolver has already been created, just return it. This lookup may
+  // yield a function declaration instead of a resolver on AArch64. That is
+  // because we didn't know whether a resolver will be generated when we first
+  // encountered a use of the symbol named after this resolver. Therefore,
+  // targets which support ifuncs should not return here unless we actually
+  // found an ifunc.
+  llvm::GlobalValue *ResolverGV = GetGlobalValue(ResolverName);
+  if (ResolverGV &&
+      (isa<llvm::GlobalIFunc>(ResolverGV) || !getTarget().supportsIFunc()))
     return ResolverGV;
 
   const CGFunctionInfo &FI = getTypes().arrangeGlobalDeclaration(GD);
@@ -4533,7 +4522,8 @@ llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
                                   "", Resolver, &getModule());
     GIF->setName(ResolverName);
     SetCommonAttributes(FD, GIF);
-
+    if (ResolverGV)
+      replaceDeclarationWith(ResolverGV, GIF);
     return GIF;
   }
 
@@ -4542,6 +4532,8 @@ llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
   assert(isa<llvm::GlobalValue>(Resolver) &&
          "Resolver should be created for the first time");
   SetCommonAttributes(FD, cast<llvm::GlobalValue>(Resolver));
+  if (ResolverGV)
+    replaceDeclarationWith(ResolverGV, Resolver);
   return Resolver;
 }
 
@@ -4571,6 +4563,7 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
     ForDefinition_t IsForDefinition) {
   const Decl *D = GD.getDecl();
 
+  std::string NameWithoutMultiVersionMangling;
   // 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)) {
@@ -4592,14 +4585,24 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
 
     if (FD->isMultiVersion()) {
       UpdateMultiVersionNames(GD, FD, MangledName);
-      if (FD->getASTContext().getTargetInfo().getTriple().isAArch64() &&
-          !FD->isUsed())
-        AddDeferredMultiVersionResolverToEmit(GD);
-      else if (!IsForDefinition)
-        return GetOrCreateMultiVersionResolver(GD);
+      if (!IsForDefinition) {
+        // On AArch64 we do not immediatelly emit an ifunc resolver when a
+        // function is used. Instead we defer the emission until we see a
+        // default definition. In the meantime we just reference the symbol
+        // without FMV mangling (it may or may not be replaced later).
+        if (getTarget().getTriple().isAArch64()) {
+          AddDeferredMultiVersionResolverToEmit(GD);
+          NameWithoutMultiVersionMangling = getMangledNameImpl(
+              *this, GD, FD, /*OmitMultiVersionMangling=*/true);
+        } else
+          return GetOrCreateMultiVersionResolver(GD);
+      }
     }
   }
 
+  if (!NameWithoutMultiVersionMangling.empty())
+    MangledName = NameWithoutMultiVersionMangling;
+
   // Lookup the entry, lazily creating it if necessary.
   llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
   if (Entry) {

clang/test/CodeGen/aarch64-fmv-resolver-emission.c

@@ -0,0 +1,111 @@
+// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -emit-llvm -o - %s | FileCheck %s
+
+// CHECK: @used_before_default_def = weak_odr ifunc void (), ptr @used_before_default_def.resolver
+// CHECK: @used_after_default_def = weak_odr ifunc void (), ptr @used_after_default_def.resolver
+// CHECK-NOT: @used_before_default_decl = weak_odr ifunc void (), ptr @used_before_default_decl.resolver
+// CHECK-NOT: @used_after_default_decl = weak_odr ifunc void (), ptr @used_after_default_decl.resolver
+// CHECK-NOT: @used_no_default = weak_odr ifunc void (), ptr @used_no_default.resolver
+// CHECK-NOT: @not_used_no_default = weak_odr ifunc void (), ptr @not_used_no_default.resolver
+// CHECK: @not_used_with_default = weak_odr ifunc void (), ptr @not_used_with_default.resolver
+
+
+// Test that an ifunc is generated and used when the default
+// version is defined after the first use of the function.
+//
+__attribute__((target_version("aes"))) void used_before_default_def(void) {}
+// CHECK-LABEL: define dso_local void @used_before_default_def._Maes(
+//
+void call_before_def(void) { used_before_default_def(); }
+// CHECK-LABEL: define dso_local void @call_before_def(
+// CHECK: call void @used_before_default_def()
+//
+__attribute__((target_version("default"))) void used_before_default_def(void) {}
+// CHECK-LABEL: define dso_local void @used_before_default_def.default(
+//
+// CHECK-NOT: declare void @used_before_default_def(
+
+
+// Test that an ifunc is generated and used when the default
+// version is defined before the first use of the function.
+//
+__attribute__((target_version("aes"))) void used_after_default_def(void) {}
+// CHECK-LABEL: define dso_local void @used_after_default_def._Maes(
+//
+__attribute__((target_version("default"))) void used_after_default_def(void) {}
+// CHECK-LABEL: define dso_local void @used_after_default_def.default(
+//
+void call_after_def(void) { used_after_default_def(); }
+// CHECK-LABEL: define dso_local void @call_after_def(
+// CHECK: call void @used_after_default_def()
+//
+// CHECK-NOT: declare void @used_after_default_def(
+
+
+// Test that an unmagled declaration is generated and used when the
+// default version is declared after the first use of the function.
+//
+__attribute__((target_version("aes"))) void used_before_default_decl(void) {}
+// CHECK-LABEL: define dso_local void @used_before_default_decl._Maes(
+//
+void call_before_decl(void) { used_before_default_decl(); }
+// CHECK-LABEL: define dso_local void @call_before_decl(
+// CHECK: call void @used_before_default_decl()
+//
+__attribute__((target_version("default"))) void used_before_default_decl(void);
+// CHECK: declare void @used_before_default_decl()
+
+
+// Test that an unmagled declaration is generated and used when the
+// default version is declared before the first use of the function.
+//
+__attribute__((target_version("aes"))) void used_after_default_decl(void) {}
+// CHECK-LABEL: define dso_local void @used_after_default_decl._Maes(
+//
+__attribute__((target_version("default"))) void used_after_default_decl(void);
+// CHECK: declare void @used_after_default_decl()
+//
+void call_after_decl(void) { used_after_default_decl(); }
+// CHECK-LABEL: define dso_local void @call_after_decl(
+// CHECK: call void @used_after_default_decl()
+
+
+// Test that an unmagled declaration is generated and used when
+// the default version is not present.
+//
+__attribute__((target_version("aes"))) void used_no_default(void) {}
+// CHECK-LABEL: define dso_local void @used_no_default._Maes(
+//
+void call_no_default(void) { used_no_default(); }
+// CHECK-LABEL: define dso_local void @call_no_default(
+// CHECK: call void @used_no_default()
+//
+// CHECK: declare void @used_no_default()
+
+
+// Test that neither an ifunc nor a declaration is generated if the default
+// definition is missing since the versioned function is not used.
+//
+__attribute__((target_version("aes"))) void not_used_no_default(void) {}
+// CHECK-LABEL: define dso_local void @not_used_no_default._Maes(
+//
+// CHECK-NOT: declare void @not_used_no_default(
+
+
+// Test that an ifunc is generated if the default version is defined but not used.
+//
+__attribute__((target_version("aes"))) void not_used_with_default(void) {}
+// CHECK-LABEL: define dso_local void @not_used_with_default._Maes(
+//
+__attribute__((target_version("default"))) void not_used_with_default(void) {}
+// CHECK-LABEL: define dso_local void @not_used_with_default.default(
+//
+// CHECK-NOT: declare void @not_used_with_default(
+
+
+// CHECK: define weak_odr ptr @used_before_default_def.resolver()
+// CHECK: define weak_odr ptr @used_after_default_def.resolver()
+// CHECK-NOT: define weak_odr ptr @used_before_default_decl.resolver(
+// CHECK-NOT: define weak_odr ptr @used_after_default_decl.resolver(
+// CHECK-NOT: define weak_odr ptr @used_no_default.resolver(
+// CHECK-NOT: define weak_odr ptr @not_used_no_default.resolver(
+// CHECK: define weak_odr ptr @not_used_with_default.resolver()

clang/test/CodeGen/aarch64-mixed-target-attributes.c

@@ -261,9 +261,9 @@ __attribute__((target_version("jscvt"))) int default_def_with_version_decls(void
 // CHECK: attributes #[[ATTR3]] = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+lse,-v9.5a" }
 // CHECK: attributes #[[ATTR4]] = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+fp-armv8,+neon,+rdm,-v9.5a" }
 // CHECK: attributes #[[ATTR5:[0-9]+]] = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+dotprod,+fp-armv8,+neon,-v9.5a" }
-// CHECK: attributes #[[ATTR6:[0-9]+]] = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+fp-armv8,+jsconv,+neon,-v9.5a" }
-// CHECK: attributes #[[ATTR7:[0-9]+]] = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="-v9.5a" }
-// CHECK: attributes #[[ATTR8:[0-9]+]] = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+lse,-v9.5a" }
+// CHECK: attributes #[[ATTR6:[0-9]+]] = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="-v9.5a" }
+// CHECK: attributes #[[ATTR7:[0-9]+]] = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+lse,-v9.5a" }
+// CHECK: attributes #[[ATTR8:[0-9]+]] = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+fp-armv8,+jsconv,+neon,-v9.5a" }
 //.
 // CHECK-NOFMV: attributes #[[ATTR0]] = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="-fmv" }
 //.