[Clang] Fix linker error for function multiversioning (#71706)

Currently target_clones attribute results in a linker error when there
are no multi-versioned function declarations in the calling TU.

In the calling TU, the call is generated with the ‘normal’ assembly
name. This does not match any of the versions or the ifunc, since
version mangling includes a .versionstring, and the ifunc includes
.ifunc suffix. The linker error is not seen with GCC since the mangling
for the ifunc symbol in GCC is the ‘normal’ assembly name for function
i.e. no ifunc suffix.

This PR removes the .ifunc suffix to match GCC. It also adds alias with
the .ifunc suffix so as to ensure backward compatibility.

The changes exclude aarch64 target because the mangling for default
versions on aarch64 does not include a .default suffix and is the
'normal' assembly name, unlike other targets. It is not clear to me what
the correct behavior for this target is.

Old Phabricator review - https://reviews.llvm.org/D158666

---------

Co-authored-by: Tom Honermann <tom@honermann.net>

Author: elizabethandrews

clang/docs/ReleaseNotes.rst

@@ -651,6 +651,9 @@ Bug Fixes in This Version
 - Fixed false positive error emitted by clang when performing qualified name
   lookup and the current class instantiation has dependent bases.
   Fixes (`#13826 <https://github.com/llvm/llvm-project/issues/13826>`_)
+- Fix the name of the ifunc symbol emitted for multiversion functions declared with the
+  ``target_clones`` attribute. This addresses a linker error that would otherwise occur
+  when these functions are referenced from other TUs.
 - Fixes compile error that double colon operator cannot resolve macro with parentheses.
   Fixes (`#64467 <https://github.com/llvm/llvm-project/issues/64467>`_)
 - Clang's ``-Wchar-subscripts`` no longer warns on chars whose values are known non-negative constants.

clang/include/clang/Basic/AttrDocs.td

@@ -2552,6 +2552,13 @@ example, the following will emit 4 versions of the function:
     __attribute__((target_clones("arch=atom,avx2","arch=ivybridge","default")))
     void foo() {}
 
+For targets that support the GNU indirect function (IFUNC) feature, dispatch
+is performed by emitting an indirect function that is resolved to the appropriate
+target clone at load time. The indirect function is given the name the
+multiversioned function would have if it had been declared without the attribute.
+For backward compatibility with earlier Clang releases, a function alias with an
+``.ifunc`` suffix is also emitted. The  ``.ifunc`` suffixed symbol is a deprecated
+feature and support for it may be removed in the future.
 }];
 }
 

clang/lib/CodeGen/CodeGenModule.cpp

@@ -4178,8 +4178,29 @@ void CodeGenModule::emitMultiVersionFunctions() {
     }
 
     llvm::Constant *ResolverConstant = GetOrCreateMultiVersionResolver(GD);
-    if (auto *IFunc = dyn_cast<llvm::GlobalIFunc>(ResolverConstant))
+    if (auto *IFunc = dyn_cast<llvm::GlobalIFunc>(ResolverConstant)) {
       ResolverConstant = IFunc->getResolver();
+      // In Aarch64, default versions of multiversioned functions are mangled to
+      // their 'normal' assembly name. This deviates from other targets which
+      // append a '.default' string. As a result we need to continue appending
+      // .ifunc in Aarch64.
+      // FIXME: Should Aarch64 mangling for 'default' multiversion function and
+      // in turn ifunc function match that of other targets?
+      if (FD->isTargetClonesMultiVersion() &&
+          !getTarget().getTriple().isAArch64()) {
+        const CGFunctionInfo &FI = getTypes().arrangeGlobalDeclaration(GD);
+        llvm::FunctionType *DeclTy = getTypes().GetFunctionType(FI);
+        std::string MangledName = getMangledNameImpl(
+            *this, GD, FD, /*OmitMultiVersionMangling=*/true);
+        // In prior versions of Clang, the mangling for ifuncs incorrectly
+        // included an .ifunc suffix. This alias is generated for backward
+        // compatibility. It is deprecated, and may be removed in the future.
+        auto *Alias = llvm::GlobalAlias::create(
+            DeclTy, 0, getMultiversionLinkage(*this, GD),
+            MangledName + ".ifunc", IFunc, &getModule());
+        SetCommonAttributes(FD, Alias);
+      }
+    }
     llvm::Function *ResolverFunc = cast<llvm::Function>(ResolverConstant);
 
     ResolverFunc->setLinkage(getMultiversionLinkage(*this, GD));
@@ -4346,10 +4367,19 @@ llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
   // 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())
+  if (getTarget().supportsIFunc()) {
+    // In Aarch64, default versions of multiversioned functions are mangled to
+    // their 'normal' assembly name. This deviates from other targets which
+    // append a '.default' string. As a result we need to continue appending
+    // .ifunc in Aarch64.
+    // FIXME: Should Aarch64 mangling for 'default' multiversion function and
+    // in turn ifunc function match that of other targets?
+    if (!FD->isTargetClonesMultiVersion() ||
+        getTarget().getTriple().isAArch64())
+      ResolverName += ".ifunc";
+  } else if (FD->isTargetMultiVersion()) {
     ResolverName += ".resolver";
+  }
 
   // If the resolver has already been created, just return it.
   if (llvm::GlobalValue *ResolverGV = GetGlobalValue(ResolverName))

clang/test/CodeGen/attr-target-clones.c

@@ -16,13 +16,23 @@
 // LINUX: @__cpu_model = external dso_local global { i32, i32, i32, [1 x i32] }
 // LINUX: @__cpu_features2 = external dso_local global [3 x i32]
 
-// LINUX: @internal.ifunc = internal ifunc i32 (), ptr @internal.resolver
-// LINUX: @foo.ifunc = weak_odr ifunc i32 (), ptr @foo.resolver
-// LINUX: @foo_dupes.ifunc = weak_odr ifunc void (), ptr @foo_dupes.resolver
-// LINUX: @unused.ifunc = weak_odr ifunc void (), ptr @unused.resolver
-// LINUX: @foo_inline.ifunc = weak_odr ifunc i32 (), ptr @foo_inline.resolver
-// LINUX: @foo_inline2.ifunc = weak_odr ifunc i32 (), ptr @foo_inline2.resolver
-// LINUX: @foo_used_no_defn.ifunc = weak_odr ifunc i32 (), ptr @foo_used_no_defn.resolver
+// LINUX: @internal.ifunc = internal alias i32 (), ptr @internal
+// LINUX: @foo.ifunc = weak_odr alias i32 (), ptr @foo
+// LINUX: @foo_dupes.ifunc = weak_odr alias void (), ptr @foo_dupes
+// LINUX: @unused.ifunc = weak_odr alias void (), ptr @unused
+// LINUX: @foo_inline.ifunc = weak_odr alias i32 (), ptr @foo_inline
+// LINUX: @foo_inline2.ifunc = weak_odr alias i32 (), ptr @foo_inline2
+// LINUX: @foo_used_no_defn.ifunc = weak_odr alias i32 (), ptr @foo_used_no_defn
+// LINUX: @isa_level.ifunc = weak_odr alias i32 (i32), ptr @isa_level
+
+// LINUX: @internal = internal ifunc i32 (), ptr @internal.resolver
+// LINUX: @foo = weak_odr ifunc i32 (), ptr @foo.resolver
+// LINUX: @foo_dupes = weak_odr ifunc void (), ptr @foo_dupes.resolver
+// LINUX: @unused = weak_odr ifunc void (), ptr @unused.resolver
+// LINUX: @foo_inline = weak_odr ifunc i32 (), ptr @foo_inline.resolver
+// LINUX: @foo_inline2 = weak_odr ifunc i32 (), ptr @foo_inline2.resolver
+// LINUX: @foo_used_no_defn = weak_odr ifunc i32 (), ptr @foo_used_no_defn.resolver
+// LINUX: @isa_level = weak_odr ifunc i32 (i32), ptr @isa_level.resolver
 
 static int __attribute__((target_clones("sse4.2, default"))) internal(void) { return 0; }
 int use(void) { return internal(); }
@@ -60,15 +70,15 @@ void bar2(void) {
   // LINUX: define {{.*}}void @bar2()
   // WINDOWS: define dso_local void @bar2()
   foo_dupes();
-  // LINUX: call void @foo_dupes.ifunc()
+  // LINUX: call void @foo_dupes()
   // WINDOWS: call void @foo_dupes()
 }
 
 int bar(void) {
   // LINUX: define {{.*}}i32 @bar() #[[DEF:[0-9]+]]
   // WINDOWS: define dso_local i32 @bar() #[[DEF:[0-9]+]]
   return foo();
-  // LINUX: call i32 @foo.ifunc()
+  // LINUX: call i32 @foo()
   // WINDOWS: call i32 @foo()
 }
 
@@ -95,8 +105,8 @@ int bar3(void) {
   // LINUX: define {{.*}}i32 @bar3()
   // WINDOWS: define dso_local i32 @bar3()
   return foo_inline() + foo_inline2();
-  // LINUX: call i32 @foo_inline.ifunc()
-  // LINUX: call i32 @foo_inline2.ifunc()
+  // LINUX: call i32 @foo_inline()
+  // LINUX: call i32 @foo_inline2()
   // WINDOWS: call i32 @foo_inline()
   // WINDOWS: call i32 @foo_inline2()
 }
@@ -134,7 +144,7 @@ int test_foo_used_no_defn(void) {
   // LINUX: define {{.*}}i32 @test_foo_used_no_defn()
   // WINDOWS: define dso_local i32 @test_foo_used_no_defn()
   return foo_used_no_defn();
-  // LINUX: call i32 @foo_used_no_defn.ifunc()
+  // LINUX: call i32 @foo_used_no_defn()
   // WINDOWS: call i32 @foo_used_no_defn()
 }
 

clang/test/CodeGenCXX/attr-target-clones.cpp

@@ -1,13 +1,20 @@
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-linux-gnu -emit-llvm %s -o - | FileCheck %s --check-prefix=LINUX
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-windows-pc -emit-llvm %s -o - | FileCheck %s --check-prefix=WINDOWS
 
+// Aliases for ifuncs
+// LINUX: @_Z10overloadedi.ifunc = weak_odr alias i32 (i32), ptr @_Z10overloadedi
+// LINUX: @_Z10overloadedPKc.ifunc = weak_odr alias i32 (ptr), ptr @_Z10overloadedPKc
+// LINUX: @_ZN1CIssE3fooEv.ifunc = weak_odr alias i32 (ptr), ptr @_ZN1CIssE3fooEv
+// LINUX: @_ZN1CIisE3fooEv.ifunc = weak_odr alias i32 (ptr), ptr @_ZN1CIisE3fooEv
+// LINUX: @_ZN1CIdfE3fooEv.ifunc = weak_odr alias i32 (ptr), ptr @_ZN1CIdfE3fooEv
+
 // Overloaded ifuncs
-// LINUX: @_Z10overloadedi.ifunc = weak_odr ifunc i32 (i32), ptr @_Z10overloadedi.resolver
-// LINUX: @_Z10overloadedPKc.ifunc = weak_odr ifunc i32 (ptr), ptr @_Z10overloadedPKc.resolver
+// LINUX: @_Z10overloadedi = weak_odr ifunc i32 (i32), ptr @_Z10overloadedi.resolver
+// LINUX: @_Z10overloadedPKc = weak_odr ifunc i32 (ptr), ptr @_Z10overloadedPKc.resolver
 // struct 'C' ifuncs, note the 'float, U' one doesn't get one.
-// LINUX: @_ZN1CIssE3fooEv.ifunc = weak_odr ifunc i32 (ptr), ptr @_ZN1CIssE3fooEv.resolver
-// LINUX: @_ZN1CIisE3fooEv.ifunc = weak_odr ifunc i32 (ptr), ptr @_ZN1CIisE3fooEv.resolver
-// LINUX: @_ZN1CIdfE3fooEv.ifunc = weak_odr ifunc i32 (ptr), ptr @_ZN1CIdfE3fooEv.resolver
+// LINUX: @_ZN1CIssE3fooEv = weak_odr ifunc i32 (ptr), ptr @_ZN1CIssE3fooEv.resolver
+// LINUX: @_ZN1CIisE3fooEv = weak_odr ifunc i32 (ptr), ptr @_ZN1CIisE3fooEv.resolver
+// LINUX: @_ZN1CIdfE3fooEv = weak_odr ifunc i32 (ptr), ptr @_ZN1CIdfE3fooEv.resolver
 
 int __attribute__((target_clones("sse4.2", "default"))) overloaded(int) { return 1; }
 // LINUX: define {{.*}}i32 @_Z10overloadedi.sse4.2.0(i32{{.+}})
@@ -37,10 +44,10 @@ int __attribute__((target_clones("arch=ivybridge", "default"))) overloaded(const
 
 void use_overloaded() {
   overloaded(1);
-  // LINUX: call noundef i32 @_Z10overloadedi.ifunc
+  // LINUX: call noundef i32 @_Z10overloadedi
   // WINDOWS: call noundef i32 @"?overloaded@@YAHH@Z"
   overloaded(nullptr);
-  // LINUX: call noundef i32 @_Z10overloadedPKc.ifunc 
+  // LINUX: call noundef i32 @_Z10overloadedPKc 
   // WINDOWS: call noundef i32 @"?overloaded@@YAHPEBD@Z"
 }
 
@@ -64,11 +71,11 @@ int __attribute__((target_clones("sse4.2", "default"))) foo(){ return 3;}
 void uses_specialized() {
   C<short, short> c;
   c.foo();
-  // LINUX: call noundef i32 @_ZN1CIssE3fooEv.ifunc(ptr
+  // LINUX: call noundef i32 @_ZN1CIssE3fooEv(ptr
   // WINDOWS: call noundef i32 @"?foo@?$C@FF@@QEAAHXZ"(ptr
   C<int, short> c2;
   c2.foo();
-  // LINUX: call noundef i32 @_ZN1CIisE3fooEv.ifunc(ptr
+  // LINUX: call noundef i32 @_ZN1CIisE3fooEv(ptr
   // WINDOWS: call noundef i32 @"?foo@?$C@HF@@QEAAHXZ"(ptr
   C<float, short> c3;
   c3.foo();
@@ -77,7 +84,7 @@ void uses_specialized() {
   // WINDOWS: call noundef i32 @"?foo@?$C@MF@@QEAAHXZ"(ptr
   C<double, float> c4;
   c4.foo();
-  // LINUX: call noundef i32 @_ZN1CIdfE3fooEv.ifunc(ptr
+  // LINUX: call noundef i32 @_ZN1CIdfE3fooEv(ptr
   // WINDOWS: call noundef i32 @"?foo@?$C@NM@@QEAAHXZ"(ptr
 }