[clang] Support __attribute__((ifunc(...))) on Darwin platforms

[jroelofs]

Unlike ELF targets, MachO does not support the same kind of dynamic symbol
resolution at load time. Instead, the corresponding MachO feature resolves
symbols lazily on first call.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Jon Roelofs (jroelofs)

Changes

Unlike ELF targets, MachO does not support the same kind of dynamic symbol
resolution at load time. Instead, the corresponding MachO feature resolves
symbols lazily on first call.

---

Full diff: https://github.com/llvm/llvm-project/pull/73687.diff

3 Files Affected:

• (modified) clang/include/clang/Basic/Attr.td (+4-1)
• (modified) clang/include/clang/Basic/AttrDocs.td (+3-1)
• (modified) clang/test/CodeGen/ifunc.c (+8)

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 03ed6accf700c4e..cef9f5578fa2baa 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -455,6 +455,9 @@ def TargetMicrosoftCXXABI : TargetArch<["x86", "x86_64", "arm", "thumb", "aarch6
 def TargetELF : TargetSpec {
   let ObjectFormats = ["ELF"];
 }
+def TargetELFOrMachO : TargetSpec {
+  let ObjectFormats = ["ELF", "MachO"];
+}
 
 def TargetSupportsInitPriority : TargetSpec {
   let CustomCode = [{ !Target.getTriple().isOSzOS() }];
@@ -1665,7 +1668,7 @@ def IBOutletCollection : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
-def IFunc : Attr, TargetSpecificAttr<TargetELF> {
+def IFunc : Attr, TargetSpecificAttr<TargetELFOrMachO> {
   let Spellings = [GCC<"ifunc">];
   let Args = [StringArgument<"Resolver">];
   let Subjects = SubjectList<[Function]>;
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index be74535e28d8a60..4c4c4eb606fb0dc 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -5408,7 +5408,9 @@ considered inline.
 Not all targets support this attribute. ELF target support depends on both the
 linker and runtime linker, and is available in at least lld 4.0 and later,
 binutils 2.20.1 and later, glibc v2.11.1 and later, and FreeBSD 9.1 and later.
-Non-ELF targets currently do not support this attribute.
+MachO targets support it, but with slightly different semantics: the resolver is
+run at first call, instead of at load time by the runtime linker. Targets other
+than ELF and MachO currently do not support this attribute.
   }];
 }
 
diff --git a/clang/test/CodeGen/ifunc.c b/clang/test/CodeGen/ifunc.c
index 0b0a0549620f8b8..99d60dc0ea85dbd 100644
--- a/clang/test/CodeGen/ifunc.c
+++ b/clang/test/CodeGen/ifunc.c
@@ -3,6 +3,10 @@
 // RUN: %clang_cc1 -triple i386-unknown-linux-gnu -fsanitize=thread -O2 -emit-llvm -o - %s | FileCheck %s --check-prefix=SAN
 // RUN: %clang_cc1 -triple i386-unknown-linux-gnu -fsanitize=address -O2 -emit-llvm -o - %s | FileCheck %s --check-prefix=SAN
 // RUN: %clang_cc1 -triple i386-unknown-linux-gnu -fsanitize=memory -O2 -emit-llvm -o - %s | FileCheck %s --check-prefix=SAN
+// RUN: %clang_cc1 -triple x86_64-apple-macosx -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx -O2 -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx -fsanitize=thread -O2 -emit-llvm -o - %s | FileCheck %s --check-prefix=MACSAN
+// RUN: %clang_cc1 -triple x86_64-apple-macosx -fsanitize=address -O2 -emit-llvm -o - %s | FileCheck %s --check-prefix=MACSAN
 
 int foo(int) __attribute__ ((ifunc("foo_ifunc")));
 
@@ -44,9 +48,13 @@ void* goo_ifunc(void) {
 // CHECK: call void @goo()
 
 // SAN: define internal nonnull ptr @foo_ifunc() #[[#FOO_IFUNC:]] {
+// MACSAN: define internal nonnull ptr @foo_ifunc() #[[#FOO_IFUNC:]] {
 
 // SAN: define dso_local noalias ptr @goo_ifunc() #[[#GOO_IFUNC:]] {
+// MACSAN: define noalias ptr @goo_ifunc() #[[#GOO_IFUNC:]] {
 
 // SAN-DAG: attributes #[[#FOO_IFUNC]] = {{{.*}} disable_sanitizer_instrumentation {{.*}}
+// MACSAN-DAG: attributes #[[#FOO_IFUNC]] = {{{.*}} disable_sanitizer_instrumentation {{.*}}
 
 // SAN-DAG: attributes #[[#GOO_IFUNC]] = {{{.*}} disable_sanitizer_instrumentation {{.*}}
+// MACSAN-DAG: attributes #[[#GOO_IFUNC]] = {{{.*}} disable_sanitizer_instrumentation {{.*}}

[echristo]

Do you need an OS version check here?

[tahonermann]

Added @erichkeane as a reviewer.

I recommend extending test coverage to all of the following tests:

• clang/test/CodeGenCXX/externc-ifunc-resolver.cpp
• clang/test/SemaCXX/externc-ifunc-resolver.cpp
• clang/test/CodeGen/ifunc.c (already done)
• clang/test/CodeGen/attr-ifunc.cpp
• clang/test/CodeGen/attr-ifunc.c

[jroelofs]

This is part of a patch series:

• [builtins][arm64] Build __init_cpu_features_resolver on Apple platforms #73685
• [llvm] Support IFuncs on Darwin platforms #73686
• [clang] Support __attribute__((ifunc(...))) on Darwin platforms #73687
• [clang] Function Multi Versioning supports IFunc lowerings on Darwin platforms #73688
• [GlobalISel] Always direct-call IFuncs and Aliases #74902

[jroelofs]

Do you need an OS version check here?

I don't think so, at least not yet. With #73686, support for this is entirely up to the compiler.

[echristo]

On Wed, Nov 29, 2023 at 9:37 AM Jon Roelofs ***@***.***> wrote: Do you need an OS version check here? I don't think so, at least not yet. With #73686 <#73686>, support for this is entirely up to the compiler.

Aha I was wondering where the implementation side was :) It looks like you don't necessarily need linker support as you're putting it into the binary on occasion as well. This could work for now, but if you plan on using this extensively it might be worth something else (as you note).