[clang] Add clang::nooutline Attribute

clang/docs/ReleaseNotes.rst

@@ -269,6 +269,9 @@ Non-comprehensive list of changes in this release
   allocation functions with a token ID can be enabled via the
   ``-fsanitize=alloc-token`` flag.
 
+- Added a new attribute, ``[[clang::nooutline]]`` to suppress outlining from
+  annotated functions. This uses the LLVM `nooutline` attribute.
+
 New Compiler Flags
 ------------------
 - New option ``-fno-sanitize-debug-trap-reasons`` added to disable emitting trap reasons into the debug info when compiling with trapping UBSan (e.g. ``-fsanitize-trap=undefined``).

clang/include/clang/Basic/Attr.td

@@ -2355,6 +2355,13 @@ def NoInline : DeclOrStmtAttr {
   let SimpleHandler = 1;
 }
 
+def NoOutline : DeclOrStmtAttr {
+  let Spellings = [Clang<"nooutline">];
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+  let Documentation = [NoOutlineDocs];
+  let SimpleHandler = 1;
+}
+
 def NoMips16 : InheritableAttr, TargetSpecificAttr<TargetMips32> {
   let Spellings = [GCC<"nomips16">];
   let Subjects = SubjectList<[Function], ErrorDiag>;

clang/include/clang/Basic/AttrDocs.td

@@ -862,6 +862,36 @@ with ``__noinline__`` defined as a macro as ``__attribute__((noinline))``.
   }];
 }
 
+def NoOutlineDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+This function attribute suppresses outlining from the annotated function.
+
+Outlining is the process where common parts of separate functions are extracted
+into a separate function (or assembly snippet), and calls to that function or
+snippet are inserted in the original functions. In this way, it can be seen as
+the opposite of inlining. It can help to reduce code size.
+
+.. code-block:: c
+
+  [[clang::nooutline]] int x1(int y) {
+    int z = COMPLEX_MACRO(y); // Not outlined
+    return z + const1;
+  }
+
+  int x2(int y) {
+    int z = COMPLEX_MACRO(y); // May be outlined
+    return z * const2;
+  }
+
+  int x3(int y) {
+    int z = COMPLEX_MACRO(y); // May be outlined
+    reutrn z / const3;
+  }
+
+  }];
+}
+
 def MustTailDocs : Documentation {
   let Category = DocCatStmt;
   let Content = [{

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2820,6 +2820,9 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
       B.addAttribute(llvm::Attribute::MinSize);
   }
 
+  if (D->hasAttr<NoOutlineAttr>())
+    B.addAttribute(llvm::Attribute::NoOutline);
+
   F->addFnAttrs(B);
 
   unsigned alignment = D->getMaxAlignment() / Context.getCharWidth();

clang/test/CodeGen/attr-nooutline.c

@@ -0,0 +1,25 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --check-attributes --version 6
+// RUN: %clang_cc1 -emit-llvm %s -triple x86_64-unknown-linux-gnu -o - | FileCheck %s --check-prefix=C
+// RUN: %clang_cc1 -emit-llvm -x c++ %s -triple x86_64-unknown-linux-gnu -o - | FileCheck %s --check-prefix=CXX
+
+// C: Function Attrs: noinline nooutline nounwind optnone
+// C-LABEL: define dso_local i32 @t1(
+// C-SAME: i32 noundef [[X:%.*]]) #[[ATTR0:[0-9]+]] {
+// C-NEXT:  [[ENTRY:.*:]]
+// C-NEXT:    [[X_ADDR:%.*]] = alloca i32, align 4
+// C-NEXT:    store i32 [[X]], ptr [[X_ADDR]], align 4
+// C-NEXT:    [[TMP0:%.*]] = load i32, ptr [[X_ADDR]], align 4
+// C-NEXT:    ret i32 [[TMP0]]
+//
+// CXX: Function Attrs: mustprogress noinline nooutline nounwind optnone
+// CXX-LABEL: define dso_local noundef i32 @_Z2t1i(
+// CXX-SAME: i32 noundef [[X:%.*]]) #[[ATTR0:[0-9]+]] {
+// CXX-NEXT:  [[ENTRY:.*:]]
+// CXX-NEXT:    [[X_ADDR:%.*]] = alloca i32, align 4
+// CXX-NEXT:    store i32 [[X]], ptr [[X_ADDR]], align 4
+// CXX-NEXT:    [[TMP0:%.*]] = load i32, ptr [[X_ADDR]], align 4
+// CXX-NEXT:    ret i32 [[TMP0]]
+//
+[[clang::nooutline]] int t1(int x) {
+  return x;
+}

clang/test/Misc/pragma-attribute-supported-attributes-list.test

@@ -123,6 +123,7 @@
 // CHECK-NEXT: NoMerge (SubjectMatchRule_function, SubjectMatchRule_variable)
 // CHECK-NEXT: NoMicroMips (SubjectMatchRule_function)
 // CHECK-NEXT: NoMips16 (SubjectMatchRule_function)
+// CHECK-NEXT: NoOutline (SubjectMatchRule_function)
 // CHECK-NEXT: NoProfileFunction (SubjectMatchRule_function)
 // CHECK-NEXT: NoRandomizeLayout (SubjectMatchRule_record)
 // CHECK-NEXT: NoSanitize (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_variable_is_global)

clang/test/Sema/attr-nooutline.c

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only
+
+[[clang::nooutline]] int a; // expected-error {{'clang::nooutline' attribute only applies to functions}}
+
+[[clang::nooutline]] void t1(void);
+
+[[clang::nooutline(2)]] void t2(void); // expected-error {{'clang::nooutline' attribute takes no arguments}}

clang/test/Sema/attr-nooutline.cpp

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s -Wno-c++17-extensions
+
+[[clang::nooutline]] int a; // expected-error {{'clang::nooutline' attribute only applies to functions}}
+
+[[clang::nooutline]] void t1(void);
+
+[[clang::nooutline(2)]] void t2(void); // expected-error {{'clang::nooutline' attribute takes no arguments}}