[clang] Add clang::nooutline Attribute

This PR:
- Adds a `[[clang::nooutline]]` function attribute for C and C++. There
  is no equivalent GNU syntax for this attribute, so no `__attribute__`
  syntax.
- Uses the presence of `[[clang::nooutline]]` to add the `nooutline`
  attribute to IR function definitions.
- Turns the `"nooutline"` attribute into an enum attribute (without
  quotes), and adds an auto-upgrader for bitcode to make that same
  change to existing IR.
- Adds test for the above.

The attribute is capable of disabling both the Machine Outliner (enabled
at Oz for some targets), and the IR Outliner (disabled by default).

Author: lenary

clang/include/clang/Basic/Attr.td

@@ -2355,6 +2355,13 @@ def NoInline : DeclOrStmtAttr {
   let SimpleHandler = 1;
 }
 
+def NoOutline : DeclOrStmtAttr {
+  let Spellings = [CXX11<"clang", "nooutline">, C23<"clang", "nooutline">];
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+  let Documentation = [Undocumented];
+  let SimpleHandler = 1;
+}
+
 def NoMips16 : InheritableAttr, TargetSpecificAttr<TargetMips32> {
   let Spellings = [GCC<"nomips16">];
   let Subjects = SubjectList<[Function], ErrorDiag>;

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 +1,16 @@
+// 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 -disable-O0-optnone -o - | FileCheck %s
+
+
+// CHECK: Function Attrs: noinline nooutline nounwind
+// CHECK-LABEL: define dso_local i32 @t1(
+// CHECK-SAME: i32 noundef [[X:%.*]]) #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    [[X_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT:    store i32 [[X]], ptr [[X_ADDR]], align 4
+// CHECK-NEXT:    [[TMP0:%.*]] = load i32, ptr [[X_ADDR]], align 4
+// CHECK-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 +1,8 @@
+// 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}}

llvm/docs/LangRef.rst

@@ -2738,7 +2738,7 @@ For example:
     to signify an unbounded maximum. The syntax `vscale_range(<val>)` can be
     used to set both `min` and `max` to the same value. Functions that don't
     include this attribute make no assumptions about the value of `vscale`.
-``"nooutline"``
+``nooutline``
     This attribute indicates that outlining passes should not modify the
     function.
 

llvm/include/llvm/Bitcode/LLVMBitCodes.h

@@ -801,6 +801,7 @@ enum AttributeKindCodes {
   ATTR_KIND_CAPTURES = 102,
   ATTR_KIND_DEAD_ON_RETURN = 103,
   ATTR_KIND_SANITIZE_ALLOC_TOKEN = 104,
+  ATTR_KIND_NOOUTLINE = 105,
 };
 
 enum ComdatSelectionKindCodes {

llvm/include/llvm/IR/Attributes.td

@@ -207,6 +207,9 @@ def NoImplicitFloat : EnumAttr<"noimplicitfloat", IntersectPreserve, [FnAttr]>;
 /// inline=never.
 def NoInline : EnumAttr<"noinline", IntersectPreserve, [FnAttr]>;
 
+/// nooutline
+def NoOutline : EnumAttr<"nooutline", IntersectPreserve, [FnAttr]>;
+
 /// Function is called early and/or often, so lazy binding isn't worthwhile.
 def NonLazyBind : EnumAttr<"nonlazybind", IntersectPreserve, [FnAttr]>;
 

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2257,6 +2257,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::Captures;
   case bitc::ATTR_KIND_DEAD_ON_RETURN:
     return Attribute::DeadOnReturn;
+  case bitc::ATTR_KIND_NOOUTLINE:
+    return Attribute::NoOutline;
   }
 }