[Clang] Implement function attribute nouwtable

To have finer control of IR uwtable attribute generation. For target code generation,
IR nounwind and uwtable may have some interaction. However, for frontend, there are
no semantic interactions so the this new `nouwtable` is marked "SimpleHandler = 1".

Differential Revision: https://reviews.llvm.org/D132592

clang/docs/ReleaseNotes.rst

@@ -149,6 +149,9 @@ Attribute Changes in Clang
   using the MinGW environment. This attribute is only available for Windows
   targets.
 
+- Introduced a new function attribute ``__attribute__((nouwtable))`` to suppress
+  LLVM IR ``uwtable`` function attribute.
+
 Windows Support
 ---------------
 

clang/include/clang/Basic/Attr.td

@@ -2092,6 +2092,13 @@ def NoThrow : InheritableAttr {
   let Documentation = [NoThrowDocs];
 }
 
+def NoUwtable : InheritableAttr {
+  let Spellings = [Clang<"nouwtable">];
+  let Subjects = SubjectList<[FunctionLike]>;
+  let Documentation = [NoUwtableDocs];
+  let SimpleHandler = 1;
+}
+
 def NvWeak : IgnoredAttr {
   // No Declspec spelling of this attribute; the CUDA headers use
   // __attribute__((nv_weak)) unconditionally. Does not receive an [[]]

clang/include/clang/Basic/AttrDocs.td

@@ -4537,6 +4537,16 @@ guaranteed to not throw an exception.
     }];
 }
 
+def NoUwtableDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+Clang supports the ``nouwtable`` attribute which skips emitting
+the unwind table entry for the specified function. This attribute is useful for
+selectively emitting the unwind table entry on some functions when building with
+``-funwind-tables`` compiler option.
+    }];
+}
+
 def InternalLinkageDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1963,7 +1963,7 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
                                                            llvm::Function *F) {
   llvm::AttrBuilder B(F->getContext());
 
-  if (CodeGenOpts.UnwindTables)
+  if ((!D || !D->hasAttr<NoUwtableAttr>()) && CodeGenOpts.UnwindTables)
     B.addUWTableAttr(llvm::UWTableKind(CodeGenOpts.UnwindTables));
 
   if (CodeGenOpts.StackClashProtector)

clang/test/CodeGen/attr-nouwtable.c

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -funwind-tables=2 -S -emit-llvm %s -o - | FileCheck %s
+
+__attribute__((nouwtable))
+int test1(void) { return 0; }
+
+// CHECK: @test1{{.*}}[[ATTR1:#[0-9]+]]
+// CHECK: attributes [[ATTR1]] = {
+// CHECK-NOT: uwtable
+// CHECK: }

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

@@ -115,6 +115,7 @@
 // CHECK-NEXT: NoStackProtector (SubjectMatchRule_function)
 // CHECK-NEXT: NoThreadSafetyAnalysis (SubjectMatchRule_function)
 // CHECK-NEXT: NoThrow (SubjectMatchRule_hasType_functionType)
+// CHECK-NEXT: NoUwtable (SubjectMatchRule_hasType_functionType)
 // CHECK-NEXT: NotTailCalled (SubjectMatchRule_function)
 // CHECK-NEXT: OSConsumed (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: OSReturnsNotRetained (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_variable_is_parameter)