[Clang] Permit noescape on non-pointer types

In modern C++ we often use span, string_view or other view objects
instead of raw pointers. This PR opens the door to mark those noescape.
This can be useful to document the API contracts, for interop with
memory safe languages like Swift or Rust, and possibly in the future to
implement take this into account in codegen.

The PR also adds a feature. The goal is to help avoiding warnings when
the code is compiler by earlier versions of clang that does not permit
this attribute on non-pointer types.

Author: Gabor Horvath

clang/docs/ReleaseNotes.rst

@@ -512,6 +512,8 @@ Attribute Changes in Clang
 
 - The ``target_version`` attribute is now only supported for AArch64 and RISC-V architectures.
 
+- Now ``__attribute__((noescape))`` can be applied to non-pointer types like ``std::span``.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 

clang/include/clang/AST/Attr.h

@@ -406,6 +406,13 @@ inline ParameterABI ParameterABIAttr::getABI() const {
     llvm_unreachable("bad parameter ABI attribute kind");
   }
 }
+
+/// Determine if type T is a valid subject for a nonnull and similar
+/// attributes. Dependent types are considered valid so they can be checked
+/// during instantiation time. By default, we look through references (the
+/// behavior used by nonnull), but if the second parameter is true, then we
+/// treat a reference type as valid.
+bool isValidPointerAttrType(QualType T, bool RefOkay = false);
 }  // end namespace clang
 
 #endif

clang/include/clang/Basic/AttrDocs.td

@@ -228,12 +228,13 @@ members, and static locals.
 def NoEscapeDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{
-``noescape`` placed on a function parameter of a pointer type is used to inform
-the compiler that the pointer cannot escape: that is, no reference to the object
-the pointer points to that is derived from the parameter value will survive
-after the function returns. Users are responsible for making sure parameters
-annotated with ``noescape`` do not actually escape. Calling ``free()`` on such
-a parameter does not constitute an escape.
+``noescape`` placed on a function parameter is used to inform the compiler that
+the pointer (or the pointer members in case of a user defined type) cannot
+escape: that is, no reference to the object the pointer points to that is
+derived from the parameter value will survive after the function returns. Users
+are responsible for making sure parameters annotated with ``noescape`` do not
+actually escape. Calling ``free()`` on such a parameter does not constitute an
+escape.
 
 For example:
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3346,6 +3346,9 @@ def warn_attribute_return_pointers_refs_only : Warning<
 def warn_attribute_pointer_or_reference_only : Warning<
   "%0 attribute only applies to a pointer or reference (%1 is invalid)">,
   InGroup<IgnoredAttributes>;
+def warn_attribute_pointer_or_reference_or_record_only : Warning<
+  "%0 attribute only applies to a pointer, reference, class, struct, or union (%1 is invalid)">,
+  InGroup<IgnoredAttributes>;
 def err_attribute_no_member_pointers : Error<
   "%0 attribute cannot be used with pointers to members">;
 def err_attribute_invalid_implicit_this_argument : Error<

clang/include/clang/Basic/Features.def

@@ -87,6 +87,7 @@ FEATURE(attribute_overloadable, true)
 FEATURE(attribute_unavailable_with_message, true)
 FEATURE(attribute_unused_on_fields, true)
 FEATURE(attribute_diagnose_if_objc, true)
+FEATURE(attribute_noescape_nonpointer, true)
 FEATURE(blocks, LangOpts.Blocks)
 FEATURE(c_thread_safety_attributes, true)
 FEATURE(cxx_exceptions, LangOpts.CXXExceptions)

clang/include/clang/Sema/Sema.h

@@ -4479,13 +4479,6 @@ class Sema final : public SemaBase {
                                       StringRef &Str,
                                       SourceLocation *ArgLocation = nullptr);
 
-  /// Determine if type T is a valid subject for a nonnull and similar
-  /// attributes. Dependent types are considered valid so they can be checked
-  /// during instantiation time. By default, we look through references (the
-  /// behavior used by nonnull), but if the second parameter is true, then we
-  /// treat a reference type as valid.
-  bool isValidPointerAttrType(QualType T, bool RefOkay = false);
-
   /// AddAssumeAlignedAttr - Adds an assume_aligned attribute to a particular
   /// declaration.
   void AddAssumeAlignedAttr(Decl *D, const AttributeCommonInfo &CI, Expr *E,

clang/lib/AST/AttrImpl.cpp

@@ -270,4 +270,30 @@ unsigned AlignedAttr::getAlignment(ASTContext &Ctx) const {
   return Ctx.getTargetDefaultAlignForAttributeAligned();
 }
 
+bool clang::isValidPointerAttrType(QualType T, bool RefOkay) {
+  if (T->isDependentType())
+    return true;
+  if (RefOkay) {
+    if (T->isReferenceType())
+      return true;
+  } else {
+    T = T.getNonReferenceType();
+  }
+
+  // The nonnull attribute, and other similar attributes, can be applied to a
+  // transparent union that contains a pointer type.
+  if (const RecordType *UT = T->getAsUnionType()) {
+    if (UT && UT->getDecl()->hasAttr<TransparentUnionAttr>()) {
+      RecordDecl *UD = UT->getDecl();
+      for (const auto *I : UD->fields()) {
+        QualType QT = I->getType();
+        if (QT->isAnyPointerType() || QT->isBlockPointerType())
+          return true;
+      }
+    }
+  }
+
+  return T->isAnyPointerType() || T->isBlockPointerType();
+}
+
 #include "clang/AST/AttrImpl.inc"

clang/lib/CodeGen/CGCall.cpp

@@ -2878,7 +2878,8 @@ void CodeGenModule::ConstructAttributeList(StringRef Name,
       break;
     }
 
-    if (FI.getExtParameterInfo(ArgNo).isNoEscape())
+    if (FI.getExtParameterInfo(ArgNo).isNoEscape() &&
+        isValidPointerAttrType(ParamType, /*RefOkay=*/true))
       Attrs.addAttribute(llvm::Attribute::NoCapture);
 
     if (Attrs.hasAttributes()) {

clang/lib/Sema/SemaChecking.cpp

@@ -3110,7 +3110,7 @@ static void CheckNonNullArguments(Sema &S,
       if (!NonNull->args_size()) {
         // Easy case: all pointer arguments are nonnull.
         for (const auto *Arg : Args)
-          if (S.isValidPointerAttrType(Arg->getType()))
+          if (isValidPointerAttrType(Arg->getType()))
             CheckNonNullArgument(S, Arg, CallSiteLoc);
         return;
       }

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1220,37 +1220,11 @@ static void handleNoSpecializations(Sema &S, Decl *D, const ParsedAttr &AL) {
       NoSpecializationsAttr::Create(S.Context, Message, AL));
 }
 
-bool Sema::isValidPointerAttrType(QualType T, bool RefOkay) {
-  if (T->isDependentType())
-    return true;
-  if (RefOkay) {
-    if (T->isReferenceType())
-      return true;
-  } else {
-    T = T.getNonReferenceType();
-  }
-
-  // The nonnull attribute, and other similar attributes, can be applied to a
-  // transparent union that contains a pointer type.
-  if (const RecordType *UT = T->getAsUnionType()) {
-    if (UT && UT->getDecl()->hasAttr<TransparentUnionAttr>()) {
-      RecordDecl *UD = UT->getDecl();
-      for (const auto *I : UD->fields()) {
-        QualType QT = I->getType();
-        if (QT->isAnyPointerType() || QT->isBlockPointerType())
-          return true;
-      }
-    }
-  }
-
-  return T->isAnyPointerType() || T->isBlockPointerType();
-}
-
 static bool attrNonNullArgCheck(Sema &S, QualType T, const ParsedAttr &AL,
                                 SourceRange AttrParmRange,
                                 SourceRange TypeRange,
                                 bool isReturnValue = false) {
-  if (!S.isValidPointerAttrType(T)) {
+  if (!isValidPointerAttrType(T)) {
     if (isReturnValue)
       S.Diag(AL.getLoc(), diag::warn_attribute_return_pointers_only)
           << AL << AttrParmRange << TypeRange;
@@ -1291,7 +1265,7 @@ static void handleNonNullAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     for (unsigned I = 0, E = getFunctionOrMethodNumParams(D);
          I != E && !AnyPointers; ++I) {
       QualType T = getFunctionOrMethodParamType(D, I);
-      if (S.isValidPointerAttrType(T))
+      if (isValidPointerAttrType(T))
         AnyPointers = true;
     }
 
@@ -1339,10 +1313,11 @@ static void handleNoEscapeAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (D->isInvalidDecl())
     return;
 
-  // noescape only applies to pointer types.
+  // noescape only applies to pointer and record types.
   QualType T = cast<ParmVarDecl>(D)->getType();
-  if (!S.isValidPointerAttrType(T, /* RefOkay */ true)) {
-    S.Diag(AL.getLoc(), diag::warn_attribute_pointers_only)
+  if (!isValidPointerAttrType(T, /* RefOkay */ true) && !T->isRecordType()) {
+    S.Diag(AL.getLoc(),
+           diag::warn_attribute_pointer_or_reference_or_record_only)
         << AL << AL.getRange() << 0;
     return;
   }

clang/test/AST/ast-dump-attr.cpp

@@ -230,8 +230,14 @@ __attribute__((external_source_symbol(generated_declaration, defined_in="module"
 // CHECK-NEXT: ExternalSourceSymbolAttr{{.*}} "Swift" "module" GeneratedDeclaration "testUSR"
 
 namespace TestNoEscape {
+  struct S { int *p; };
   void noescapeFunc(int *p0, __attribute__((noescape)) int *p1) {}
-  // CHECK: `-FunctionDecl{{.*}} noescapeFunc 'void (int *, __attribute__((noescape)) int *)'
+  // CHECK: |-FunctionDecl{{.*}} noescapeFunc 'void (int *, __attribute__((noescape)) int *)'
+  // CHECK-NEXT: ParmVarDecl
+  // CHECK-NEXT: ParmVarDecl
+  // CHECK-NEXT: NoEscapeAttr
+  void noescapeFunc2(int *p0, __attribute__((noescape)) S p1) {}
+  // CHECK: `-FunctionDecl{{.*}} noescapeFunc2 'void (int *, __attribute__((noescape)) S)'
   // CHECK-NEXT: ParmVarDecl
   // CHECK-NEXT: ParmVarDecl
   // CHECK-NEXT: NoEscapeAttr

clang/test/CodeGenCXX/noescape.cpp

@@ -6,6 +6,7 @@ struct S {
   S &operator=(int * __attribute__((noescape)));
   void m0(int *, int * __attribute__((noescape)));
   virtual void vm1(int *, int * __attribute__((noescape)));
+  virtual void vm2(int *, int __attribute__((noescape)));
 };
 
 // CHECK: define{{.*}} void @_ZN1SC2EPiS0_(ptr {{.*}}, {{.*}}, {{.*}} nocapture noundef {{%.*}})
@@ -23,6 +24,9 @@ void S::m0(int *, int * __attribute__((noescape))) {}
 // CHECK: define{{.*}} void @_ZN1S3vm1EPiS0_(ptr {{.*}}, {{.*}} nocapture noundef {{%.*}})
 void S::vm1(int *, int * __attribute__((noescape))) {}
 
+// CHECK-NOT: nocapture
+void S::vm2(int *, int __attribute__((noescape))) {}
+
 // CHECK-LABEL: define{{.*}} void @_Z5test0P1SPiS1_(
 // CHECK: call void @_ZN1SC1EPiS0_(ptr {{.*}}, {{.*}}, {{.*}} nocapture noundef {{.*}})
 // CHECK: call {{.*}} ptr @_ZN1SaSEPi(ptr {{.*}}, {{.*}} nocapture noundef {{.*}})

clang/test/CodeGenObjC/noescape.m

@@ -13,6 +13,7 @@
 void noescapeFunc1(__attribute__((noescape)) int *);
 void noescapeFunc2(__attribute__((noescape)) id);
 void noescapeFunc3(__attribute__((noescape)) union U);
+void noescapeFunc4(__attribute__((noescape)) int);
 
 // Block descriptors of non-escaping blocks don't need pointers to copy/dispose
 // helper functions.
@@ -53,6 +54,11 @@ void test3(union U u) {
   noescapeFunc3(u);
 }
 
+// CHECK-NOT: nocapture
+void testNonPtr(int i) {
+  noescapeFunc4(i);
+}
+
 // CHECK: define internal void @"\01-[C0 m0:]"({{.*}}, {{.*}}, {{.*}} nocapture {{.*}})
 
 // CHECK-LABEL: define{{.*}} void @test4(

clang/test/SemaCXX/noescape-attr.cpp

@@ -3,5 +3,11 @@
 template<typename T>
 void test1(T __attribute__((noescape)) arr, int size);
 
-// expected-warning@+1 {{'noescape' attribute only applies to pointer arguments}}
-void test2(int __attribute__((noescape)) arr, int size);
+void test2(int __attribute__((noescape)) a, int b); // expected-warning {{'noescape' attribute only applies to a pointer, reference, class, struct, or union (0 is invalid)}}
+
+struct S { int *p; };
+void test3(S __attribute__((noescape)) s);
+
+#if !__has_feature(attribute_noescape_nonpointer)
+  #error "attribute_noescape_nonpointer should be supported"
+#endif

clang/test/SemaObjCXX/noescape.mm

@@ -23,11 +23,11 @@
 template <class T>
 void noescapeFunc7(__attribute__((noescape)) T &&);
 
-void invalidFunc0(int __attribute__((noescape))); // expected-warning {{'noescape' attribute only applies to pointer arguments}}
+void invalidFunc0(int __attribute__((noescape))); // expected-warning {{'noescape' attribute only applies to a pointer, reference, class, struct, or union (0 is invalid)}}
 void invalidFunc1(int __attribute__((noescape(0)))); // expected-error {{'noescape' attribute takes no arguments}}
 void invalidFunc2(int0 *__attribute__((noescape))); // expected-error {{use of undeclared identifier 'int0'; did you mean 'int'?}}
-void invalidFunc3(__attribute__((noescape)) int (S::*Ty)); // expected-warning {{'noescape' attribute only applies to pointer arguments}}
-void invalidFunc4(__attribute__((noescape)) void (S::*Ty)()); // expected-warning {{'noescape' attribute only applies to pointer arguments}}
+void invalidFunc3(__attribute__((noescape)) int (S::*Ty)); // expected-warning {{'noescape' attribute only applies to a pointer, reference, class, struct, or union (0 is invalid)}}
+void invalidFunc4(__attribute__((noescape)) void (S::*Ty)()); // expected-warning {{'noescape' attribute only applies to a pointer, reference, class, struct, or union (0 is invalid)}}
 int __attribute__((noescape)) g; // expected-warning {{'noescape' attribute only applies to parameters}}
 
 struct S1 {