[Clang] Introduce malloc_span attribute

clang/docs/ReleaseNotes.rst

@@ -327,6 +327,10 @@ Attribute Changes in Clang
 - New format attributes ``gnu_printf``, ``gnu_scanf``, ``gnu_strftime`` and ``gnu_strfmon`` are added
   as aliases for ``printf``, ``scanf``, ``strftime`` and ``strfmon``. (#GH16219)
 
+- New function attribute `malloc_span` is added. Its semantics is similar to that of the `malloc`
+  attribute, but `malloc_span` applies not to functions returning pointers, but to functions returning
+  span-like structures (i.e. those that contain a pointer field and a size integer field).
+
 Improvements to Clang's diagnostics
 -----------------------------------
 - Diagnostics messages now refer to ``structured binding`` instead of ``decomposition``,

clang/include/clang/Basic/Attr.td

@@ -2068,6 +2068,12 @@ def Restrict : InheritableAttr {
   let Documentation = [RestrictDocs];
 }
 
+def MallocSpan : InheritableAttr {
+  let Spellings = [Clang<"malloc_span">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [MallocSpanDocs];
+}
+
 def LayoutVersion : InheritableAttr, TargetSpecificAttr<TargetMicrosoftRecordLayout> {
   let Spellings = [Declspec<"layout_version">];
   let Args = [UnsignedArgument<"Version">];

clang/include/clang/Basic/AttrDocs.td

@@ -5247,6 +5247,21 @@ yet implemented in clang.
   }];
 }
 
+def MallocSpanDocs : Documentation {
+  let Category = DocCatFunction;
+  let Heading = "malloc_span";
+  let Content = [{
+The ``malloc_span`` attribute can be used to mark that a function which acts
+like a system memory allocation function and returns a span-like structure
+where the returned memory range does not alias storage from any other object
+accessible to the caller.
+
+In this context, a span-like structure is assumed to have a pointer to the
+allocated memory as its first field and any integer type containing the size
+of the actually allocated memory as the second field.
+  }];
+}
+
 def ReturnsNonNullDocs : Documentation {
   let Category = NullabilityDocs;
   let Content = [{

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3449,6 +3449,10 @@ def err_attribute_integers_only : Error<
 def warn_attribute_return_pointers_only : Warning<
   "%0 attribute only applies to return values that are pointers">,
   InGroup<IgnoredAttributes>;
+def warn_attribute_return_span_only
+    : Warning<"%0 attribute only applies to return values that are span-like "
+              "structures">,
+      InGroup<IgnoredAttributes>;
 def warn_attribute_return_pointers_refs_only : Warning<
   "%0 attribute only applies to return values that are pointers or references">,
   InGroup<IgnoredAttributes>;

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1839,6 +1839,38 @@ static void handleRestrictAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
                  RestrictAttr(S.Context, AL, DeallocE, DeallocPtrIdx));
 }
 
+static bool isSpanLikeType(const QualType &Ty) {
+  // Check that the type is a plain record with the first field being a pointer
+  // type and the second field being an integer. This matches the common
+  // implementation of std::span or sized_allocation_t in P0901R11.
+  // Note that there may also be numerous cases of pointer+integer structures
+  // not actually exhibiting a span-like semantics, so sometimes
+  // this heuristic expectedly leads to false positive results.
+  const RecordDecl *RD = Ty->getAsRecordDecl();
+  if (!RD || RD->isUnion())
+    return false;
+  const RecordDecl *Def = RD->getDefinition();
+  if (!Def)
+    return false; // This is an incomplete type.
+  auto FieldsBegin = Def->field_begin();
+  if (std::distance(FieldsBegin, Def->field_end()) != 2)
+    return false;
+  const FieldDecl *FirstField = *FieldsBegin;
+  const FieldDecl *SecondField = *std::next(FieldsBegin);
+  return FirstField->getType()->isAnyPointerType() &&
+         SecondField->getType()->isIntegerType();
+}
+
+static void handleMallocSpanAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  QualType ResultType = getFunctionOrMethodResultType(D);
+  if (!isSpanLikeType(ResultType)) {
+    S.Diag(AL.getLoc(), diag::warn_attribute_return_span_only)
+        << AL << getFunctionOrMethodResultSourceRange(D);
+    return;
+  }
+  D->addAttr(::new (S.Context) MallocSpanAttr(S.Context, AL));
+}
+
 static void handleCPUSpecificAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // Ensure we don't combine these with themselves, since that causes some
   // confusing behavior.
@@ -7278,6 +7310,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_Restrict:
     handleRestrictAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_MallocSpan:
+    handleMallocSpanAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_Mode:
     handleModeAttr(S, D, AL);
     break;

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

@@ -102,6 +102,7 @@
 // CHECK-NEXT: MIGServerRoutine (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_block)
 // CHECK-NEXT: MSConstexpr (SubjectMatchRule_function)
 // CHECK-NEXT: MSStruct (SubjectMatchRule_record)
+// CHECK-NEXT: MallocSpan (SubjectMatchRule_function)
 // CHECK-NEXT: MaybeUndef (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: MicroMips (SubjectMatchRule_function)
 // CHECK-NEXT: MinSize (SubjectMatchRule_function, SubjectMatchRule_objc_method)

clang/test/Sema/attr-malloc_span.c

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s
+// RUN: %clang_cc1 -emit-llvm -o %t %s
+
+#include <stddef.h>
+
+typedef struct {
+  void *ptr;
+  size_t n;
+} sized_ptr;
+sized_ptr  returns_sized_ptr  (void) __attribute((malloc_span)); // no-warning
+
+// The first struct field must be pointer and the second must be an integer.
+// Check the possible ways to violate it.
+typedef struct {
+  size_t n;
+  void *ptr;
+} invalid_span1;
+invalid_span1  returns_non_std_span1  (void) __attribute((malloc_span)); // expected-warning {{attribute only applies to return values that are span-like structures}}
+
+typedef struct {
+  void *ptr;
+  void *ptr2;
+} invalid_span2;
+invalid_span2  returns_non_std_span2  (void) __attribute((malloc_span)); // expected-warning {{attribute only applies to return values that are span-like structures}}
+
+typedef struct {
+  void *ptr;
+  size_t n;
+  size_t n2;
+} invalid_span3;
+invalid_span3  returns_non_std_span3  (void) __attribute((malloc_span)); // expected-warning {{attribute only applies to return values that are span-like structures}}