[clang] Fix gnu::init_priority attribute handling for reserved values (#121577)

- Added a new diagnostic group `InitPriorityReserved`
- Allow values within the range 0-100 of `init_priority` to be used
outside system library, but with a warning
- Updated relavant tests

Fixes #121108

clang/docs/ReleaseNotes.rst

@@ -147,6 +147,9 @@ related warnings within the method body.
   ``__attribute__((model("large")))`` on non-TLS globals in x86-64 compilations.
   This forces the global to be considered small or large in regards to the
   x86-64 code model, regardless of the code model specified for the compilation.
+- Clang now emits a warning ``-Wreserved-init-priority`` instead of a hard error 
+  when ``__attribute__((init_priority(n)))`` is used with values of n in the 
+  reserved range [0, 100]. The warning will be treated as an error by default.
 
 - There is a new ``format_matches`` attribute to complement the existing
   ``format`` attribute. ``format_matches`` allows the compiler to verify that

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3343,6 +3343,9 @@ def err_attribute_argument_out_of_range : Error<
 def err_init_priority_object_attr : Error<
   "can only use 'init_priority' attribute on file-scope definitions "
   "of objects of class type">;
+def warn_init_priority_reserved : Warning<
+  "requested 'init_priority' %0 is reserved for internal use">,
+  InGroup<DiagGroup<"init-priority-reserved">>, DefaultError;
 def err_attribute_argument_out_of_bounds : Error<
   "%0 attribute parameter %1 is out of bounds">;
 def err_attribute_only_once_per_parameter : Error<

clang/lib/Sema/SemaDeclAttr.cpp

@@ -3720,16 +3720,18 @@ static void handleInitPriorityAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     return;
   }
 
-  // Only perform the priority check if the attribute is outside of a system
-  // header. Values <= 100 are reserved for the implementation, and libc++
-  // benefits from being able to specify values in that range.
-  if ((prioritynum < 101 || prioritynum > 65535) &&
-      !S.getSourceManager().isInSystemHeader(AL.getLoc())) {
+  if (prioritynum > 65535) {
     S.Diag(AL.getLoc(), diag::err_attribute_argument_out_of_range)
-        << E->getSourceRange() << AL << 101 << 65535;
+        << E->getSourceRange() << AL << 0 << 65535;
     AL.setInvalid();
     return;
   }
+
+  // Values <= 100 are reserved for the implementation, and libc++
+  // benefits from being able to specify values in that range.
+  if (prioritynum < 101)
+    S.Diag(AL.getLoc(), diag::warn_init_priority_reserved)
+        << E->getSourceRange() << prioritynum;
   D->addAttr(::new (S.Context) InitPriorityAttr(S.Context, AL, prioritynum));
 }
 

clang/test/SemaCXX/init-priority-attr.cpp

@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -triple=x86_64-unknown-unknown -fsyntax-only -verify %s
 // RUN: %clang_cc1 -triple=x86_64-unknown-unknown -fsyntax-only -DSYSTEM -verify %s
+// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -fsyntax-only -DNOERROR -Wno-error=init-priority-reserved -verify %s
 // RUN: %clang_cc1 -triple=s390x-none-zos -fsyntax-only -verify=unknown %s
 // RUN: %clang_cc1 -triple=s390x-none-zos -fsyntax-only -DSYSTEM -verify=unknown-system %s
 
@@ -24,24 +25,32 @@ extern Two goo;
 extern Two coo[];
 extern Two koo[];
 
+// unknown-system-no-diagnostics
+
 Two foo __attribute__((init_priority(101))) ( 5, 6 );
- // unknown-system-no-diagnostics
- // unknown-warning@-2 {{unknown attribute 'init_priority' ignored}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
+
+Two loo __attribute__((init_priority(65535))) ( 5, 6 );
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 
 Two goo __attribute__((init_priority(2,3))) ( 5, 6 ); // expected-error {{'init_priority' attribute takes one argument}}
 // unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 
 Two coo[2]  __attribute__((init_priority(100)));
 #if !defined(SYSTEM)
-  // expected-error@-2 {{'init_priority' attribute requires integer constant between 101 and 65535 inclusive}}
-  // unknown-warning@-3 {{unknown attribute 'init_priority' ignored}}
-#endif
+#if !defined(NOERROR)
+  // expected-error@-3 {{requested 'init_priority' 100 is reserved for internal use}}
+#else  // defined(NOERROR)
+  // expected-warning@-5 {{requested 'init_priority' 100 is reserved for internal use}}
+#endif // !defined(NOERROR)
+  // unknown-warning@-7 {{unknown attribute 'init_priority' ignored}}
+#endif // !defined(SYSTEM)
 
-Two boo[2]  __attribute__((init_priority(65536)));
-#if !defined(SYSTEM)
- // expected-error@-2 {{'init_priority' attribute requires integer constant between 101 and 65535 inclusive}}
- // unknown-warning@-3 {{unknown attribute 'init_priority' ignored}}
-#endif
+Two zoo[2]  __attribute__((init_priority(-1))); // expected-error {{'init_priority' attribute requires integer constant between 0 and 65535 inclusive}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
+
+Two boo[2]  __attribute__((init_priority(65536))); // expected-error {{'init_priority' attribute requires integer constant between 0 and 65535 inclusive}}
+// unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 
 Two koo[4]  __attribute__((init_priority(1.13))); // expected-error {{'init_priority' attribute requires an integer constant}}
 // unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
@@ -49,7 +58,6 @@ Two koo[4]  __attribute__((init_priority(1.13))); // expected-error {{'init_prio
 Two func()  __attribute__((init_priority(1001))); // expected-error {{'init_priority' attribute only applies to variables}}
 // unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}
 
-
 int i  __attribute__((init_priority(1001))); // expected-error {{can only use 'init_priority' attribute on file-scope definitions of objects of class type}}
 // unknown-warning@-1 {{unknown attribute 'init_priority' ignored}}