[PATCH] D50805: Don't warn on returning the address of a label

[Reid Kleckner via Phabricator via cfe-commits]

rnk created this revision.
rnk added reviewers: niravd, rsmith, nickdesaulniers.

There aren't any lifetime issues inherent in returning a local label.
Hypothetically, it could be used to drive a state machine driven by
computed goto the next time that same scope is re-entered. In any case,
the Linux kernel uses this code pattern to get PCs, and we don't want to
warn on that code pattern.

Fixes PR38569


https://reviews.llvm.org/D50805

Files:
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/Sema/SemaInit.cpp
  clang/test/Analysis/stack-addr-ps.cpp
  clang/test/Sema/statements.c


Index: clang/test/Sema/statements.c
===================================================================
--- clang/test/Sema/statements.c
+++ clang/test/Sema/statements.c
@@ -29,9 +29,22 @@
 }
 
 
-void *test10() { 
+// PR38569: Clang used to warn on returning the address of a label, but we don't
+// anymore. Labels aren't exactly destroyed when they go out of scope, and the
+// Linux kernel uses this functionality.
+void *test10() {
 bar:
-  return &&bar;  // expected-warning {{returning address of label, which is local}}
+  return &&bar;
+}
+
+// Linux actually does something more like this, and we don't want to warn on
+// it.
+void test10_logpc(void*);
+void test10a() {
+  test10_logpc(({
+    my_pc:
+      &&my_pc;
+  }));
 }
 
 // PR6034
Index: clang/test/Analysis/stack-addr-ps.cpp
===================================================================
--- clang/test/Analysis/stack-addr-ps.cpp
+++ clang/test/Analysis/stack-addr-ps.cpp
@@ -74,10 +74,12 @@
   return &x; // expected-warning{{Address of stack memory associated with local variable 's1' returned}} expected-warning {{address of stack memory associated with local variable 's1' returned}}
 }
 
+// PR38569: Clang used to warn when returning label addresses, but now it
+// doesn't.
 void *lf() {
     label:
-    void *const &x = &&label; // expected-note {{binding reference variable 'x' here}}
-    return x; // expected-warning {{returning address of label, which is local}}
+    void *const &x = &&label;
+    return x;
 }
 
 template <typename T>
Index: clang/lib/Sema/SemaInit.cpp
===================================================================
--- clang/lib/Sema/SemaInit.cpp
+++ clang/lib/Sema/SemaInit.cpp
@@ -6291,7 +6291,6 @@
 /// A temporary or local variable. This will be one of:
 ///  * A MaterializeTemporaryExpr.
 ///  * A DeclRefExpr whose declaration is a local.
-///  * An AddrLabelExpr.
 ///  * A BlockExpr for a block with captures.
 using Local = Expr*;
 
@@ -6735,11 +6734,6 @@
     }
     break;
 
-  case Stmt::AddrLabelExprClass:
-    // We want to warn if the address of a label would escape the function.
-    Visit(Path, Local(cast<AddrLabelExpr>(Init)), RK_ReferenceBinding);
-    break;
-
   default:
     break;
   }
@@ -6923,8 +6917,6 @@
             << isa<ParmVarDecl>(DRE->getDecl()) << DiagRange;
       } else if (isa<BlockExpr>(L)) {
         Diag(DiagLoc, diag::err_ret_local_block) << DiagRange;
-      } else if (isa<AddrLabelExpr>(L)) {
-        Diag(DiagLoc, diag::warn_ret_addr_label) << DiagRange;
       } else {
         Diag(DiagLoc, diag::warn_ret_local_temp_addr_ref)
          << Entity.getType()->isReferenceType() << DiagRange;
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7869,9 +7869,6 @@
 def warn_ret_local_temp_addr_ref : Warning<
   "returning %select{address of|reference to}0 local temporary object">,
   InGroup<ReturnStackAddress>;
-def warn_ret_addr_label : Warning<
-  "returning address of label, which is local">,
-  InGroup<ReturnStackAddress>;
 def err_ret_local_block : Error<
   "returning block that lives on the local stack">;
 def note_local_var_initializer : Note<


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