Re: [PATCH] c++: Avoid strict_aliasing_warning on dependent types or expressions [PR94951]

[Jason Merrill via Gcc-patches]

On 5/5/20 6:28 PM, Jakub Jelinek wrote:

Hi!

The following testcase gets a bogus warning during build_base_path,
when cp_build_indirect_ref* calls strict_aliasing_warning with a dependent
expression.  IMHO calling get_alias_set etc. on dependent types feels wrong
to me, we should just defer the warnings in those cases until instantiation
and only handle the cases where neither type nor expr are dependent.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

2020-05-05  Jakub Jelinek  <ja...@redhat.com>

        PR c++/94951
        * typeck.c (cp_strict_aliasing_warning): New function.
        (cp_build_indirect_ref_1, build_reinterpret_cast_1): Use
        it instead of strict_aliasing_warning.

        * g++.dg/warn/Wstrict-aliasing-bogus-tmpl.C: New test.

--- gcc/cp/typeck.c.jj  2020-03-19 22:54:38.393744684 +0100
+++ gcc/cp/typeck.c     2020-05-05 18:12:21.964090624 +0200
@@ -3318,6 +3318,22 @@ build_x_indirect_ref (location_t loc, tr
      return rval;
  }

+/* Like c-family strict_aliasing_warning, but don't warn for dependent

+   types or expressions.  */
+
+static bool
+cp_strict_aliasing_warning (location_t loc, tree type, tree expr)
+{
+  if (processing_template_decl)
+    {
+      tree e = expr;
+      STRIP_NOPS (e);
+      if (dependent_type_p (type) || type_dependent_expression_p (e))
+       return false;
+    }
+  return strict_aliasing_warning (loc, type, expr);
+}
+
  /* The implementation of the above, and of indirection implied by other
     constructs.  If DO_FOLD is true, fold away INDIRECT_REF of ADDR_EXPR.  */

@@ -3360,10 +3376,10 @@ cp_build_indirect_ref_1 (location_t loc,

          /* If a warning is issued, mark it to avoid duplicates from
             the backend.  This only needs to be done at
             warn_strict_aliasing > 2.  */
-         if (warn_strict_aliasing > 2)
-           if (strict_aliasing_warning (EXPR_LOCATION (ptr),
-                                        type, TREE_OPERAND (ptr, 0)))
-             TREE_NO_WARNING (ptr) = 1;
+         if (warn_strict_aliasing > 2
+             && cp_strict_aliasing_warning (EXPR_LOCATION (ptr),
+                                            type, TREE_OPERAND (ptr, 0)))
+           TREE_NO_WARNING (ptr) = 1;
        }

if (VOID_TYPE_P (t))

@@ -7777,7 +7793,7 @@ build_reinterpret_cast_1 (location_t loc
        expr = cp_build_addr_expr (expr, complain);

if (warn_strict_aliasing > 2)

-       strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);
+       cp_strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);

if (expr != error_mark_node)

        expr = build_reinterpret_cast_1
@@ -7891,7 +7907,7 @@ build_reinterpret_cast_1 (location_t loc

if (warn_strict_aliasing <= 2)

        /* strict_aliasing_warning STRIP_NOPs its expr.  */
-       strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);
+       cp_strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);

return build_nop_reinterpret (type, expr);

      }
--- gcc/testsuite/g++.dg/warn/Wstrict-aliasing-bogus-tmpl.C.jj  2020-05-05 
18:18:43.408320000 +0200
+++ gcc/testsuite/g++.dg/warn/Wstrict-aliasing-bogus-tmpl.C     2020-05-05 
18:17:35.178352212 +0200
@@ -0,0 +1,12 @@
+// PR c++/94951
+// { dg-do compile }
+// { dg-options "-O2 -Wall" }
+
+struct A { int a; };
+template <int N>
+struct B : public A
+{
+  static B<N> foo () { B<N> t; t.a = 4; return t; }        // { dg-bogus 
"dereferencing type-punned pointer will break strict-aliasing rules" }
+};
+
+B<0> b = B<0>::foo ();

        Jakub