In r11-6900 (PR 98646 - static_cast confuses -Wnonnull) we decided that issuing -Wnonnull for dereferencing the result of dynamic_cast was helpful despite the false positives it causes when the pointer is guaranteed not to be null because of a prior test.
The test case in PR 99251 along with the feedback I got from Martin Liska have convinced me it was not the right decision. The attached patch arranges for dynamic_cast to also suppress -Wnonnull analogously to static_cast. Since there already is a helper function that builds the if-not-null test (ifnonnull) and sets TREE_NO_WARNING, I factored out the corresponding code from build_base_path that sets the additional TREE_NO_WARNING bit for static_cast into the function and called it from both places. I also renamed the function to make its purpose clearer and for consistency with other build_xxx APIs. Tested on x86_64-linux. Martin
PR c++/99251 - inconsistent -Wnonnull warning behaviour with dynamic_cast gcc/cp/ChangeLog: PR c++/99251 * class.c (build_base_path): Call build_nonnull_test. * cp-tree.h (build_nonnull_test): Declare. * rtti.c (ifnonnull): Rename... (build_nonnull_test): ...to this. Set no-warning bit on COND_EXPR. (build_dynamic_cast_1): Adjust to name change. gcc/testsuite/ChangeLog: PR c++/99251 * g++.dg/warn/Wnonnull9.C: Expect no warnings. * g++.dg/warn/Wnonnull12.C: New test. diff --git a/gcc/cp/class.c b/gcc/cp/class.c index 40f5fef7baa..6c6e0564bf9 100644 --- a/gcc/cp/class.c +++ b/gcc/cp/class.c @@ -402,16 +402,9 @@ build_base_path (enum tree_code code, if (TREE_SIDE_EFFECTS (expr) && (null_test || virtual_access)) expr = save_expr (expr); - /* Now that we've saved expr, build the real null test. */ + /* Store EXPR and build the real null test just before returning. */ if (null_test) - { - tree zero = cp_convert (TREE_TYPE (expr), nullptr_node, complain); - null_test = build2_loc (input_location, NE_EXPR, boolean_type_node, - expr, zero); - /* This is a compiler generated comparison, don't emit - e.g. -Wnonnull-compare warning for it. */ - TREE_NO_WARNING (null_test) = 1; - } + null_test = expr; /* If this is a simple base reference, express it as a COMPONENT_REF. */ if (code == PLUS_EXPR && !virtual_access @@ -516,14 +509,8 @@ build_base_path (enum tree_code code, out: if (null_test) - { - expr = fold_build3_loc (input_location, COND_EXPR, target_type, null_test, - expr, build_zero_cst (target_type)); - /* Avoid warning for the whole conditional expression (in addition - to NULL_TEST itself -- see above) in case the result is used in - a nonnull context that the front end -Wnonnull checks. */ - TREE_NO_WARNING (expr) = 1; - } + /* Wrap EXPR in a null test. */ + expr = build_nonnull_test (null_test, expr, complain); return expr; } diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h index 38b31e3908f..8c6cda8d1a6 100644 --- a/gcc/cp/cp-tree.h +++ b/gcc/cp/cp-tree.h @@ -7271,6 +7271,7 @@ extern void emit_support_tinfos (void); extern bool emit_tinfo_decl (tree); extern unsigned get_pseudo_tinfo_index (tree); extern tree get_pseudo_tinfo_type (unsigned); +extern tree build_nonnull_test (tree, tree, tsubst_flags_t); /* in search.c */ extern tree get_parent_with_private_access (tree decl, tree binfo); diff --git a/gcc/cp/rtti.c b/gcc/cp/rtti.c index b41d95469c6..84482743392 100644 --- a/gcc/cp/rtti.c +++ b/gcc/cp/rtti.c @@ -121,7 +121,6 @@ vec<tree, va_gc> *unemitted_tinfo_decls; and are generated as needed. */ static GTY (()) vec<tinfo_s, va_gc> *tinfo_descs; -static tree ifnonnull (tree, tree, tsubst_flags_t); static tree tinfo_name (tree, bool); static tree build_dynamic_cast_1 (location_t, tree, tree, tsubst_flags_t); static tree throw_bad_cast (void); @@ -529,16 +528,23 @@ get_typeid (tree type, tsubst_flags_t complain) /* Check whether TEST is null before returning RESULT. If TEST is used in RESULT, it must have previously had a save_expr applied to it. */ -static tree -ifnonnull (tree test, tree result, tsubst_flags_t complain) +tree +build_nonnull_test (tree test, tree result, tsubst_flags_t complain) { - tree cond = build2 (NE_EXPR, boolean_type_node, test, - cp_convert (TREE_TYPE (test), nullptr_node, complain)); + tree null_ptr = cp_convert (TREE_TYPE (test), nullptr_node, complain); + tree cond = build2 (NE_EXPR, boolean_type_node, test, null_ptr); + /* This is a compiler generated comparison, don't emit e.g. -Wnonnull-compare warning for it. */ TREE_NO_WARNING (cond) = 1; - return build3 (COND_EXPR, TREE_TYPE (result), cond, result, - cp_convert (TREE_TYPE (result), nullptr_node, complain)); + + null_ptr = cp_convert (TREE_TYPE (result), nullptr_node, complain); + cond = build3 (COND_EXPR, TREE_TYPE (result), cond, result, null_ptr); + + /* Likewise, don't emit -Wnonnull for using the result to call + a member function. */ + TREE_NO_WARNING (cond) = 1; + return cond; } /* Execute a dynamic cast, as described in section 5.2.6 of the 9/93 working @@ -671,7 +677,7 @@ build_dynamic_cast_1 (location_t loc, tree type, tree expr, expr1 = build_headof (expr); if (TREE_TYPE (expr1) != type) expr1 = build1 (NOP_EXPR, type, expr1); - return ifnonnull (expr, expr1, complain); + return build_nonnull_test (expr, expr1, complain); } else { @@ -786,7 +792,7 @@ build_dynamic_cast_1 (location_t loc, tree type, tree expr, /* Now back to the type we want from a void*. */ result = cp_convert (type, result, complain); - return ifnonnull (expr, result, complain); + return build_nonnull_test (expr, result, complain); } } else diff --git a/gcc/testsuite/g++.dg/warn/Wnonnull12.C b/gcc/testsuite/g++.dg/warn/Wnonnull12.C new file mode 100644 index 00000000000..7b2606302f5 --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wnonnull12.C @@ -0,0 +1,29 @@ +/* PR c++/99251 - inconsistent -Wnonnull warning behaviour with dynamic_cast + { dg-do compile } + { dg-options "-Wall" } */ + +struct A +{ + virtual ~A (); +}; + +struct B: A +{ + int f (int); +}; + +int f1 (A *p) +{ + if (!p) + return 0; + + return (dynamic_cast<B *>(p))->f (1); +} + +int f2 (A *p) +{ + if (!p) + return 0; + + return dynamic_cast<B *>(p)->f (2); // { dg-bogus "\\\[-Wnonnull" } +} diff --git a/gcc/testsuite/g++.dg/warn/Wnonnull9.C b/gcc/testsuite/g++.dg/warn/Wnonnull9.C index b6135c4a624..e7c0dda2502 100644 --- a/gcc/testsuite/g++.dg/warn/Wnonnull9.C +++ b/gcc/testsuite/g++.dg/warn/Wnonnull9.C @@ -38,13 +38,16 @@ void static_cast_const_C_ptr (B *p) void dynamic_cast_C_ptr (B *p) { - // The dynamic_cast might fail so a warning is justified. - dynamic_cast<C*>(p->bptr ())->g (); // { dg-warning "\\\[-Wnonnull" } + /* The dynamic_cast might fail but so a warning might be justified + but not when the operand is guarded. See pr99251. */ + if (B *q = p->bptr ()) + dynamic_cast<C*>(q)->g (); // { dg-bogus "\\\[-Wnonnull" } } void dynamic_cast_const_C_ptr (B *p) { - dynamic_cast<const C*>(p->bptr ())->g (); // { dg-warning "\\\[-Wnonnull" } + if (B *q = p->bptr ()) + dynamic_cast<const C*>(q)->g (); // { dg-bogus "\\\[-Wnonnull" } } @@ -107,11 +110,14 @@ void static_cast_const_D_ptr (B *p) void dynamic_cast_D_ptr (B *p) { - // The dynamic_cast might fail so a warning is justified. - dynamic_cast<D*>(p->bptr ())->g (); // { dg-warning "\\\[-Wnonnull" } + /* The dynamic_cast might fail but so a warning might be justified + but not when the operand is guarded. See pr99251. */ + if (B *q = p->bptr ()) + dynamic_cast<D*>(q)->g (); // { dg-bogus "\\\[-Wnonnull" } } void dynamic_cast_const_D_ptr (B *p) { - dynamic_cast<const D*>(p->bptr ())->g (); // { dg-warning "\\\[-Wnonnull" } + if (B *q = p->bptr ()) + dynamic_cast<const D*>(q)->g (); // { dg-bogus "\\\[-Wnonnull" } }