Re: [PATCH] Emit -Waddress warnings for comparing address of reference against NULL

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Sun, Apr 26, 2015 at 8:56 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> Here is an updated version of the patch with, hopefully, all your
> suggestions made.  I decided to add calls to STRIP_NOPS before emitting
> the warning so that we properly warn for cases where there's a cast in
> between the whole thing, e.g.
>
>   if (!&(int &)a)
>
> I also added guards to emit the warnings only when the stripped operand
> is actually a decl so that we don't pass a non-decl argument to
> warning_at() which can happen in a case like
>
>   if (!&(int &)*(int *)0)
>
> Does this look OK now after testing?
>
> gcc/c-family/ChangeLog:
>
>         PR c++/65168
>         * c-common.c (c_common_truthvalue_conversion): Warn when
>         converting an address of a reference to a truth value.
>
> gcc/cp/ChangeLog:
>
>         PR c++/65168
>         * typeck.c (cp_build_binary_op): Warn when comparing an address
>         of a reference against NULL.
>
> gcc/testsuite/ChangeLog:
>
>         PR c++/65168
>         g++.dg/warn/Walways-true-3.C: New test.
> ---
>  gcc/c-family/c-common.c                    | 14 ++++++++++
>  gcc/cp/typeck.c                            | 34 ++++++++++++++++++++++
>  gcc/testsuite/g++.dg/warn/Walways-true-3.C | 45 ++++++++++++++++++++++++++++++
>  3 files changed, 93 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/warn/Walways-true-3.C
>
> diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
> index 9797e17..c353c72 100644
> --- a/gcc/c-family/c-common.c
> +++ b/gcc/c-family/c-common.c
> @@ -4806,6 +4806,20 @@ c_common_truthvalue_conversion (location_t location, tree expr)
>         tree totype = TREE_TYPE (expr);
>         tree fromtype = TREE_TYPE (TREE_OPERAND (expr, 0));
>
> +       if (POINTER_TYPE_P (totype)
> +           && TREE_CODE (fromtype) == REFERENCE_TYPE)
> +         {
> +           tree inner = expr;
> +           STRIP_NOPS (inner);
> +
> +           if (DECL_P (inner))
> +             warning_at (location,
> +                         OPT_Waddress,
> +                         "the compiler can assume that the address of "
> +                         "%qD will always evaluate to %<true%>",
> +                         inner);
> +         }
> +
>         /* Don't cancel the effect of a CONVERT_EXPR from a REFERENCE_TYPE,
>            since that affects how `default_conversion' will behave.  */
>         if (TREE_CODE (totype) == REFERENCE_TYPE
> diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
> index 91db32a..13fb401 100644
> --- a/gcc/cp/typeck.c
> +++ b/gcc/cp/typeck.c
> @@ -4423,6 +4423,23 @@ cp_build_binary_op (location_t location,
>                 warning (OPT_Waddress, "the address of %qD will never be NULL",
>                          TREE_OPERAND (op0, 0));
>             }
> +
> +         if (CONVERT_EXPR_P (op0)
> +             && TREE_CODE (TREE_TYPE (TREE_OPERAND (op0, 0)))
> +                == REFERENCE_TYPE)
> +           {
> +             tree inner_op0 = op0;
> +             STRIP_NOPS (inner_op0);
> +
> +             if ((complain & tf_warning)
> +                 && c_inhibit_evaluation_warnings == 0
> +                 && !TREE_NO_WARNING (op0)
> +                 && DECL_P (inner_op0))
> +               warning (OPT_Waddress,
> +                        "the compiler can assume that the address of "
> +                        "%qD will never be NULL",
> +                        inner_op0);
> +           }
>         }
>        else if (((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
>                 && null_ptr_cst_p (op0))
> @@ -4445,6 +4462,23 @@ cp_build_binary_op (location_t location,
>                 warning (OPT_Waddress, "the address of %qD will never be NULL",
>                          TREE_OPERAND (op1, 0));
>             }
> +
> +         if (CONVERT_EXPR_P (op1)
> +             && TREE_CODE (TREE_TYPE (TREE_OPERAND (op1, 0)))
> +                == REFERENCE_TYPE)
> +           {
> +             tree inner_op1 = op1;
> +             STRIP_NOPS (inner_op1);
> +
> +             if ((complain & tf_warning)
> +                 && c_inhibit_evaluation_warnings == 0
> +                 && !TREE_NO_WARNING (op1)
> +                 && DECL_P (inner_op1))
> +               warning (OPT_Waddress,
> +                        "the compiler can assume that the address of "
> +                        "%qD will never be NULL",
> +                        inner_op1);
> +           }
>         }
>        else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
>                || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
> diff --git a/gcc/testsuite/g++.dg/warn/Walways-true-3.C b/gcc/testsuite/g++.dg/warn/Walways-true-3.C
> new file mode 100644
> index 0000000..0d13d3f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Walways-true-3.C
> @@ -0,0 +1,45 @@
> +// { dg-options "-Waddress" }
> +
> +void foo (void);
> +
> +int d;
> +int &c = d;
> +
> +void
> +bar (int &a)
> +{
> +  int &b = a;
> +
> +  if ((int *)&a) // { dg-warning "address of" }
> +    foo ();
> +
> +  if (&b) // { dg-warning "address of" }
> +    foo ();
> +
> +  if (!&c) // { dg-warning "address of" }
> +    foo ();
> +
> +  if (!&(int &)(int &)a) // { dg-warning "address of" }
> +    foo ();
> +
> +  if (&a == 0) // { dg-warning "never be NULL" }
> +    foo ();
> +
> +  if (&b != 0) // { dg-warning "never be NULL" }
> +    foo ();
> +
> +  if (0 == &(int &)(int &)c) // { dg-warning "never be NULL" }
> +    foo ();
> +
> +  if (&a != (int *)0) // { dg-warning "never be NULL" }
> +    foo ();
> +}
> +
> +bool
> +bar_1 (int &a)
> +{
> +  if (d == 5)
> +    return &a; // { dg-warning "address of" }
> +  else
> +    return !&(int &)(int &)a; // { dg-warning "address of" }
> +}
> --
> 2.4.0.rc2.31.g7c597ef
>

Ping.