Hi Martin,
On Sat, Nov 09, 2024 at 06:15:35PM GMT, Martin Uecker wrote:
>
> This patch enables the Wzero-as-null-pointer-constant for C.
> The second version added more tests and fixes one condition
> to not incorrectly warn for nullptr.
>
>
> Bootstrapped and regression tested on x86_64.
>
>
> c: add Wzero-as-null-pointer-constant [PR117059]
>
> Add warnings for the use of zero as a null pointer constant to the C FE.
>
> PR c/117059
>
> gcc/c-family/ChangeLog:
> * c.opt (Wzero-as-null-pointer-constant): Enable for C and ObjC.
>
> gcc/c/ChangeLog:
> * c-typeck.cc (parse_build_binary_op): Add warning.
> (build_conditional_expr): Add warning.
> (convert_for_assignment): Add warning.
>
> gcc/testsuite/ChangeLog:
> * gcc.dg/Wzero-as-null-pointer-constant.c: New test.
>
>
> Suggested-by: Alejandro Colomar <a...@kernel.org>
> Acked-by: Alejandro Colomar <a...@kernel.org>
>
>
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 9b9f5e744f6..b4e967ce000 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -1632,7 +1632,7 @@ C ObjC C++ ObjC++ Var(warn_xor_used_as_pow) Warning
> Init(1)
> Warn about xor operators where it appears the user meant exponentiation.
>
> Wzero-as-null-pointer-constant
> -C++ ObjC++ Var(warn_zero_as_null_pointer_constant) Warning
> +C ObjC C++ ObjC++ Var(warn_zero_as_null_pointer_constant) Warning
> Warn when a literal '0' is used as null pointer.
>
> Wzero-length-bounds
> diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
> index 3fdf3437bf5..ced58c2be84 100644
> --- a/gcc/c/c-typeck.cc
> +++ b/gcc/c/c-typeck.cc
> @@ -4600,15 +4600,24 @@ parser_build_binary_op (location_t location, enum
> tree_code code,
> "comparison with string literal results in unspecified "
> "behavior");
>
> - if (true
> - && TREE_CODE_CLASS (code) == tcc_comparison
> - && ((TREE_CODE (type1) == POINTER_TYPE
> - && TREE_CODE (type2) == INTEGER_TYPE
> - && null_pointer_constant_p (arg2.value))
> - || (TREE_CODE (type2) == POINTER_TYPE
> - && TREE_CODE (type1) == INTEGER_TYPE
> - && null_pointer_constant_p (arg1.value))))
> - warning_at (location, 0, "zero as null pointer constant");
> + if (warn_zero_as_null_pointer_constant
> + && c_inhibit_evaluation_warnings == 0
> + && TREE_CODE_CLASS (code) == tcc_comparison)
> + {
> + if ((TREE_CODE (type1) == POINTER_TYPE
> + || TREE_CODE (type1) == NULLPTR_TYPE)
> + && TREE_CODE (type2) == INTEGER_TYPE
> + && null_pointer_constant_p (arg2.value))
> + warning_at (arg2.get_location(), OPT_Wzero_as_null_pointer_constant,
> + "zero as null pointer constant");
> +
> + if ((TREE_CODE (type2) == POINTER_TYPE
> + || TREE_CODE (type2) == NULLPTR_TYPE)
> + && TREE_CODE (type1) == INTEGER_TYPE
> + && null_pointer_constant_p (arg1.value))
> + warning_at (arg1.get_location(), OPT_Wzero_as_null_pointer_constant,
> + "zero as null pointer constant");
> + }
>
> if (warn_array_compare
> && TREE_CODE_CLASS (code) == tcc_comparison
> @@ -5975,6 +5984,20 @@ build_conditional_expr (location_t colon_loc, tree
> ifexp, bool ifexp_bcp,
> t1, t2);
> }
>
> + if (warn_zero_as_null_pointer_constant
> + && c_inhibit_evaluation_warnings == 0)
> + {
> + if ((code1 == POINTER_TYPE || code1 == NULLPTR_TYPE)
> + && code2 == INTEGER_TYPE && null_pointer_constant_p (orig_op2))
> + warning_at (op2_loc, OPT_Wzero_as_null_pointer_constant,
> + "zero as null pointer constant");
> +
> + if ((code2 == POINTER_TYPE || code2 == NULLPTR_TYPE)
> + && code1 == INTEGER_TYPE && null_pointer_constant_p (orig_op1))
> + warning_at (op1_loc, OPT_Wzero_as_null_pointer_constant,
> + "zero as null pointer constant");
> + }
> +
> /* Quickly detect the usual case where op1 and op2 have the same type
> after promotion. */
> if (TYPE_MAIN_VARIANT (type1) == TYPE_MAIN_VARIANT (type2))
> @@ -8406,8 +8429,10 @@ convert_for_assignment (location_t location,
> location_t expr_loc, tree type,
> || coder == NULLPTR_TYPE
> || coder == BITINT_TYPE))
> {
> - if (null_pointer_constant)
> - warning_at (location, 0, "zero as null pointer constant");
> + if (null_pointer_constant && c_inhibit_evaluation_warnings == 0
> + && coder == INTEGER_TYPE)
> + warning_at (location, OPT_Wzero_as_null_pointer_constant,
> + "zero as null pointer constant");
>
> /* An explicit constant 0 or type nullptr_t can convert to a pointer,
> or one that results from arithmetic, even including a cast to
> diff --git a/gcc/testsuite/gcc.dg/Wzero-as-null-pointer-constant.c
> b/gcc/testsuite/gcc.dg/Wzero-as-null-pointer-constant.c
> new file mode 100644
> index 00000000000..372756faeb9
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/Wzero-as-null-pointer-constant.c
> @@ -0,0 +1,37 @@
> +/* { dg-do compile } */
> +/* { dg-options "-std=c23 -Wzero-as-null-pointer-constant" } */
> +
> +#define NULL (void*)0
> +
> +void foo(void*);
> +void bar()
> +{
> + foo(0); /* { dg-warning "zero as null pointer
> constant" } */
> + foo(NULL);
> + foo(nullptr);
> +
> + void *p = 0; /* { dg-warning "zero as null pointer
> constant" } */
> + void *r = NULL;
> + void *t = nullptr;
> + void *q = { 0 }; /* { dg-warning "zero as null pointer
> constant" } */
> + void *s = { NULL };
> + void *u = { nullptr };
> + struct { void *q; } x = { 0 }; /* { dg-warning "zero as null pointer
> constant" } */
> + struct { void *q; } y = { NULL };
> + struct { void *q; } z = { nullptr };
> + struct { int a; void *b; } a = { 0 };
> + struct { int a; int b; void *c; } b = { 0, 0 };
> +
> + 1 ? 0 : p; /* { dg-warning "zero as null pointer
> constant" } */
> + 1 ? p : 0; /* { dg-warning "zero as null pointer
> constant" } */
> + 1 ? 0 : NULL; /* { dg-warning "zero as null pointer
> constant" } */
> + 1 ? NULL : 0; /* { dg-warning "zero as null pointer
> constant" } */
> +
> + if (p == 0); /* { dg-warning "zero as null pointer
> constant" } */
> + if (0 == p); /* { dg-warning "zero as null pointer
> constant" } */
> + if (NULL == 0); /* { dg-warning "zero as null pointer
> constant" } */
> + if (0 == NULL); /* { dg-warning "zero as null pointer
> constant" } */
> + if (0 == nullptr); /* { dg-warning "zero as null pointer
> constant" } */
> + if (nullptr == 0); /* { dg-warning "zero as null pointer
> constant" } */
Maybe we should also check a few obvious cases that shouldn't be warned
because 0 is an int? If some bug causes these to trigger, we'd know
something is very wrong.
if (0 == 0);
1 ? 0 : 0;
int i = 0;
It might also be interesting to have at least one test where we use a
hardcoded (void*)0 without the name NULL.
Cheers,
Alex
> +}
> +
>
--
<https://www.alejandro-colomar.es/>
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