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From: Tatsuo Ishii <[email protected]>
To: [email protected]
To: [email protected]
Cc: [email protected]
Subject: Re: warning: dereferencing type-punned pointer
Date: Fri, 26 Jul 2024 16:49:10 +0900 (JST)
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

Sorry, I forgot to attach the file...

> I don't agree. I think the compiler just dislike that nodeTag macro's
> argument is a pointer created by '&' operator in this case:
> 
> #define nodeTag(nodeptr)  (((const Node*)(nodeptr))->type)
> 
> If we just give a pointer variable either it's type is Node * or
> ErrorSaveContext * to nodeTag macro, the compiler becomes happy.
> 
> Moreover I think whether the object is inline or not is
> irrelevant. Attached is a self contained test case. In the program:
> 
> 	if (IsA(&f, List))
> 
> produces the strict aliasing rule violation but
> 
> 	if (IsA(fp, List))
> 
> does not. Here "f" is an object defined as:
> 
> typedef struct Foo
> {
> 	NodeTag		type;
> 	int		d;
> } Foo;
> 
> Foo f;
> 
> and fp is defined as:
> 
> 	Foo	*fp = &f;
> 
> $ gcc -Wall -O2 -c strict2.c
> strict2.c: In function ‘sub’:
> strict2.c:1:29: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
>     1 | #define nodeTag(nodeptr)  (((const Node*)(nodeptr))->type)
>       |                            ~^~~~~~~~~~~~~~~~~~~~~~~
> strict2.c:2:31: note: in expansion of macro ‘nodeTag’
>     2 | #define IsA(nodeptr,_type_)  (nodeTag(nodeptr) == T_##_type_)
>       |                               ^~~~~~~
> strict2.c:26:6: note: in expansion of macro ‘IsA’
>    26 |  if (IsA(&f, List))
>       |      ^~~
> At top level:
> strict2.c:21:12: warning: ‘sub’ defined but not used [-Wunused-function]
>    21 | static int sub(void)
>       |            ^~~
> 
>> If you change the member to a pointer
>> 
>> -   ErrorSaveContext escontext;
>> +   ErrorSaveContext *escontext;
>>  } JsonExprState;
>> 
>> and make the required adjustments elsewhere in the code, the warning
>> goes away.
> 
> I think this is not necessary. Just my patch in the upthread is enough.
> 
> Best reagards,
> --
> Tatsuo Ishii
> SRA OSS LLC
> English: http://www.sraoss.co.jp/index_en/
> Japanese:http://www.sraoss.co.jp

/*
 * Minimum definitions copied from PostgreSQL to make the
 * test self-contained.
 */
#define nodeTag(nodeptr)  (((const Node*)(nodeptr))->type)
#define IsA(nodeptr,_type_)  (nodeTag(nodeptr) == T_##_type_)

typedef enum NodeTag
{
	T_Invalid = 0,
	T_List = 1
} NodeTag;

typedef struct Node
{
	NodeTag		type;
} Node;

/* Home brew node */
typedef struct Foo
{
	NodeTag		type;
	int		d;
} Foo;

static int	sub(void)
{
	Foo	f;
	Foo	*fp = &f;
	f.type = T_List;

	/* strict aliasing rule error */
	if (IsA(&f, List))
		return 1;

	/* This is ok */
	if (IsA(fp, List))
		return 1;
	return 0;
}

	


Attachments:

  [text/plain] strict2.c (594B, ../[email protected]/2-strict2.c)
  download | inline:
/*
 * Minimum definitions copied from PostgreSQL to make the
 * test self-contained.
 */
#define nodeTag(nodeptr)  (((const Node*)(nodeptr))->type)
#define IsA(nodeptr,_type_)  (nodeTag(nodeptr) == T_##_type_)

typedef enum NodeTag
{
	T_Invalid = 0,
	T_List = 1
} NodeTag;

typedef struct Node
{
	NodeTag		type;
} Node;

/* Home brew node */
typedef struct Foo
{
	NodeTag		type;
	int		d;
} Foo;

static int	sub(void)
{
	Foo	f;
	Foo	*fp = &f;
	f.type = T_List;

	/* strict aliasing rule error */
	if (IsA(&f, List))
		return 1;

	/* This is ok */
	if (IsA(fp, List))
		return 1;
	return 0;
}

	

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