strict alias analysis question

strict alias analysis question

[Manfred Spraul]

Hi,

the latest gcc versions generate warnings about potentially dangerous 
pointer casts if strict alias analysis is enabled.
Right now two groups of  warnings are generated: several from the 
inlined MemSet macros:

#define MemSetLoop(start, val, len) \
        do \
        { \
                int32 * _start = (int32 *) (start); \
                int32 * _stop = (int32 *) ((char *) \
                            _start + (Size) (len)); \
        \
                while (_start < _stop) \
                        *_start++ = 0; \
        } while (0)

And a few from structures with common headers:

struct hdr {
    ...;
}
struct b {
    struct hdr info;
    ...
};

void handler(struct hdr* parm)
{
    struct b* ptr = (struct b*)parm;
}

Does MemSet break the rules for strict alias analysis? And what about 
manual base classes? The actual use it a bit longer (the cast to (Node 
*) in src/backend/commands/tablecmds.c generates the warning).

--
    Manfred

Re: strict alias analysis question

[Mike Stump]

On Tuesday, October 14, 2003, at 09:52 AM, Manfred Spraul wrote:
> the latest gcc versions generate warnings about potentially dangerous 
> pointer casts if strict alias analysis is enabled.
> Right now two groups of  warnings are generated: several from the 
> inlined MemSet macros:
>
> #define MemSetLoop(start, val, len) \
>        do \
>        { \
>                int32 * _start = (int32 *) (start); \
>                int32 * _stop = (int32 *) ((char *) \
>                            _start + (Size) (len)); \
>        \
>                while (_start < _stop) \
>                        *_start++ = 0; \
>        } while (0)

> Does MemSet break the rules for strict alias analysis?

No, it'll work fine for all int32 arrays.  Will it work for anything 
else, no, sorry.  Well, it might work for a few other things, like char 
arrays that are a multiple of around 4 bytes...  I'd recommend reading 
up, the standard is fairly clear and easy to read in this area.

Hint, use memcpy/memmove instead, it works.