From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Ensuring hash tuples are properly maxaligned
Date: Tue, 2 Jan 2018 17:09:31 -0800
Message-ID: <20180103010931.6vsirlb5pdapdroq@alap3.anarazel.de> (raw)
In-Reply-To: <14483.1514938129@sss.pgh.pa.us>
References: <14483.1514938129@sss.pgh.pa.us>
Hi,
On 2018-01-02 19:08:49 -0500, Tom Lane wrote:
> Now, the existing definition of the struct seems safe on all
> architectures we support, but it would not take much to break it.
> I think we ought to do what we did recently in the memory-context
> code: insert an explicit padding calculation and a static assertion
> that it's right. Hence, the attached proposed patch (in which
> I also failed to resist the temptation to make the two code paths
> in dense_alloc() look more alike). Any objections?
Generally no objections.
> + /*
> + * It's required that data[] start at a maxaligned offset. This padding
> + * calculation is correct as long as int is not wider than size_t and
> + * dsa_pointer is not wider than a regular pointer, but there's a static
> + * assertion to check things in nodeHash.c.
> + */
But note that dsa_pointer can be wider than a regular pointer on
platforms without atomics support.
> +#define HASHMEMORYCHUNKDATA_RAWSIZE (SIZEOF_SIZE_T * 3 + SIZEOF_VOID_P)
> +
> + /* ensure proper alignment by adding padding if needed */
> +#if (HASHMEMORYCHUNKDATA_RAWSIZE % MAXIMUM_ALIGNOF) != 0
> + char padding[MAXIMUM_ALIGNOF - HASHMEMORYCHUNKDATA_RAWSIZE % MAXIMUM_ALIGNOF];
> +#endif
> +
> char data[FLEXIBLE_ARRAY_MEMBER]; /* buffer allocated at the end */
> } HashMemoryChunkData;
Wonder if this specific case wouldn't be easier handled with a union?
Greetings,
Andres Freund
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To: pgsql-hackers@postgresql.org
Cc: andres@anarazel.de, tgl@sss.pgh.pa.us, thomas.munro@enterprisedb.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Ensuring hash tuples are properly maxaligned
In-Reply-To: <20180103010931.6vsirlb5pdapdroq@alap3.anarazel.de>
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