----- On Mar 1, 2016, at 1:25 PM, H. Peter Anvin hpa@xxxxxxxxx wrote:
> On 02/27/16 16:39, Mathieu Desnoyers wrote:
>>
>> Very good points! Would the following interfaces be acceptable ?
>>
>> /* This structure needs to be aligned cache line size. */
>> struct thread_local_abi {
>> int32_t cpu_id; /* Aligned on
>> 32-bit. */
>> uint32_t rseq_seqnum; /* Aligned on 32-bit. */
>> uint64_t rseq_post_commit_ip; /* Aligned on 64-bit. */
>> /* Add new fields at the end. */
>> } __attribute__((packed));
>>
>
> First of all, DO NOT use __attribute__((packed)). First of all, it
> buggers up the alignment of the *entire structure* (the alignment of a
> packed structure defaults to 1, and gcc will assume the whole structure
> is misaligned, generating unaligned access instructions on architectures
> which need them.)
>
> Sadly gcc doesn't currently have an __attribute__ to express "error out
> on padding" which is what you actually want here.
Good point.
>
> You may, however, want to add an explicit alignment attribute to make
> sure it is cache line aligned.
Good idea, will do!
>
> Second, as far as the 32/64 bit issue is concerned, you have to order
> the fields so you always access the LSB. This is probably the best way
> to do it:
>
> #ifdef __LP64__
> # define __FIELD_32_64(field,n) uint64_t field;
> #elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> # define __FIELD_32_64(field,n) uint32_t field, _unused ## n;
> #else
> # define __FIELD_32_64(field,n) uint32_t _unused ## n, field;
> #endif
>
> All these macros are intrinsic to gcc (and hopefully to gcc-compatible
> compilers) so there are no header file dependencies.
Thanks for the hint. I'll try it out.
Mathieu
>
> -hpa
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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