Re: [PATCH] scsi: fc: use get/put_unaligned64 for wwn access

[Hannes Reinecke]

On 03/16/2016 05:39 PM, Arnd Bergmann wrote:
> A bug in the gcc-6.0 prerelease version caused at least one
> driver (lpfc) to have excessive stack usage when dealing with
> wwn data, on the ARM architecture.
> 
> lpfc_scsi.c: In function 'lpfc_find_next_oas_lun':
> lpfc_scsi.c:117:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]
> 
> I have reported this as a gcc regression in
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70232
> 
> However, using a better implementation of wwn_to_u64() not only
> helps with the particular gcc problem but also leads to better
> object code for any version or architecture.
> 
> The kernel already provides get_unaligned_be64() and
> put_unaligned_be64() helper functions that provide an
> optimized implementation with the desired semantics.
> 
> The lpfc_find_next_oas_lun() function in the example that
> grew from 1146 bytes to 5144 bytes when moving from gcc-5.3
> to gcc-6.0 is now 804 bytes, as the optimized
> get_unaligned_be64() load can be done in three instructions.
> The stack usage is now down to 28 bytes from 128 bytes with
> gcc-5.3 before.
> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  include/scsi/scsi_transport_fc.h | 15 +++------------
>  1 file changed, 3 insertions(+), 12 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@xxxxxxxx>

Cheers,

Hannes
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