Re: [PATCH] RDMA/irdma: Slightly optimize irdma_form_ah_cm_frame()

Jason Gunthorpe
2023-04-12T23:06:56Z

On Thu, Feb 02, 2023 at 09:23:24PM +0100, Christophe JAILLET wrote:
> There is no need to zero 'pktsize' bytes of 'buf', only the header needs
> to be cleared, to be safe.
> All the other bytes are already written with some memcpy() at the end of
> the function.
>
> Doing so also gives the opportunity to the compiler to avoid the memset()
> call. It can be inlined now that the length is known as compile time.
>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> Reviewed-by: Shiraz Saleem <shiraz.saleem@intel.com>
> ---
> Just in case, here is the diff of what is generated by gcc 11.3.0 before
> and after the patch.
>
> .L736:
> -# drivers/infiniband/hw/irdma/cm.c:340: memset(buf, 0, pktsize);
> +# drivers/infiniband/hw/irdma/cm.c:340: memset(buf, 0, sizeof(*tcph));
> call __sanitizer_cov_trace_pc #
> - xorl %esi, %esi #
> - movzwl %r13w, %edx # _194, __fortify_size
> - movq %rbp, %rdi # buf,
> - call memset #
> - leaq 104(%r12), %rax #, _259
> + movl $0, 16(%rbp) #, MEM <char[1:20]> [(void *)buf_114]
> + leaq 104(%r12), %rax #, _295
> +# drivers/infiniband/hw/irdma/cm.c:342: sqbuf->totallen = pktsize;
> + movzwl %r13w, %r13d # _192, _192
> +# drivers/infiniband/hw/irdma/cm.c:340: memset(buf, 0, sizeof(*tcph));
> + movq $0, 0(%rbp) #, MEM <char[1:20]> [(void *)buf_114]
> +# drivers/infiniband/hw/irdma/cm.c:342: sqbuf->totallen = pktsize;
> + movq %rax, %rdi # _295,
> +# drivers/infiniband/hw/irdma/cm.c:340: memset(buf, 0, sizeof(*tcph));
> + movq $0, 8(%rbp) #, MEM <char[1:20]> [(void *)buf_114]
> + movq %rax, 64(%rsp) # _295, %sfp
> # drivers/infiniband/hw/irdma/cm.c:342: sqbuf->totallen = pktsize;
> ---
> drivers/infiniband/hw/irdma/cm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)

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Thanks,
Jason