[dpdk-dev] rte_ether_addr_copy() strange comment

[Jerin Jacob]

On Fri, Jun 26, 2020 at 6:05 PM Morten Brørup <mb at smartsharesystems.com> wrote:
>
> > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ferruh Yigit
> > Sent: Friday, June 26, 2020 2:08 PM
> >
> > On 6/25/2020 4:45 PM, Morten Brørup wrote:
> > > The function rte_ether_addr_copy() checks for __INTEL_COMPILER and
> > has a comment about "a strange gcc warning". It says:
> > >
> > > static inline void rte_ether_addr_copy(const struct rte_ether_addr
> > *ea_from,
> > >                                struct rte_ether_addr *ea_to)
> > > {
> > > #ifdef __INTEL_COMPILER
> > >     uint16_t *from_words = (uint16_t *)(ea_from->addr_bytes);
> > >     uint16_t *to_words   = (uint16_t *)(ea_to->addr_bytes);
> > >
> > >     to_words[0] = from_words[0];
> > >     to_words[1] = from_words[1];
> > >     to_words[2] = from_words[2];
> > > #else
> > >     /*
> > >      * Use the common way, because of a strange gcc warning.
> > >      */
> > >     *ea_to = *ea_from;
> > > #endif
> > > }
> > >
> > > I can see that from_words discards the const qualifier. Is that the
> > "strange" gcc warning the comment is referring to?
> > >
> > > This goes back to before the first public release of DPDK in 2013,
> > ref. https://git.dpdk.org/dpdk/log/lib/librte_ether/rte_ether.h
> > >
> > >
> > > It should be fixed as follows:
> > >
> > > -   uint16_t *from_words = (uint16_t *)(ea_from->addr_bytes);
> > > -   uint16_t *to_words   = (uint16_t *)(ea_to->addr_bytes);
> > > +   const uint16_t *from_words = (const uint16_t *)ea_from;
> > > +   uint16_t       *to_words   = (uint16_t *)ea_to;
> > >
> > > And the consequential question: Is copying the three shorts faster
> > than copying the struct? In other words: Should we get rid of the
> > #ifdef and use the first method only?
> >
> >
> > I tried to investigate this in godbolt: https://godbolt.org/z/YSmvDn
> >
> > First I don't see the "strange" gcc warning with various gcc versions
> > there.
> >
> > Related to the struct copy vs word copy, struct copy seems with less
> > instruction
> > [1],[2],
> > my vote to remove ifdef and keep struct copy.
> >
> >
> > [1] copy as individual function
> > [1a] gcc 10.1, struct copy:
> > copy:
> >         movdqa  (%rsi), %xmm0
> >         movaps  %xmm0, (%rdi)
> >         ret
> >
> > [1b] gcc 10.1, word copy:
> > copy:
> >         movzwl  (%rsi), %eax
> >         movw    %ax, (%rdi)
> >         movzwl  2(%rsi), %eax
> >         movw    %ax, 2(%rdi)
> >         movzwl  4(%rsi), %eax
> >         movw    %ax, 4(%rdi)
> >         ret
> >
> > [1c] icc 19.0.1, struct copy
> > copy:
> >         movups    (%rsi), %xmm0                                 #19.13
> >         movups    %xmm0, (%rdi)                                 #19.13
> >         ret
> >
> >
> > [2] gcc 10.1, copy as inline function that knows the data, both seems
> > similar
> > // .addr = {1, 1, 1, 1, 1, 1},
> > [2a] struct copy:
> > ...
> >         movl    $257, %eax
> >         movw    %ax, 4(%rsp)
> >         leaq    16(%rsp), %rdi
> >         movl    $16843009, (%rsp)
> >         movdqa  (%rsp), %xmm0
> >         movaps  %xmm0, 16(%rsp)
> > ...
> >
> > [2b] word copy:
> >         movl    $257, %eax
> >         movq    %rsp, %rdi
> >         movw    %ax, 4(%rsp)
> >         movl    $16843009, (%rsp)
> >
>
> Thank you for the detailed response, Ferruh.
>
> I didn't know about godbolt, so thank you for that reference too.
>
> The address struct is 2 byte aligned, not 16 byte aligned. Modifying your test in godbolt to use 2 byte alignment gives a similar result, i.e. fewer instructions on both icc and gcc.
>
> [1c-modified] icc 19.0.1, struct copy
>
> copy:
>         movl      (%rsi), %eax                                  #19.13
>         movl      %eax, (%rdi)                                  #19.13
>         movzwl    4(%rsi), %edx                                 #19.13
>         movw      %dx, 4(%rdi)                                  #19.13
>         ret                                                     #28.1
>
> [1d-modified] icc 19.0.1, word copy
> copy:
>         movzwl    (%rsi), %eax                                  #24.12
>         movw      %ax, (%rdi)                                   #24.5
>         movzwl    2(%rsi), %edx                                 #25.12
>         movw      %dx, 2(%rdi)                                  #25.5
>         movzwl    4(%rsi), %ecx                                 #26.12
>         movw      %cx, 4(%rdi)                                  #26.5
>         ret                                                     #28.1
>
> Testing for ARM64 on godbolt gives a similar result: more instructions using word copy than struct copy.
>
> In conclusion, I will proceed with the struct copy.

Since you are up to changing the code, Could you add __restrict
keyword for the further hint to the compiler for struct copy case?

http://mails.dpdk.org/archives/dev/2020-June/169876.html