[PATCH v9 1/8] eal: generic 64 bit counter

[Morten Brørup]

> From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> Sent: Wednesday, 22 May 2024 17.38
> 
> On Wed, 22 May 2024 10:31:39 +0200
> Morten Brørup <mb at smartsharesystems.com> wrote:
> 
> > > +/* On 32 bit platform, need to use atomic to avoid load/store
> tearing */
> > > +typedef RTE_ATOMIC(uint64_t) rte_counter64_t;
> >
> > As shown by Godbolt experiments discussed in a previous thread [2],
> non-tearing 64 bit counters can be implemented without using atomic
> instructions on all 32 bit architectures supported by DPDK. So we should
> use the counter/offset design pattern for RTE_ARCH_32 too.
> >
> > [2]:
> https://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35E9F433@smarts
> erver.smartshare.dk/
> 
> 
> This code built with -O3 and -m32 on godbolt shows split problem.
> 
> #include <stdint.h>
> 
> typedef uint64_t rte_counter64_t;
> 
> void
> rte_counter64_add(rte_counter64_t *counter, uint32_t val)
> {
> 	*counter += val;
> }
> …	*counter = val;
> }
> 
> rte_counter64_add:
>         push    ebx
>         mov     eax, DWORD PTR [esp+8]
>         xor     ebx, ebx
>         mov     ecx, DWORD PTR [esp+12]
>         add     DWORD PTR [eax], ecx
>         adc     DWORD PTR [eax+4], ebx
>         pop     ebx
>         ret
> 
> rte_counter64_read:
>         mov     eax, DWORD PTR [esp+4]
>         mov     edx, DWORD PTR [eax+4]
>         mov     eax, DWORD PTR [eax]
>         ret
> rte_counter64_set:
>         movq    xmm0, QWORD PTR [esp+8]
>         mov     eax, DWORD PTR [esp+4]
>         movq    QWORD PTR [eax], xmm0
>         ret

Sure, atomic might be required on some 32 bit architectures and/or with some compilers.

I envision a variety of 32 bit implementations, optimized for certain architectures/compilers.

Some of them can provide non-tearing 64 bit load/store, so we should also use the counter/offset design pattern for those.