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From: Luc Pelletier <lucp.at.work@gmail.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
Cc: "Richardson, Bruce" <bruce.richardson@intel.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	 "Li, Xiaoyun" <xiaoyun.li@intel.com>,
	"stable@dpdk.org" <stable@dpdk.org>
Subject: Re: [PATCH v5] eal: fix unaligned loads/stores in rte_memcpy_generic
Date: Thu, 10 Feb 2022 11:56:30 -0500	[thread overview]
Message-ID: <CAFeRdtAdYmKOAr+1eSbMRcVR=OcAnoTmZvMR8twdNtbJygBsZA@mail.gmail.com> (raw)
In-Reply-To: <DM6PR11MB4491E6A7BA3167268FD189B59A2F9@DM6PR11MB4491.namprd11.prod.outlook.com>

Hi Konstantin,

> > Thanks for your input Konstantin. Much appreciated. Just to make sure
> > I understand, can you please confirm that we do not want to fix the
> > fact that unaligned access in C is undefined behaviour?
>
> Yes, I don't think it is a real problem in that particular case.

Perfect. Thank you.

> > Also, do you have any opinion on the strict aliasing violation (as was
> > pointed out by Georg)? I suggested adding __may_alias__ thinking it
> > would likely fix the issue, but I'd really like to get confirmation
> > from someone who has better knowledge of how the attribute works
> > exactly.
>
> Not sure I understand the problem you are referring to.
> Are you saying that original rte_memcpy() code breaks strict aliasing?
> If so, could you point where exactly?

As far as I understand, yes, it does break strict aliasing. For
example, in the following line:

*(uint64_t *)dstu = *(const uint64_t *)srcu;

IIUC, both casts break strict aliasing rules. While the src/dst
parameters are void* and can therefore be cast to something else
without breaking strict aliasing rules, the type of src/dst in the
calling code might be something other than uint64_t*. This can result
in src/dst pointers being cast to different unrelated types. AFAICT,
the fact that rte_memcpy is "always inline" increases the risk of the
compiler making an optimization that results in broken code.

I was able to come up with an example where the latest version of GCC
produces broken code when strict aliasing is enabled:

https://godbolt.org/z/3Yzvjr97c

With -fstrict-aliasing, it reorders a write and results in broken
code. If you change the compiler flags to -fno-strict-aliasing, it
produces the expected result.

  reply	other threads:[~2022-02-10 16:56 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-15 19:41 [PATCH] " Luc Pelletier
2022-01-15 21:39 ` [PATCH v2] " Luc Pelletier
2022-01-15 22:13   ` Stephen Hemminger
2022-01-16 14:09     ` Luc Pelletier
2022-01-16 16:32       ` Stephen Hemminger
2022-01-16 14:13 ` [PATCH v3] " Luc Pelletier
2022-01-16 14:33   ` Luc Pelletier
2022-01-16 16:34     ` Stephen Hemminger
2022-01-16 17:59       ` Morten Brørup
2022-01-16 20:33 ` [PATCH v4] " Luc Pelletier
2022-01-17 15:37 ` [PATCH v5] " Luc Pelletier
2022-02-04 16:42   ` Luc Pelletier
2022-02-04 17:16   ` Ananyev, Konstantin
2022-02-08 16:53     ` Thomas Monjalon
2022-02-09 15:05     ` Luc Pelletier
2022-02-10 14:04       ` Ananyev, Konstantin
2022-02-10 16:56         ` Luc Pelletier [this message]
2022-02-11 15:51           ` Ananyev, Konstantin
2022-02-13 22:31             ` Luc Pelletier
2022-02-14 13:41               ` Ananyev, Konstantin
2022-02-25 15:51 ` [PATCH v6] eal: fix rte_memcpy strict aliasing/alignment bugs Luc Pelletier
2022-02-25 16:38 ` [PATCH v7] " Luc Pelletier
2022-03-10 14:55   ` Ananyev, Konstantin
2022-04-07 15:24     ` David Marchand
2022-04-07 15:32       ` David Marchand
2022-04-07 15:40         ` David Marchand
2022-05-13 19:15           ` Luc Pelletier
2022-05-19 16:41             ` David Marchand
2022-04-08 13:47       ` Luc Pelletier
2022-05-19 16:47       ` David Marchand

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