[llvm-dev] @llvm.memcpy not honoring volatile?
Guillaume Chatelet via llvm-dev
llvm-dev at lists.llvm.org
Wed Jun 12 06:37:33 PDT 2019
Thx for the reply.
I've created https://bugs.llvm.org/show_bug.cgi?id=42254 and will work on a
On Tue, Jun 11, 2019 at 6:08 PM JF Bastien <jfbastien at apple.com> wrote:
> On Jun 11, 2019, at 6:27 AM, Guillaume Chatelet <gchatelet at google.com>
> I spent some time reading the C standard
> 220.127.116.11 Program execution
> 2. Accessing a volatile object, modifying an object, modifying a file, or
> calling a function that does any of those operations are all side effects,
> which are changes in the state of the execution environment...
> 6.7.3 Type qualifiers
> 8. An object that has volatile-qualified type may be modified in ways
> unknown to the implementation or have other unknown side effects. Therefore
> any expression referring to such an object shall be evaluated strictly
> according to the rules of the abstract machine, as described in 18.104.22.168.
> Furthermore, at every sequence point the value last stored in the object
> shall agree with that prescribed by the abstract machine, except as
> modified by the unknown factors mentioned previously. What constitutes an
> access to an object that has volatile-qualified type is
> My intuition is that it's unreasonable to do the overlap because it may
> create side effects in the middle of the read. At the same time, "accessing
> a volatile object" is implementation defined...
> As such "volatile" does not provide a lot of guarantees. I guess you have
> to resort on assembly if you want to really control what's going on.
> gcc, icc, msvc do not implement the overlap trick. They either read the
> memory region sequentially or use rep movs.
> Let's note that rep movs does not provide any guarantees on the memory
> access patterns either.
> Besides I'm not even sure that volatile in the context of @llvm.memcpy
> intrinsics really relates to the C volatile semantic.
> Now, I see several ways to move forward:
> 1. Do nothing and call it implementation defined
> 2. Fix the code that generates the loads/stores for the isVolatile case so
> no overlap can occur.
> 3. Remove "volatile" argument from @llvm.memcpy, @llvm.memmove
> and @llvm.memset and generates the code in the front end using volatile
> 3 is probably controversial and involves a lot a changes, it would move
> some complexity from backend to frontends.
> I'm interested in thoughts from other developers here.
> Volatile isn’t specified in any decent normative way. There’s substantial
> documentation of intent, but C and C++ standards are lacking. We should
> therefore implement something that’s sensible and works kind-of as expected.
> I’ve documented specification and intent here: wg21.link/P1152R0
> If you want to follow-on (without all that documentation):
> I think we want option 2.: keep volatile memcpy, and implement it as
> touching each byte exactly once. That’s unlikely to be particularly useful
> for every direct-to-hardware uses, but it behaves intuitively enough that I
> think it’s desirable.
> On Mon, Jun 10, 2019 at 10:51 PM John Regehr via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> I agree, this is a bug.
>> On 6/7/19 11:48 AM, JF Bastien via llvm-dev wrote:
>> >> On Jun 5, 2019, at 2:28 PM, Tim Northover via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> >> On Wed, 5 Jun 2019 at 13:49, Eli Friedman via llvm-dev
>> >> <llvm-dev at lists.llvm.org> wrote:
>> >>> I don’t see any particular reason to guarantee that a volatile memcpy
>> will access each byte exactly once. How is that useful?
>> >> I agree it's probably not that useful, but I think the non-duplicating
>> >> property of volatile is ingrained strongly enough that viewing a
>> >> memcpy as a single load and store to each unit (in an unspecified
>> >> order) should be legitimate; so I think this actually is a bug.
>> >> As the documentation says though, it's unwise to depend on the
>> >> behaviour of a volatile memcpy.
>> > I agree with Tim, this seems like a bug. My expectation is that
>> volatile touch each memory location exactly once, unless absolutely
>> impossible (e.g. bitfields on most ISAs).
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