[llvm-dev] @llvm.memcpy not honoring volatile?

Tue Jun 11 06:27:35 PDT 2019

I spent some time reading the C standard
<https://web.archive.org/web/20181230041359if_/http://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf>
:

5.1.2.3 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 5.1.2.3.
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
implementation-defined.

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
load/stores.

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.

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.
>
> John
>
>
> 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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> >
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