[Bug libstdc++/101274] [11/12 Regression] std::execution::seq has incorrect behaviour under GCC 11.1.0

2021-07-02 Thread rodgertq at gcc dot gnu.org via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

--- Comment #6 from Thomas Rodgers  ---
It does raise an issue which I'm going to follow up on separately on the SG1
(concurrency and parallelism study group) mailing list.

While it is indeed the case that standard says you can't count on deterministic
sequencing for std::reduce(), I can't find any wording that directly supports
the wording on cppreference.com std::execution::sequenced_policy, though that
wording is consistent with my recollection of SG1's discussions regarding
execution policies.

There is wording that says 'Unless otherwise specified, the semantics of
ExecutionPolicy algorithm overloads are identical to their overloads without."

The wording for std::for_each() for the non-execution policy overload the
standard says -

Effects: Applies f to the result of dereferencing every iterator in the range
[first, last), starting from first and proceeding to last - 1.

And for one that takes an execution policy -

Effects: Applies f to the result of dereferencing every iterator in the range
[first, last).

So it omits the ordering constraint, which makes sense, but I wonder if we
shouldn't be more explicit in the documentation of the policies themselves.

[Bug libstdc++/101274] [11/12 Regression] std::execution::seq has incorrect behaviour under GCC 11.1.0

2021-07-02 Thread general at yhf8377 dot me via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

--- Comment #5 from yhf8377  ---
Thank you Thomas for the detailed information and links to the references.

I guess I will need to move this bug report into our own bug tracker. :-) This
is indeed a bug in our code as we had incorrect assumptions on how
`std::reduce()` and `std::execution::seq` work.

Thanks again!

[Bug libstdc++/101274] [11/12 Regression] std::execution::seq has incorrect behaviour under GCC 11.1.0

2021-07-01 Thread rodgertq at gcc dot gnu.org via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

--- Comment #4 from Thomas Rodgers  ---
I did some more reading of
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4878.pdf and it would
seem that you are not even guaranteed a deterministic ordering of the
application of binary_op on the non-execution policy accepting overloads of
std::reduce().

See [reduce] the note at bullet 9 -

"[Note 1 : The difference between reduce and accumulate is that reduce applies
binary_op in an unspecified order, which yields a nondeterministic result for
non-associative or non-commutative binary_op such as floating-point addition. —
end note]"

[Bug libstdc++/101274] [11/12 Regression] std::execution::seq has incorrect behaviour under GCC 11.1.0

2021-07-01 Thread rodgertq at gcc dot gnu.org via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

Thomas Rodgers  changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 Resolution|--- |INVALID

--- Comment #3 from Thomas Rodgers  ---
The standard only requires that the the parallel algorithm's execution, when
invoked with std::sequenced_policy, be invoked on the calling thread, and (as
with the other execution policies) allows for the invocations of element access
functions to be indeterminately sequenced on the execution agent.

This is also spelled at -

https://en.cppreference.com/w/cpp/algorithm/execution_policy_tag_t

"1) The execution policy type used as a unique type to disambiguate parallel
algorithm overloading and require that a parallel algorithm's execution may not
be parallelized. The invocations of element access functions in parallel
algorithms invoked with this policy (usually specified as std::execution::seq)
are indeterminately sequenced in the calling thread."

[Bug libstdc++/101274] [11/12 Regression] std::execution::seq has incorrect behaviour under GCC 11.1.0

2021-07-01 Thread marxin at gcc dot gnu.org via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

Martin Liška  changed:

   What|Removed |Added

 CC||marxin at gcc dot gnu.org
 Ever confirmed|0   |1
 Status|UNCONFIRMED |NEW
   Last reconfirmed||2021-07-01

--- Comment #2 from Martin Liška  ---
Confirmed.

[Bug libstdc++/101274] [11/12 Regression] std::execution::seq has incorrect behaviour under GCC 11.1.0

2021-07-01 Thread rguenth at gcc dot gnu.org via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

Richard Biener  changed:

   What|Removed |Added

   Keywords||wrong-code
Summary|std::execution::seq has |[11/12 Regression]
   |incorrect behaviour under   |std::execution::seq has
   |GCC 11.1.0  |incorrect behaviour under
   ||GCC 11.1.0
   Target Milestone|--- |11.2