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

[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 c++/101274] std::execution::seq has incorrect behaviour under GCC 11.1.0

[general at yhf8377 dot me via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

--- Comment #1 from yhf8377  ---
Sorry, forgot to list the compiler options we used.

g++ -std=c++17 -g -m64 -O0 -Wall -fmessage-length=0 -pthread -o gcc_test
src/main.cpp

[Bug c++/101274] New: std::execution::seq has incorrect behaviour under GCC 11.1.0

[general at yhf8377 dot me via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101274

Bug ID: 101274
   Summary: std::execution::seq has incorrect behaviour under GCC
11.1.0
   Product: gcc
   Version: 11.1.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: general at yhf8377 dot me
  Target Milestone: ---

Created attachment 51091
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51091=edit
Minimal code to reproduce the error

We have a unit test that passes when compiled with GCC 10 but fails when
compiled with GCC 11. Further digging shows that calling `std::reduce()` with
execution policy `std::execution::seq` does not execute the provided function
object in sequential order.

Attached is the minimal test case that shows the different behaviour under GCC
10 and GCC 11.

Under GCC 10 (10.3.0) the output is:

plus_squared_ignore_nan: a = nan, b = 336.88
plus_squared_ignore_nan: a = 113488, b = nan
plus_squared_ignore_nan: a = 113488, b = 818.6
plus_squared_ignore_nan: a = 783594, b = nan
plus_squared_ignore_nan: a = 783594, b = 521.16
plus_squared_ignore_nan: a = 1.0552e+06, b = 451.78
plus_squared_ignore_nan: a = 1.25931e+06, b = 601.78
plus_squared_ignore_nan: a = 1.62145e+06, b = 709.2
plus_squared_ignore_nan: a = 2.12441e+06, b = 262.55
plus_squared_ignore_nan: a = 2.19334e+06, b = 659.67
plus_squared_ignore_nan: a = 2.62851e+06, b = 56.6
plus_squared_ignore_nan: a = 2.63171e+06, b = 780.31
plus_squared_ignore_nan: a = 3.2406e+06, b = 398.74

Correct! sum_squared == 3399588.6719

Under GCC 11 (11.1.0) the output is:

plus_squared_ignore_nan: a = 336.88, b = nan
plus_squared_ignore_nan: a = 818.6, b = nan
plus_squared_ignore_nan: a = 336.88, b = 818.6
plus_squared_ignore_nan: a = nan, b = 670443
plus_squared_ignore_nan: a = 521.16, b = 451.78
plus_squared_ignore_nan: a = 601.78, b = 709.2
plus_squared_ignore_nan: a = 204626, b = 503566
plus_squared_ignore_nan: a = 4.49494e+11, b = 2.53579e+11
plus_squared_ignore_nan: a = 262.55, b = 659.67
plus_squared_ignore_nan: a = 56.6, b = 780.31
plus_squared_ignore_nan: a = 435427, b = 608940
plus_squared_ignore_nan: a = 6.43025e+22, b = 3.70809e+11
plus_squared_ignore_nan: a = 2.01802e+23, b = 398.74
gcc_test: src/main.cpp:24: int main(int, char**): Assertion `sum_squared ==
3399588.6719' failed.
Aborted