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&action=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
