commit d1472711efc77d5ddc2fa6d5eff57baca584c8ef
author Cassio Neri <cassio.neri@gmail.com> Sun Nov 12 01:33:52 2023 +0000
committer Jonathan Wakely <jwakely@redhat.com> Tue Mar 12 11:53:05 2024 +0000
tree 72b4e9d52b0d59bb510a0c87f35bef74c8346393
parent 779563cff2e18e7891abf57aeee90e8db5035eb5

libstdc++: Fix UB in weekday::weekday(sys_days) and add test

The following has undefined behaviour (signed overflow) [1]:
    weekday max{sys_days{days{numeric_limits<days::rep>::max()}}};

The issue is in this line when __n is very large and __n + 4 overflows:
    return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6);

In addition to fixing this bug, the new implementation makes the compiler emit
shorter and branchless code for x86-64 and ARM [2].

[1] https://godbolt.org/z/1s5bv7KfT
[2] https://godbolt.org/z/zKsabzrhs

libstdc++-v3/ChangeLog:

	* include/std/chrono (weekday::_S_from_days): Fix UB.
	* testsuite/std/time/weekday/1.cc: Add test for overflow.

(cherry picked from commit f6ce081d0ffb5f25d71eb2f30fcfdff7f20dba22)