Date: Thu, 14 Jan 2021 13:44:34 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Thierry Thomas <thierry@freebsd.org> Cc: FreeBSD Hackerz <hackers@freebsd.org> Subject: Re: Some fun with -O2 Message-ID: <YAAuouAngRyT7Dw0@kib.kiev.ua> In-Reply-To: <YAAqI08bgBtfpssP@graf.pompo.net> References: <YAAqI08bgBtfpssP@graf.pompo.net>
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On Thu, Jan 14, 2021 at 12:25:23PM +0100, Thierry Thomas wrote: > Let´s check a small C program, check_mktime.c, fetchable from > <https://people.freebsd.org/~thierry/check_mktime.c>. > > $CC -o check_mktime -O2 check_mktime.c > ./check_mktime > > It is OK with clang90. > It loops forever with gcc9, gcc10, clang10 and clang11, > but OK without -O2. > > It is part of the configure script of ports/math/oleo, and I guess that > the same part could be found in other configure scripts. > > Note a similar program is part of glibc: see > <https://code.woboq.org/userspace/glibc/time/tst-mktime2.c.html>. There is no fun with this stuff. The time_t type is signed, then the loop for (time_t_max = 1; 0 < time_t_max; time_t_max *= 2) continue; intent is to get signed overflow, which is UB. Modern compilers prefer to shoot into your foot instead of following common sense. Workaround is to add -fwrapv compiler switch.
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