Some fun with -O2

[Dimitry Andric]

On 14 Jan 2021, at 12:25, Thierry Thomas <thierry at FreeBSD.org> 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

$ gcc -fsanitize=undefined check_mktime.c -o check_mktime
<...lots of warnings snipped...>

$ ./check_mktime
check_mktime.c:109:51: runtime error: signed integer overflow: 4611686018427387904 * 2 cannot be represented in type 'long int'
check_mktime.c:111:13: runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long int'
check_mktime.c:123:28: runtime error: signed integer overflow: 1073741824 * 2 cannot be represented in type 'int'
check_mktime.c:125:7: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'

This program indeed relies on -fwrapv.

-Dimitry

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