Build fails with strict-aliasing violations

[eli-schwartz]

Description

FAILED: tests/CMakeFiles/test_pfx.dir/test_pfx.c.o 
/usr/bin/x86_64-pc-linux-gnu-gcc  -I/var/tmp/portage/net-libs/rtrlib-0.8.0/work/rtrlib-0.8.0/.  -march=native -fstack-protector-all -O2 -pipe -fdiagnostics-color=always -frecord-gcc-switches -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-clash-protection -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing  -Wformat -Werror=format-security -Werror=implicit-function-declaration -Werror=implicit-int -Werror=int-conversion -Werror=incompatible-pointer-types -Wall -std=gnu99 -fstack-protector-all -MD -MT tests/CMakeFiles/test_pfx.dir/test_pfx.c.o -MF tests/CMakeFiles/test_pfx.dir/test_pfx.c.o.d -o tests/CMakeFiles/test_pfx.dir/test_pfx.c.o -c /var/tmp/portage/net-libs/rtrlib-0.8.0/work/rtrlib-0.8.0/tests/test_pfx.c
/var/tmp/portage/net-libs/rtrlib-0.8.0/work/rtrlib-0.8.0/tests/test_pfx.c: In function ‘mass_test’:
/var/tmp/portage/net-libs/rtrlib-0.8.0/work/rtrlib-0.8.0/tests/test_pfx.c:132:54: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  132 |                 ((uint64_t *)pfx.prefix.u.addr6.addr)[0] = min_i + i;
      |                                                      ^
/var/tmp/portage/net-libs/rtrlib-0.8.0/work/rtrlib-0.8.0/tests/test_pfx.c:152:54: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  152 |                 ((uint64_t *)pfx.prefix.u.addr6.addr)[0] = min_i + i;
      |                                                      ^
/var/tmp/portage/net-libs/rtrlib-0.8.0/work/rtrlib-0.8.0/tests/test_pfx.c:176:54: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  176 |                 ((uint64_t *)pfx.prefix.u.addr6.addr)[0] = min_i + i;
      |                                                      ^
cc1: some warnings being treated as errors

Steps to reproduce the issue

I tried to compile with the following flags for LTO: -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing

The -Werror's here are very important, because LTO can optimize based on the assumption that certain language requirements are met, and if they aren't met, a miscompilation happens.

Actually, strict-aliasing can cause issues even without LTO ;) but LTO does tend to make them worse.

Downstream report: https://bugs.gentoo.org/861581
Full build logs: build.log

Versions

Operating system: Gentoo Linux
Build environment: GCC
rtrlib: 0.8.0