Build fails with strict-aliasing violations

[eli-schwartz]

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

The -Werror=* flags are important to detect cases where the compiler can try to optimize based on assuming UB cannot happen, and miscompile code that has UB in it. strict-aliasing issues are always bad but LTO can make them even worse.

I got this error:

x86_64-pc-linux-gnu-g++ -DHAVE_CONFIG_H -I.  -DCONFDIR='"/etc"'    -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 -c -o portspoof-connection.o `test -f 'connection.cpp' || echo './'`connection.cpp
connection.cpp: In function ‘void* process_connection(void*)’:
connection.cpp:92:22: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
   92 |         int tid =  *((int*)(&arg));
      |                     ~^~~~~~~~~~~~~
cc1plus: some warnings being treated as errors
make[2]: *** [Makefile:455: portspoof-connection.o] Error 1

Downstream report: https://bugs.gentoo.org/861698
Full build log: build.log