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:

/bin/sh ../../libtool  --tag=CC   --mode=compile x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I../..  -include ../../config.h -I ../../include  -Wall -Wextra -Wno-unused -Wno-unused-result -Wno-strict-aliasing -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 -c -o built-in/libply_la-buffer.lo `test -f 'built-in/buffer.c' || echo './'`built-in/buffer.c
libtool: compile:  x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I../.. -include ../../config.h -I ../../include -Wall -Wextra -Wno-unused -Wno-unused-result -Wno-strict-aliasing -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 -c built-in/buffer.c  -fPIC -DPIC -o built-in/.libs/libply_la-buffer.o
built-in/buffer.c: In function '__get_head':
built-in/buffer.c:83:36: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
   83 |         volatile uint64_t head = *((volatile uint64_t *)&mem->data_head);
      |                                   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

Downstream report: https://bugs.gentoo.org/858458
Full build log: build.log