[BUILD] Tests fail with strict-aliasing violations

[eli-schwartz]

I tried to build with the following *FLAGS to optimize the build: -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:

FAILED: nanovdb/nanovdb/unittest/CMakeFiles/nanovdb_test_nanovdb.dir/TestNanoVDB.cc.o 
/usr/bin/ccache /usr/bin/x86_64-pc-linux-gnu-g++ -DBOOST_IOSTREAMS_DYN_LINK -DBOOST_IOSTREAMS_NO_LIB -DGTEST_LINKED_AS_SHARED_LIBRARY=1 -DNANOVDB_USE_BLOSC -DNANOVDB_USE_INTRINSICS -DNANOVDB_USE_OPENVDB -DNANOVDB_USE_TBB -DNANOVDB_USE_ZIP -DOPENVDB_DLL -DOPENVDB_USE_AVX -DOPENVDB_USE_DELAYED_LOADING -DOPENVDB_USE_LOG4CPLUS -DOPENVDB_USE_SSE42 -I/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/nanovdb/nanovdb/.. -I/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/openvdb/openvdb/.. -I/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0_build-install/openvdb/openvdb -I/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0_build-install/openvdb/openvdb/openvdb -isystem /usr/include/Imath  -march=native -fstack-protector-all -O2 -pipe -fdiagnostics-color=always -frecord-gcc-switches -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing  -Wformat -Werror=format-security -std=c++17 -fdiagnostics-color=always -mavx -msse4.2 -Wno-invalid-offsetof -MD -MT nanovdb/nanovdb/unittest/CMakeFiles/nanovdb_test_nanovdb.dir/TestNanoVDB.cc.o -MF nanovdb/nanovdb/unittest/CMakeFiles/nanovdb_test_nanovdb.dir/TestNanoVDB.cc.o.d -o nanovdb/nanovdb/unittest/CMakeFiles/nanovdb_test_nanovdb.dir/TestNanoVDB.cc.o -c /var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/nanovdb/nanovdb/unittest/TestNanoVDB.cc
In file included from /var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/nanovdb/nanovdb/unittest/TestNanoVDB.cc:40:
/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/nanovdb/nanovdb/../nanovdb/PNanoVDB.h: In function ‘pnanovdb_uint32_t pnanovdb_float_as_uint32(float)’:
/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/nanovdb/nanovdb/../nanovdb/PNanoVDB.h:347:86: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  347 | PNANOVDB_FORCE_INLINE pnanovdb_uint32_t pnanovdb_float_as_uint32(float v) { return *((pnanovdb_uint32_t*)(&v)); }
      |                                                                                     ~^~~~~~~~~~~~~~~~~~~~~~~~~
/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/nanovdb/nanovdb/../nanovdb/PNanoVDB.h: In function ‘pnanovdb_uint64_t pnanovdb_double_as_uint64(double)’:
/var/tmp/portage/media-gfx/openvdb-11.0.0/work/openvdb-11.0.0/nanovdb/nanovdb/../nanovdb/PNanoVDB.h:349:88: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  349 | PNANOVDB_FORCE_INLINE pnanovdb_uint64_t pnanovdb_double_as_uint64(double v) { return *((pnanovdb_uint64_t*)(&v)); }
      |                                                                                       ~^~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: some warnings being treated as errors
ninja: build stopped: subcommand failed.

Downstream report: https://bugs.gentoo.org/926820
Full build log: build.log

It should be noted that this error only occurs when building the testsuite.