Fails to build on Debian/m68k due to alignment issues

[glaubitz]

The zimg package fails to build on the m68k architecture in Debian due to the fact that the native alignment of this target is 16 bits despite being a 32 bits architecture.

Tail of build log:

libtool: compile:  g++ -DPACKAGE_NAME=\"zimg\" -DPACKAGE_TARNAME=\"zimg\" -DPACKAGE_VERSION=\"3.0.3\" "-DPACKAGE_STRING=\"zimg 3.0.3\"" -DPACKAGE_BUGREPORT=\"https://github.com/sekrit-twc/zimg/pulls\" -DPACKAGE_URL=\"https://github.com/sekrit-twc/zimg\" -DHAVE_STDIO_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_STRINGS_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_UNISTD_H=1 -DSTDC_HEADERS=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=\".libs/\" -DHAVE_PTHREAD_PRIO_INHERIT=1 -DHAVE_PTHREAD=1 -I. -I./src/zimg -Wdate-time -D_FORTIFY_SOURCE=2 -std=c++14 -O2 -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -DNDEBUG -fvisibility=hidden -g -O2 -ffile-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -c src/zimg/depth/dither.cpp  -fPIC -DPIC -o src/zimg/depth/.libs/libzimg_internal_la-dither.o
libtool: compile:  g++ -DPACKAGE_NAME=\"zimg\" -DPACKAGE_TARNAME=\"zimg\" -DPACKAGE_VERSION=\"3.0.3\" "-DPACKAGE_STRING=\"zimg 3.0.3\"" -DPACKAGE_BUGREPORT=\"https://github.com/sekrit-twc/zimg/pulls\" -DPACKAGE_URL=\"https://github.com/sekrit-twc/zimg\" -DHAVE_STDIO_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_STRINGS_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_UNISTD_H=1 -DSTDC_HEADERS=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=\".libs/\" -DHAVE_PTHREAD_PRIO_INHERIT=1 -DHAVE_PTHREAD=1 -I. -I./src/zimg -Wdate-time -D_FORTIFY_SOURCE=2 -std=c++14 -O2 -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -DNDEBUG -fvisibility=hidden -g -O2 -ffile-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -c src/zimg/depth/depth.cpp -o src/zimg/depth/libzimg_internal_la-depth.o >/dev/null 2>&1
src/zimg/depth/dither.cpp: In member function ‘virtual std::tuple<const float*, unsigned int, unsigned int> zimg::depth::{anonymous}::NoneDitherTable::get_dither_coeffs(unsigned int, unsigned int) const’:
src/zimg/depth/dither.cpp:177:76: error: requested alignment ‘12’ is not a positive power of 2
  177 |   static constexpr float table alignas(ALIGNMENT)[AlignmentOf<float>::value] = {};
      | 

Full log available here: https://buildd.debian.org/status/fetch.php?pkg=zimg&arch=m68k&ver=3.0.3%2Bds1-1&stamp=1631269317&raw=0

I haven't had the time yet to start looking into the issue, but a proper alignas() at the right place would certainly fix the issue. If you want to have a look at the issue yourself, see below for instructions on getting an emulated Debian/m68k environment.

To set up a QEMU-based m68k system, see: https://wiki.debian.org/M68k/QemuSystemM68k

To use an m68k qemu-user chroot, see: https://wiki.debian.org/M68k/sbuildQEMU