There are some amd64 computers that only support to boot from UEFI in x86 (32-bit) mode but allow to load an amd64 operating system. On GNU/Linux that is usually achieved by issuing
--target=i386-efi when installing the GRUB boot loader.
Nowadays GRUB (v2.06) allows to boot Linux from a LUKS-encrypted
/boot partition (even one inside a LVM) with the
GRUB_ENABLE_CRYPTODISK=y key and the appropriate LUKS or LUKS2 modules loaded.
If we boot a system with this 32-bit EFI setup, will LUKS be decrypting the contents of the partition using the full 64-bit instruction set (and AES-NI when applicable) or will it stick to 32-bit instructions when the operating system is running?