It's rare for programs to contain integer constants that don't fit in 32 bits, and they aren't needed for 64-bit code/global memory references due to the introd

It's rare for programs to contain integer constants that don't fit in 32 bits, and they aren't needed for 64-bit code/global memory references due to the introduction of addressing relative to the instruction pointer. Also there is a 15 bytes instruction length limit that would have to be extended if 64-bit immediates were allowed on all instructions.