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You can't use mov. I am not an x86 expert, but after quick googling: Recall that immediates are normally restricted to 32 bits. To load a larger constant into a quad register, use movabsq, which takes a full 64-bit immediate as its source [https://www.lri.fr/~filliatr/ens/compil/x86-64.pdf] Even in your example, assembler replaces mov by movq, which, say, hints: >>>>> $ echo "mov r8, 0x12345678"|./bin/llvm-mc -assemble -show-encoding -x86-asm-syntax=intel -print-imm-hex -triple=x86_64 >>>>> .text >>>>>...