They're the same only for unsigned values. Modulo of negative numbers since C++11 has an expected behavior and generates more code: %rax = %rax & 7: andl $7, %eax %rax = %rax % 8 with GCC: cqto shrq $61, %rdx addq %rdx, %rax andl $7, %eax subq %rdx, %rax [read as ((%rax + (%rax < 0 ? 7 : 0)) & 7) - (%rax < 0 ? 7 : 0))] With Clang: movq %rax, %rcx sarq $63, %rcx shrq $61, %rcx addq %rax, %rcx andq $-8, %rcx subq %rcx, %rax Change-Id: Ife213d861bb14c1787e1fffd15b83b004be7eba0 Reviewed-by: Lars Knoll <lars.knoll@qt.io>