Why is this C++ code faster than my hand-written assembly (2016)

Re:Why is this C++ code faster than my hand-written assembly... by colejohnson66 on Tuesday August 22, 2017 @01:23PM (#15074512)

But on x86, division by 2 on an unsigned integer is the same as shifting to the right by 1. So instead of

    even:
        mov rbx, 2
        xor rdx, rdx
        div rbx

the OP should've done something like

    even:
        xor rdx, rdx % possible to remove?
        shr rbx, 1

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Re:Why is this C++ code faster than my hand-written assembly... by pwdisswordfish on Tuesday August 22, 2017 @05:19PM (#15076497)

    shr rax, 1

div rbx will divide the 128-bit value in (rdx, rax) by rbx, then store the quotient in rax and the remainder in rdx.

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Re:Why is this C++ code faster than my hand-written assembly... by hrydgard on Tuesday August 22, 2017 @01:45PM (#15074740)

Your second snippet will just zero rdx - don't confuse the registers (rbx is in the mix in the original :)

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Re:Why is this C++ code faster than my hand-written assembly... by colejohnson66 on Tuesday August 22, 2017 @02:26PM (#15075119)

Thanks :) Fixed.

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