A leap year check in three instructions

Re:A leap year check in three instructions by npendleton on Friday May 16, 2025 @06:48AM (#44003801)

This is so cool!

Terrible nitpick, but this is actually 3 operations, not instructions. On x86 you get 4:

  is_leap_year_fast:
        imul    eax, edi, 1073750999
        and     eax, -1073614833
        cmp     eax, 126977
        setb    al
        ret

On ARM you get a bit more due to instruction encoding:

  is_leap_year_fast:
        ldr     r1, .LCPI0_0
        mul     r0, r0, r1
        ldr     r1, .LCPI0_1
        and     r1, r0, r1
        mov     r0, #0
        cmp     r1, #126976
        movwls  r0, #1
        bx      lr
  .LCPI0_0:
        .long   1073750999
  .LCPI0_1:
        .long   3221352463

Compiler explorer reference: https://godbolt.org/z/7ajYqbT9z

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Re:A leap year check in three instructions by gpderetta on Friday May 16, 2025 @07:51AM (#44004256)

You could argue that the setb and ret are not part of the leap year check itself. For example if the compiled inlined the call into a caller doing:

    if(is_leap_year_fast()) {...}

Then the ret would obviously go away and the setb wouldn't be necessary as it could generate directly a conditional jmp from the result of the cmp.

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Re:A leap year check in three instructions by npendleton on Friday May 16, 2025 @08:22AM (#44004560)

Hah, great point!

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