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
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
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()) {...}