On Thu, Aug 21, 2025 at 4:29 PM David Hildenbrand [off-list ref] wrote:

quoted

Because doing a 64-bit shift on x86-32 is like three cycles. Doing a
64-bit signed division by a simple constant is something like ten
strange instructions even if the end result is only 32-bit.

I would have thought that the compiler is smart enough to optimize that?
PAGE_SIZE is a constant.

Oh, the compiler optimizes things. But dividing a 64-bit signed value
with a constant is still quite complicated.

It doesn't generate a 'div' instruction, but it generates something like this:

    movl %ebx, %edx
    sarl $31, %edx
    movl %edx, %eax
    xorl %edx, %edx
    andl $4095, %eax
    addl %ecx, %eax
    adcl %ebx, %edx

and that's certainly a lot faster than an actual 64-bit divide would be.

An unsigned divide - or a shift - results in just

    shrdl $12, %ecx, %eax

which is still not the fastest instruction (I think shrld gets split
into two uops), but it's certainly simpler and easier to read.

           Linus