netdev - Re: [PATCH RFC 31/35] crypto: remove nth

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Message-ID: <CAADWXX_5AJxTsk5m_RvP58d=quRMqT4-XbnQQx=obBTKjHr1Og@mail.gmail.com>
Date: Thu, 21 Aug 2025 16:40:13 -0400
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>, 
	"David S. Miller" <davem@...emloft.net>, Alexander Potapenko <glider@...gle.com>, 
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	x86@...nel.org, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH RFC 31/35] crypto: remove nth_page() usage within SG entry

On Thu, Aug 21, 2025 at 4:29 PM David Hildenbrand <david@...hat.com> wrote:
> > 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