linux-kernel - Re: [git pull] drm build fix

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Message-ID: <CA+55aFwZGzy6GJQn_2pJAnFTMe4LLmzLUpHYNxz-YyJOhmhxFg@mail.gmail.com>
Date:	Sat, 3 Aug 2013 20:50:33 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Airlie <airlied@...ux.ie>,
	Alex Deucher <alexander.deucher@....com>
Cc:	DRI mailing list <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [git pull] drm build fix

On Sat, Aug 3, 2013 at 6:08 PM, Dave Airlie <airlied@...ux.ie> wrote:
>
> Alex Deucher (1):
>       drm/radeon: fix 64 bit divide in SI spm code

That code is stupid. You're asking for a 64-by-64 divide, when the
divisor is clearly an "int" (100 and 1000 respectively).

Why is it doing "div64_s64()" instead of the simpler and faster "div_s64()"?

A full 64-by-64 divide is _expensive_ on 32-bit architecture (up to
four divide instructions, each potentially expensive in its own
right), which is the whole reason why we have that "math64.h" to begin
with - to make people aware of it.

Now, our lib/div64.c routines do notice that the upper bits of the
divisor are zero and end up using the simpler 64-by-32 divide
functions, but why explicitly ask for those more complex functions to
begin with?

So the code is likely not performance critical, and hey, our library
routines do the simple optimizations to avoid the trivially excessive
divide instructions, so this "doesn't matter". Except for the
annoyance factor of you using a more complicated function for no
reason.

                   Linus
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