From mboxrd@z Thu Jan 1 00:00:00 1970 From: Afzal Mohammed Date: Sat, 22 Aug 2015 07:53:34 +0000 Subject: Re: [RFC] fbdev/riva:change to use generice function to implement reverse_order() Message-Id: <20150822075310.GA2337@afzalpc> List-Id: References: <55D5B3A9.6040901@ti.com> <867D66CD-9A3B-4536-B537-8C065C85E497@gmail.com> <55D6C812.6080400@ti.com> <4DCC50F3-9B6D-4A3A-9693-E7A7196564A8@gmail.com> <55D6DAE5.20304@ti.com> In-Reply-To: <55D6DAE5.20304@ti.com> MIME-Version: 1.0 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: quoted-printable To: Tomi Valkeinen Cc: yalin wang , adaplas@gmail.com, plagnioj@jcrosoft.com, linux-fbdev@vger.kernel.org, open list Hi, On Fri, Aug 21, 2015 at 11:01:41AM +0300, Tomi Valkeinen wrote: > >> Possibly the patches are still good for x86 also, but that needs to be > >> proven. > >> > > not exactly, because x86_64 don=E2=80=99t have hardware instruction to = do rbit OP, > > i compile by test : >=20 > For old drivers i386 may be more relevant than x86_64. It seems asm bit reversal is supported in Kernel on arm & arm64 only, not sure whether any other arch even provide asm bit reversal instruction. > These kind of optimizations should have some real world measurements, Not for this case, but once measured on ARM, iirc, a 32-bit asm bit reversal as compared to doing it in C was taking 1 cycle as opposed to ~225 cycles!, of course writing optimized C could have made it fare better, but still would reach no-way near asm bit reversal. Regards Afzal