lkml.org 
[lkml]   [2014]   [Oct]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [RFC V5 3/3] arm64:add bitrev.h file to support rbit instruction
From
On 30 October 2014 13:01, Will Deacon <will.deacon@arm.com> wrote:
> On Wed, Oct 29, 2014 at 05:52:00AM +0000, Wang, Yalin wrote:
>> This patch add bitrev.h file to support rbit instruction,
>> so that we can do bitrev operation by hardware.
>> Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
>> ---
>> arch/arm64/Kconfig | 1 +
>> arch/arm64/include/asm/bitrev.h | 28 ++++++++++++++++++++++++++++
>> 2 files changed, 29 insertions(+)
>> create mode 100644 arch/arm64/include/asm/bitrev.h
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 9532f8d..b1ec1dd 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -35,6 +35,7 @@ config ARM64
>> select HANDLE_DOMAIN_IRQ
>> select HARDIRQS_SW_RESEND
>> select HAVE_ARCH_AUDITSYSCALL
>> + select HAVE_ARCH_BITREVERSE
>> select HAVE_ARCH_JUMP_LABEL
>> select HAVE_ARCH_KGDB
>> select HAVE_ARCH_TRACEHOOK
>> diff --git a/arch/arm64/include/asm/bitrev.h b/arch/arm64/include/asm/bitrev.h
>> new file mode 100644
>> index 0000000..292a5de
>> --- /dev/null
>> +++ b/arch/arm64/include/asm/bitrev.h
>> @@ -0,0 +1,28 @@
>> +#ifndef __ASM_ARM64_BITREV_H
>> +#define __ASM_ARM64_BITREV_H
>> +
>> +static __always_inline __attribute_const__ u32 __arch_bitrev32(u32 x)
>> +{
>> + if (__builtin_constant_p(x)) {
>> + x = (x >> 16) | (x << 16);
>> + x = ((x & 0xFF00FF00) >> 8) | ((x & 0x00FF00FF) << 8);
>> + x = ((x & 0xF0F0F0F0) >> 4) | ((x & 0x0F0F0F0F) << 4);
>> + x = ((x & 0xCCCCCCCC) >> 2) | ((x & 0x33333333) << 2);
>> + return ((x & 0xAAAAAAAA) >> 1) | ((x & 0x55555555) << 1);
>
> Shouldn't this part be in the generic code?
>
>> + }
>> + __asm__ ("rbit %w0, %w1" : "=r" (x) : "r" (x));
>
> You can write this more neatly as:
>
> asm ("rbit %w0, %w0" : "+r" (x));
>

This forces GCC to use the same register as input and output, which
doesn't necessarily result in the fastest code. (e.g., if the
un-bitrev()'ed value is reused again afterwards).
On the other hand, the original notation does allow GCC to use the
same register, but doesn't force it to, so I prefer the original one.

--
Ard.


\
 
 \ /
  Last update: 2014-10-30 14:01    [W:1.202 / U:0.136 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site