linux-kernel - Re: [RFC] fbdev/riva:change to use generice function to implement reverse

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Date:	Fri, 21 Aug 2015 15:46:50 +0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	adaplas@...il.com, plagnioj@...osoft.com,
	linux-fbdev@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] fbdev/riva:change to use generice function to implement reverse_order()


> On Aug 21, 2015, at 14:41, Tomi Valkeinen <tomi.valkeinen@...com> wrote:
> 
> 
> 
> On 20/08/15 14:30, yalin wang wrote:
>> 
>>> On Aug 20, 2015, at 19:02, Tomi Valkeinen <tomi.valkeinen@...com> wrote:
>>> 
>>> 
>>> On 10/08/15 13:12, yalin wang wrote:
>>>> This change to use swab32(bitrev32()) to implement reverse_order()
>>>> function, have better performance on some platforms.
>>> 
>>> Which platforms? Presuming you tested this, roughly how much better
>>> performance? If you didn't, how do you know it's faster?
>> 
>> i investigate on arm64 platforms:
> 
> Ok. So is any arm64 platform actually using these devices? If these
> devices are mostly used by 32bit x86 platforms, optimizing them for
> arm64 doesn't make any sense.
> 
> Possibly the patches are still good for x86 also, but that needs to be
> proven.
> 
not exactly, because x86_64 don’t have hardware instruction to do rbit OP,
i compile by test :

use the patch:
  use swab32(bitrev32()):
  2775:       0f b6 d0                movzbl %al,%edx                                                                                                                                                    
  2778:       0f b6 c4                movzbl %ah,%eax
  277b:       0f b6 92 00 00 00 00    movzbl 0x0(%rdx),%edx
  2782:       0f b6 80 00 00 00 00    movzbl 0x0(%rax),%eax
  2789:       c1 e2 08                shl    $0x8,%edx
  278c:       09 d0                   or     %edx,%eax
  278e:       0f b6 d5                movzbl %ch,%edx
  2791:       0f b6 c9                movzbl %cl,%ecx
  2794:       0f b6 89 00 00 00 00    movzbl 0x0(%rcx),%ecx
  279b:       0f b6 92 00 00 00 00    movzbl 0x0(%rdx),%edx
  27a2:       0f b7 c0                movzwl %ax,%eax
  27a5:       c1 e1 08                shl    $0x8,%ecx
  27a8:       09 ca                   or     %ecx,%edx
  27aa:       c1 e2 10                shl    $0x10,%edx
  27ad:       09 d0                   or     %edx,%eax
  27af:       45 85 ff                test   %r15d,%r15d
  27b2:       0f c8                   bswap  %eax
4 memory access instructions,



without the patch:
use
do {                            \
-       u8 *a = (u8 *)(l);      \
-       a[0] = bitrev8(a[0]);   \
-       a[1] = bitrev8(a[1]);   \
-       a[2] = bitrev8(a[2]);   \
-       a[3] = bitrev8(a[3]);   \
-} while(0)



    277b:       45 0f b6 80 00 00 00    movzbl 0x0(%r8),%r8d
    2782:       00 
    2783:       c1 ee 10                shr    $0x10,%esi
    2786:       89 f2                   mov    %esi,%edx
    2788:       0f b6 f4                movzbl %ah,%esi
    278b:       c1 e8 18                shr    $0x18,%eax
    278e:       0f b6 d2                movzbl %dl,%edx
    2791:       48 98                   cltq   
    2793:       45 85 ed                test   %r13d,%r13d
    2796:       0f b6 92 00 00 00 00    movzbl 0x0(%rdx),%edx
    279d:       0f b6 80 00 00 00 00    movzbl 0x0(%rax),%eax
    27a4:       44 88 85 54 ff ff ff    mov    %r8b,-0xac(%rbp)
    27ab:       44 0f b6 86 00 00 00    movzbl 0x0(%rsi),%r8d
    27b2:       00 
    27b3:       88 95 56 ff ff ff       mov    %dl,-0xaa(%rbp)
    27b9:       88 85 57 ff ff ff       mov    %al,-0xa9(%rbp)
    27bf:       44 88 85 55 ff ff ff    mov    %r8b,-0xab(%rbp)

6 memory access instructions, and generate more code that the patch .

because the original code use byte access 4 times , i don’t
think have better performance. :)

Thanks






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