[Beignet] a question about default optimize option when building

Thu Feb 12 16:47:29 PST 2015

Thanks for your patient reply, I will try.

ÅíÏ¯ºº
2015-02-13

·¢¼þÈË£º Zhigang Gong
·¢ËÍÊ±¼ä£º 2015-02-12 18:26:54
ÊÕ¼þÈË£º 'ÅíÏ¯ºº'
³ËÍ£º 'å½å¸æ±?peng_xihan at dahuatec'@domain.invalid; beignet at lists.freedesktop.org; 'Song, Ruiling'
Ö÷Ìâ£º RE: [Beignet] a question about default optimize option when building

One correction:
Just checked the C99 standard, the restricted pointer is officially supported.
So if should be safe for you to use this in your opencl kernel.

> -----Original Message-----
> From: Zhigang Gong [mailto:zhigang.gong at linux.intel.com]
> Sent: Thursday, February 12, 2015 6:21 PM
> To: 'ÅíÏ¯ºº'
> Cc: 'å½å¸æ±?peng_xihan at dahuatec'; 'beignet at lists.freedesktop.org'; 'Song,
> Ruiling'
> Subject: RE: [Beignet] a question about default optimize option when building
> 
> I just found a non-standard performance hint keyword __restrict__ is
> supported by CLANG.
> You could try it this way
> 
> __global unsigned char __restrict__ *p;
>  int a, b, c, d;
> 
> res1 = *p * (a*b + c*d);
>  <some code here  >
> res2 = *p * (a*b + c*d + 1);
> 
> 
> Then the compiler will treat pointer p pointing to an unique chunk of memory
> which could only be accessed by p. Then if there is no storing to pointer p
> between the res1 and res2 assignment, the res2 will not generate an extra
> load.
> 
> Please have a try at your kernel and check the difference by enable the LLVM
> output as below:
> 
> export OCL_OUTPUT_LLVM_AFTER_GEN=1
> 
> Please be warned, this is not a portable way even it works for beignet, may not
> work with other OpenCL library.
> 
>  > -----Original Message-----
>  > From: Beignet [mailto:beignet-bounces at lists.freedesktop.org] On Behalf
>  > Of Zhigang Gong
>  > Sent: Thursday, February 12, 2015 4:57 PM
>  > To: ÅíÏ¯ºº
>  > Cc: å½å¸æ±?peng_xihan at dahuatec; beignet at lists.freedesktop.org; Song,
>  > Ruiling
>  > Subject: Re: [Beignet] a question about default optimize option when
>  > building
>  >
>  > Athough theoretically this is doable for OpenCL, if user could
>  > guarantee all the buffers are not overlapped, then the compiler could
>  > do more aggresive optimization to merge duplicate loads.
>  >
>  > But you can check the OpenCL C spec or C99 spec, there are no such
>  > type of attribute qualifiers defined, so there is no safe/portable way
>  > to do this type of optimization from compiler side currently.
>  >
>  > On Thu, Feb 12, 2015 at 05:32:08PM +0800, ÅíÏ¯ºº wrote:
>  >  > Any way to tell compiler I have no side effect for memory pointer p
>  >  > and I don't
>  > want to load it again?
>  >  >
>  >  >
>  >  >
>  >  >
>  >  > ÅíÏ¯ºº
>  >  > 2015-02-12
>  >  >
>  >  >
>  >  >
>  >  > ·¢¼þÈË£º Zhigang Gong
>  >  > ·¢ËÍÊ±¼ä£º 2015-02-12 17:14:36
>  >  > ÊÕ¼þÈË£º Song, Ruiling
>  >  > ³ËÍ£º beignet at lists.freedesktop.org; ÅíÏ¯›T
>  > peng_xihan at dahuatech.com >
>  >  > Ö÷Ìâ£º Re: [Beignet] a question about default optimize option when
>  >  > building
>  >  >
>  >  > Some additional analysis based on ruiling's comment.
>  >  >
>  >  > The second load from p(to calculate res2) may or may not be issued.
>  >  > It depends on whether there are some side effect instructions
>  >  > between
>  >  > res1 and res2's assignment. For example, if there is a store
>  >  > instruction or there is a barrier, the second load will be issued
>  >  > and you will see two loads for the same pointer in the final instruction
> stream.
>  >  >
>  >  > As to the a * b + c*d, it will always be optimized and be reused
>  >  > when calculate for res2 which means at the res2 assignment it will
>  >  > only generate one add instruction to add the 1 to the previous
>  >  > calculated value.
>  >  >
>  >  > On Thu, Feb 12, 2015 at 08:56:57AM +0000, Song, Ruiling wrote:
>  >  >  > It should not read global memory again. We already enable such
>  >  >  > kind of
>  > optimization pass in LLVM.
>  >  >  > And (a*b+c*d) should not calculate again. This is common-subexpression.
>  > Clang should do it easily. But I am not quite sure whether clang is
>  > affected by
>  > -O2 or -O0. Anyone know details?
>  >  >  >
>  >  >  > To check specific kernel. You may need to ¡®export
>  > OCL_OUTPUT_LLVM_AFTER_GEN=1¡¯ and build your program again to get
> the
>  > LLVM IR.
>  >  >  >
>  >  >  > From: Beignet [mailto:beignet-bounces at lists.freedesktop.org] On
>  >  >  > Behalf Of ÅíÏ¯ºº
>  >  >  > Sent: Thursday, February 12, 2015 4:40 PM
>  >  >  > To: beignet at lists.freedesktop.org
>  >  >  > Subject: [Beignet] a question about default optimize option when
>  >  >  > building
>  >  >  >
>  >  >  > Hi:
>  >  >  >
>  >  >  > My CL kernel program looks like as follow:
>  >  >  >
>  >  >  > __global unsigned char *p;
>  >  >  > int a, b, c, d;
>  >  >  >
>  >  >  > res1 = *p * (a*b + c*d);
>  >  >  >
>  >  >  >   <some code here  >
>  >  >  >
>  >  >  > res2 = *p * (a*b + c*d + 1);
>  >  >  >
>  >  >  >
>  >  >  > If I use default build option, for res2, what will EU do? read
>  >  >  > global memory
>  > for pointer p again and do computing of "a*b + c*d" again?
>  >  >
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               ¡¡2015-02-13
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   ¹«Ë¾Ãû³Æ£ºÕã½´ó»ª¼¼Êõ¹É·ÝÓÐÏÞ¹«Ë¾
   ZheJiang Dahua Technology CO.,LTD.
   µØÖ·£ºº¼ÖÝ±õ½Çø±õ°²Â·1199ºÅ
   ²¿ÃÅ£º´æ´¢²úÆ·Ïß-NVR²úÆ·Ïß
   ÊÖ»ú£º18969076807
   ÓÊÕþ±àÂë£º310053
   E-mail: peng_xihan at dahuatech.com
   Http: //www.dahuatech.com

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