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:[email protected]] On Behalf Of > Zhigang Gong > Sent: Thursday, February 12, 2015 4:57 PM > To: 彭席汉 > Cc: 褰腑姹?peng_xihan@dahuatec; [email protected]; 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, 彭席汉 wrote: > > Any way to tell compiler I have no side effect for memory pointer p and I > > don't > want to load it again? > > > > > > > > > > 彭席汉 > > 2015-02-12 > > > > > > > > 发件人： Zhigang Gong > > 发送时间： 2015-02-12 17:14:36 > > 收件人： Song, Ruiling > > 抄送： [email protected]; 彭席汿 > [email protected]> > > 主题： 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:[email protected]] On > > > Behalf Of 彭席汉 > > > Sent: Thursday, February 12, 2015 4:40 PM > > > To: [email protected] > > > 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? > > > > > _______________________________________________ > > > Beignet mailing list > > > [email protected] > > > http://lists.freedesktop.org/mailman/listinfo/beignet > > > > _______________________________________________ > > Beignet mailing list > > [email protected] > > http://lists.freedesktop.org/mailman/listinfo/beignet > > = = = = = = = = = = = = = = = = = = = = > > > > 致 > > 礼！ > > > > > > > > 2015-02-12 > > > **************************************************************** > ****** > > ****************** > > > > 公司名称：浙江大华技术股份有限公司 > > ZheJiang Dahua Technology CO.,LTD. > > 地址：杭州滨江区滨安路1199号 > > 部门：存储产品线-NVR产品线 > > 手机：18969076807 > > 邮政编码：310053 > > E-mail: [email protected] > > Http: //www.dahuatech.com > > > > > **************************************************************** > ****** > > ******************** > > > _______________________________________________ > Beignet mailing list > [email protected] > http://lists.freedesktop.org/mailman/listinfo/beignet _______________________________________________ Beignet mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/beignet
