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<DIV><FONT color=#0000ff size=2 face=Verdana>Any way to tell compiler I have no
side effect for memory pointer p and I don't want to load it again?</FONT></DIV>
<DIV><FONT size=2 face=Verdana></FONT> </DIV>
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<DIV><FONT color=#c0c0c0><FONT size=2 face=Verdana>彭席汉</FONT></DIV>
<DIV><FONT size=2 face=Verdana>2015-02-12</FONT></FONT></DIV></DIV>
<DIV><FONT size=2 face=Verdana>
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<DIV><FONT face=Verdana><FONT size=2><STRONG>发件人：</STRONG> Zhigang
Gong</FONT></FONT></DIV>
<DIV><FONT face=Verdana><FONT size=2><STRONG>发送时间：</STRONG>
2015-02-12 17:14:36</FONT></FONT></DIV>
<DIV><FONT face=Verdana><FONT size=2><STRONG>收件人：</STRONG> Song,
Ruiling</FONT></FONT></DIV>
<DIV><FONT face=Verdana><FONT size=2><STRONG>抄送：</STRONG>
beignet@lists.freedesktop.org;
彭席汿peng_xihan@dahuatech.com></FONT></FONT></DIV>
<DIV><FONT face=Verdana><FONT size=2><STRONG>主题：</STRONG> Re: [Beignet] a
question about default optimize option when building</FONT></FONT></DIV>
<DIV><FONT size=2 face=Verdana></FONT> </DIV>
<DIV><FONT size=2 face=Verdana>
<DIV>Some additional analysis based on ruiling's comment.</DIV>
<DIV> </DIV>
<DIV>The second load from p(to calculate res2) may or may not be issued.</DIV>
<DIV>It depends on whether there are some side effect instructions between</DIV>
<DIV>res1 and res2's assignment. For example, if there is a store instruction</DIV>
<DIV>or there is a barrier, the second load will be issued and you will see</DIV>
<DIV>two loads for the same pointer in the final instruction stream.</DIV>
<DIV> </DIV>
<DIV>As to the a * b + c*d, it will always be optimized and be reused when</DIV>
<DIV>calculate for res2 which means at the res2 assignment it will only</DIV>
<DIV>generate one add instruction to add the 1 to the previous calculated</DIV>
<DIV>value.</DIV>
<DIV> </DIV>
<DIV>On Thu, Feb 12, 2015 at 08:56:57AM +0000, Song, Ruiling wrote:</DIV>
<DIV>> It should not read global memory again. We already enable such kind of optimization pass in LLVM.</DIV>
<DIV>> 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?</DIV>
<DIV>> </DIV>
<DIV>> 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.</DIV>
<DIV>> </DIV>
<DIV>> From: Beignet [mailto:beignet-bounces@lists.freedesktop.org] On Behalf Of 彭席汉</DIV>
<DIV>> Sent: Thursday, February 12, 2015 4:40 PM</DIV>
<DIV>> To: beignet@lists.freedesktop.org</DIV>
<DIV>> Subject: [Beignet] a question about default optimize option when building</DIV>
<DIV>> </DIV>
<DIV>> Hi:</DIV>
<DIV>> </DIV>
<DIV>> My CL kernel program looks like as follow:</DIV>
<DIV>> </DIV>
<DIV>> __global unsigned char *p;</DIV>
<DIV>> int a, b, c, d;</DIV>
<DIV>> </DIV>
<DIV>> res1 = *p * (a*b + c*d);</DIV>
<DIV>> </DIV>
<DIV>> <some code here ></DIV>
<DIV>> </DIV>
<DIV>> res2 = *p * (a*b + c*d + 1);</DIV>
<DIV>> </DIV>
<DIV>> </DIV>
<DIV>> 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?</DIV>
<DIV> </DIV>
<DIV>> _______________________________________________</DIV>
<DIV>> Beignet mailing list</DIV>
<DIV>> Beignet@lists.freedesktop.org</DIV>
<DIV>> <A
href="http://lists.freedesktop.org/mailman/listinfo/beignet">http://lists.freedesktop.org/mailman/listinfo/beignet</A></DIV>
<DIV> </DIV>
<DIV>_______________________________________________</DIV>
<DIV>Beignet mailing list</DIV>
<DIV>Beignet@lists.freedesktop.org</DIV>
<DIV><A
href="http://lists.freedesktop.org/mailman/listinfo/beignet">http://lists.freedesktop.org/mailman/listinfo/beignet</A></DIV></FONT></DIV></FONT></DIV>
<DIV>= = = = = = = = = = = = = = = = = = = =</DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2> 致<BR>礼！<BR> </FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>
<BR>
2015-02-12</FONT></DIV>
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