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<p class="MsoNormal"><span lang="EN-US">Hi Jakob,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I’ve a problem related to the commit #155362.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Consider the following snippet:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">void bar(float* f) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">…<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">void foo(float* f, int idx) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">int hi = idx>>3;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">int lo = idx&7;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">bar(&f[hi*8+lo]); // hi*8 + lo == idx<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">bar(&f[hi*10+lo]);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Before 155362 revision InstCombine was able to optimize hi*8+lo to idx by applying following patterns:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo1"><![if !supportLists]><span lang="EN-US"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US">hi*8 -> hi << 3<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo1"><![if !supportLists]><span lang="EN-US"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US">((idx >> 3) << 3) -> idx & -8<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo1"><![if !supportLists]><span lang="EN-US"><span style="mso-list:Ignore">3.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US">hi*8+lo -> hi*8 | lo<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo1"><![if !supportLists]><span lang="EN-US"><span style="mso-list:Ignore">4.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US">(idx & -8) | (idx & 7) -> idx & (-8 | 7) -> idx<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">After 155362 pattern #2 is deferred to DAGCombine stage, so InstCombine is unable to apply pattern #4:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> 4*. ((idx >> 3) << 3) | (idx & 7) -> idx // SimplifyOr can’t handle it.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">So now LLVM IR contains a couple of redundant operations:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %mul312 = shl nsw i32 %shr, 3 ; hi*8<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> %add313 = or i32 %mul312, %and ; hi*8 + lo == idx<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">These few additional operations over index prevent our analysis pass from recognizing memory access patterns and I believe could harm not only us.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I think 4* optimization can be safely done at LLVM IR level.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Can you suggest the best way to fix this issue? I suppose adding new pattern just for particular 4* case is not the best way.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Aleksey<o:p></o:p></span></p>
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