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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I would not expect a JIT to produce as good of code as a static compiler. A JIT is supposed to run relatively fast, whereas a static compiler may take a lot
longer.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Micah<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu]
<b>On Behalf Of </b>???<br>
<b>Sent:</b> Tuesday, August 20, 2013 12:23 AM<br>
<b>To:</b> llvmdev@cs.uiuc.edu<br>
<b>Subject:</b> [LLVMdev] Memory optimizations for LLVM JIT<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Hello. </span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">I'm new to LLVM and I've got some problems with LLVM JIT.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">I have set ExecutionEngine as CodeGenOpt::Aggressive and PassManagerBuilder.OptLevel as 3. (mem2reg and GVN are included)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">However, the machine code generated by JIT is not as good as that generated by clang or llc.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Here is an example:</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">--------------------------------------------------------------------</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> source fragment ==> clang or llc</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">struct {</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> uint64_t a[10];</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">} *p;</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> mov 0x8(%rax),%rdx</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">p->a[2] = p->a[1]; mov %rdx,0x10(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">p->a[3] = p->a[1]; ==> mov %rdx,0x18(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">p->a[4] = p->a[2]; mov %rdx,0x20(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">p->a[5] = p->a[4]; mov %rdx,0x28(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">--------------------------------------------------------------------</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> JIT (map p to GlobalVariable) ==> JIT (map p to constant GlobalVariable)</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 1* movabsq $0x18c6b88, %rax 1* movabsq $0x18c6b88, %rax</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 2* movq (%rax), %rcx // p 2* movq (%rax), %rax</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 3* movq 0x8(%rcx), %rdx // a[1] 3* movq 0x8(%rax), %rcx</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 4* movq %rdx, 0x10(%rcx) // a[2] 4* movq %rcx, 0x10(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 5 movq (%rax), %rcx 5</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 6 movq 0x8(%rcx), %rdx 6 movq 0x8(%rax), %rcx</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 7* movq %rdx, 0x18(%rcx) ==> 7* movq %rcx, 0x18(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 8 movq (%rax), %rcx 8</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> 9 movq 0x10(%rcx), %rdx 9 movq 0x10(%rax), %rcx</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">10* movq %rdx, 0x20(%rcx) 10* movq %rcx, 0x20(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">11 movq (%rax), %rax 11</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">12 movq 0x20(%rax), %rcx 12</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">13* movq %rcx, 0x28(%rax) 13* movq %rcx, 0x28(%rax)</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">----------------------------------------------------------------------</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">A GlobalValue was declared and mapped to the variable p.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Some LLVM IR instructions were created according to those generated by LLVM from source.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">I.e., load p, load a[1] based on p, load p again, store a[2] based on p, etc.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">The machine code turned out to be slightly optmized, as shown on the left.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Things were getting better after the GlobalVariable of p was set as a constant.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Redundant Loads of p (line 5, 8 and 11) were removed, and so was line 12 because of line 10.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">However, I could not make it better any more, although optimal machine code just need those marked with '*'.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">It seems that store instructions block the optimizations across them.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">I.e., line 3&6 or 4&9 are similar to line 10&12, but they are not optimized.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">The store (line 4 or 7) between them obviously has no alias problem.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">My question is: how to make LLVM JIT optimize this code? </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Did I miss anything, or need I write some kind of optimization pass?</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">I will be grateful for any help you can provide.</span><o:p></o:p></p>
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