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<p class="MsoNormal">Hi Jose,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The langref is correct for a 32-bit architecture, but indeed forgets to mention that in the example.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Also see: <a href="https://www.godbolt.org/z/PseehvTdf">https://www.godbolt.org/z/PseehvTdf</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">For a 64-bit architecture, the inner_a is at a 16-byte boundary. (Just omit the '-m32' in the godbolt link)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Greetings,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Jeroen Dobbelaere<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>José Wesley Magalhães via llvm-dev<br>
<b>Sent:</b> Monday, April 26, 2021 15:43<br>
<b>To:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [llvm-dev] Offsets in TBAA Access Tags<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Dear LLVMers,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I'm following the TBAA section from the LLVM <o:p></o:p></p>
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<p class="MsoNormal">Language Reference Manual, and I found a minor<o:p></o:p></p>
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<p class="MsoNormal">detail in the offsets in the access tags. <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">For example:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<pre><span class="gmail-kt">void</span> <span class="gmail-nf">f</span><span class="gmail-p">(</span><span class="gmail-k">struct</span> <span class="gmail-nc">Outer</span><span class="gmail-o">*</span> <span class="gmail-n">outer</span><span class="gmail-p">,</span> <span class="gmail-k">struct</span> <span class="gmail-nc">Inner</span><span class="gmail-o">*</span> <span class="gmail-n">inner</span><span class="gmail-p">,</span> <span class="gmail-kt">float</span><span class="gmail-o">*</span> <span class="gmail-n">f</span><span class="gmail-p">,</span> <span class="gmail-kt">int</span><span class="gmail-o">*</span> <span class="gmail-n">i</span><span class="gmail-p">,</span> <span class="gmail-kt">char</span><span class="gmail-o">*</span> <span class="gmail-n">c</span><span class="gmail-p">)</span> <span class="gmail-p">{</span><o:p></o:p></pre>
<pre> <span class="gmail-n">outer</span><span class="gmail-o">-></span><span class="gmail-n">f</span> <span class="gmail-o">=</span> <span class="gmail-mi">0</span><span class="gmail-p">;</span> <span class="gmail-c1">// tag0: (OuterStructTy, FloatScalarTy, 0)</span><o:p></o:p></pre>
<pre> <span class="gmail-n">outer</span><span class="gmail-o">-></span><span class="gmail-n">inner_a</span><span class="gmail-p">.</span><span class="gmail-n">i</span> <span class="gmail-o">=</span> <span class="gmail-mi">0</span><span class="gmail-p">;</span> <span class="gmail-c1">// tag1: (OuterStructTy, IntScalarTy, 12)</span><o:p></o:p></pre>
<pre> <span class="gmail-n">outer</span><span class="gmail-o">-></span><span class="gmail-n">inner_a</span><span class="gmail-p">.</span><span class="gmail-n">f</span> <span class="gmail-o">=</span> <span class="gmail-mf">0.0</span><span class="gmail-p">;</span> <span class="gmail-c1">// tag2: (OuterStructTy, FloatScalarTy, 16)</span><o:p></o:p></pre>
<pre> <span class="gmail-o">*</span><span class="gmail-n">f</span> <span class="gmail-o">=</span> <span class="gmail-mf">0.0</span><span class="gmail-p">;</span> <span class="gmail-c1">// tag3: (FloatScalarTy, FloatScalarTy, 0)</span><o:p></o:p></pre>
<pre><span class="gmail-p">}</span><o:p></o:p></pre>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">When this program is compiled to the IR, the<o:p></o:p></p>
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<p class="MsoNormal">offsets in tag1 and tag2 are actually 4 bytes more<o:p></o:p></p>
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<p class="MsoNormal">than the indicated in the text. (x86_64 architecture)<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"">store i32 0, i32* %i2, align 8, !dbg !46, !tbaa !47</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">!47 = !{!38, !44, i64 16} (tag1)</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"">store float 0.000000e+00, float* %f4, align 4, !dbg !49, !tbaa !50</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">!50 = !{!38, !39, i64 20} (tag2)</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">Is this an error in the manual, or is just some padding<o:p></o:p></p>
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<p class="MsoNormal">within structs Outer and Inner that are yielding<o:p></o:p></p>
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<p class="MsoNormal">such a difference?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Best regards,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">José Wesley de Souza Magalhães<o:p></o:p></p>
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<p class="MsoNormal">MSc Student in Computer Science<o:p></o:p></p>
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<p class="MsoNormal">Belo Horizonte/MG - Brazil<o:p></o:p></p>
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