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<p class="MsoNormal"><span lang="EN-CA">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">I am looking at the Type Based Alias Analysis and I am trying to understand why, from what I see,
<i>pointsToConstantMemory()</i> never returns true. <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">It seems that this information should come from the TBAA metadata, in which the Access Tag has an optional 4th field to specify this information.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span lang="EN-CA">“Access tags are represented as MDNode s with either 3 or 4 operands. The first operand is an MDNode pointing to the node representing the base type. The second operand is an MDNode pointing
to the node representing the access type. The third operand is a ConstantInt that states the offset of the access.
<u>If a fourth field is present, it must be a ConstantInt valued at 0 or 1. If it is 1 then the access tag states that the location being accessed is “constant” (meaningpointsToConstantMemory should return true; see <a href="https://llvm.org/docs/AliasAnalysis.html#OtherItfs"><span style="color:windowtext">other
useful AliasAnalysis methods</span></a>). </u>The TBAA root of the access type and the base type of an access tag must be the same, and that is the TBAA root of the access tag.” (<a href="https://llvm.org/docs/LangRef.html#tbaa-metadata">https://llvm.org/docs/LangRef.html#tbaa-metadata</a>)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">But, from what I see, the CodeGenTBAA pass never sets this field.
</span><span style="font-size:9.5pt;font-family:Consolas;color:black">CodeGenTBAA</span><span lang="EN-CA">::</span><span style="font-size:9.5pt;font-family:Consolas;color:black">getAccessTagInfo()</span><span lang="EN-CA"> calls
</span><span style="font-size:9.5pt;font-family:Consolas;color:black">createTBAAStructTagNode()
</span><span lang="EN-CA">to create the access tag, but it does not pass the IsConstant parameter.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black">
</span><span style="font-size:9.5pt;font-family:Consolas;color:darkgreen">/// \brief Return metadata for a TBAA tag node with the given</span><span style="font-size:9.5pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black">
</span><span style="font-size:9.5pt;font-family:Consolas;color:darkgreen">/// base type, access type and offset relative to the base type.</span><span style="font-size:9.5pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt;text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black">
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF">MDNode</span><span style="font-size:9.5pt;font-family:Consolas;color:black"> *createTBAAStructTagNode(</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF">MDNode</span><span style="font-size:9.5pt;font-family:Consolas;color:black">
*</span><span style="font-size:9.5pt;font-family:Consolas;color:gray">BaseType</span><span style="font-size:9.5pt;font-family:Consolas;color:black">,
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF">MDNode</span><span style="font-size:9.5pt;font-family:Consolas;color:black"> *</span><span style="font-size:9.5pt;font-family:Consolas;color:gray">AccessType</span><span style="font-size:9.5pt;font-family:Consolas;color:black">,<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:9.5pt;font-family:Consolas;color:black">
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF">uint64_t</span><span style="font-size:9.5pt;font-family:Consolas;color:black">
</span><span style="font-size:9.5pt;font-family:Consolas;color:gray">Offset</span><span style="font-size:9.5pt;font-family:Consolas;color:black">,
</span><span style="font-size:9.5pt;font-family:Consolas;color:blue">bool</span><span style="font-size:9.5pt;font-family:Consolas;color:black">
</span><span style="font-size:9.5pt;font-family:Consolas;color:gray">IsConstant</span><span style="font-size:9.5pt;font-family:Consolas;color:black"> =
</span><span style="font-size:9.5pt;font-family:Consolas;color:blue">false</span><span style="font-size:9.5pt;font-family:Consolas;color:black">);</span><span lang="EN-CA"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">So my questions are:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo2"><![if !supportLists]><span lang="EN-CA"><span style="mso-list:Ignore">-<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-CA"> Is there any reason <i>IsConstant</i> is never set ?
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo2"><![if !supportLists]><span lang="EN-CA"><span style="mso-list:Ignore">-<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-CA"> Would it be valid to set this information according to the const-qualifier of the data accessed, maybe by getting it from
</span><span style="font-size:9.5pt;font-family:Consolas;color:#2B91AF">QualType</span><span style="font-size:9.5pt;font-family:Consolas;color:black">::isConstQualified()</span><span lang="EN-CA">somehow ?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Marianne<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
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