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<p><span id="ms-rterangepaste-start"></span><span id="docs-internal-guid-a663f343-143d-ef81-9e99-9c08bb7392cd">Hi,
</span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">I’m writing an analysis pass which is supposed to find instructions
in a function that modify function’s by-ref arguments. For that I’m using llvm AliasAnalysis pass, particularly querying for ModRef info for instructions and function arguments, to see if a given instruction modifies a given argument. However, for functions
with more than one by-ref argument, I get strange results. Instructions, which modify one of the arguments, are reported as modifying the others too - saying, ModRef info of those instructions is Mod for all arguments.</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">For this example</span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>void function(int& n, int& m)</i></span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>{</i></span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>int tmp = m;</i></span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>n = tmp;</i></span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>}</i></span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><br>
</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">Arguments
<i>n</i> and <i>m</i> are marked as MayAlias, and storing <i>tmp</i> into <i>n</i> is reported to modify
<i>m</i> too.</span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">I found a few discussions about this problem. In one of them,
it was suggested to use cfl-aa, I tried it and it did not solve the problem.</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">In another one they said that this happens on purpose. llvm
Alias Analysis marks function’s by-ref arguments MayAlias, as function may be called with arguments referencing the same variable.</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><br>
</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">So I would like to ask whether this is the case. And if it
is, is there a way to make llvm ignore this consideration?</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><br>
</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">Thanks,</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">Anahit.</span></p>
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<span id="ms-rterangepaste-end"></span><br>
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