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<div class="moz-cite-prefix">On 9/17/17 10:24 AM, Gnanambikai
Krishnakumar via llvm-dev wrote:<br>
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cite="mid:CAHsZv_dvaPWzsN5WO+vzKn9_7BEZKU58UFzDWGd1au+amzq2SA@mail.gmail.com"
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<div>Hello All,<br>
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We want to write a transformation pass such that
the array accesses use a base register other than ebp
(assuming that the code is compiled for x86
architecture). For example,<br>
<br>
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The below C code<br>
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->int a[5];<br>
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->a[2] = 2<br>
<br>
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by default, gets compiled to something like this:<br>
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<div>->mov dword ptr [ebp - 28], eax<br>
<br>
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<div>However, we want our pass to emit the something similar to
the following instead:<br>
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<div>->mov ecx, "base of array"<br>
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<div>->mov dword ptr [ecx - "index from base of array a"],
eax<br>
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<div><br>
Is there any way we could modify the llvm code so as
to achieve the required instruction format? Please
let us know if there is some code that already does
this or if it is documented elsewhere. <br>
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There are two ways that I can see to do this:<br>
<br>
<ul>
<li>Modify the register allocator so that stack offsets are
referenced using another register</li>
<li>Write a separate MachineFunctionPass that scans over the code
and changes the register used to index into the array. The SVA
project has some code which searches for free registers. If you
need it, let me know, and I can point you to the relevant code
on Github.</li>
</ul>
<br>
I think the more interesting question is why you want to do this.
What is your end goal?<br>
<br>
Regards,<br>
<br>
John Criswell<br>
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<div>Regards,<br>
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<div>Gnanambikai<br>
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<pre class="moz-signature" cols="72">--
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
<a class="moz-txt-link-freetext" href="http://www.cs.rochester.edu/u/criswell">http://www.cs.rochester.edu/u/criswell</a></pre>
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