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<div class="moz-cite-prefix">On 03/16/2017 12:32 PM, Hal Finkel via
llvm-dev wrote:<br>
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<div class="moz-cite-prefix">On 03/16/2017 12:51 PM, Arsenault,
Matthew via llvm-dev wrote:<br>
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<p>LLVM makes several assumptions about address space 0.
However,</p>
<p>alloca is presently constrained to always return this
address space.</p>
<p>There's no real way to avoid using alloca, so without this</p>
<p>there is no way to opt out of these assumptions.</p>
<p><span style="font-size:12pt"><br>
</span></p>
<p><span style="font-size:12pt">The problematic assumptions
include:</span></p>
<ul>
<li><span style="font-size:12pt">That the pointer size used
for the stack is the same size as </span><span
style="font-size:12pt">the code size pointer, which is
also the maximum sized pointer.</span></li>
<li><span style="font-size:12pt">That 0 is an invalid,
non-dereferencable pointer value.</span></li>
</ul>
<span style="font-size:12pt">These are problems for AMDGPU
because alloca is used to </span><span
style="font-size:12pt">implement the private address space,
which uses a 32-bit </span><span style="font-size:12pt">index
as the pointer value. Other pointers are 64-bit </span><span
style="font-size:12pt">and behave more like LLVM's notion of
generic address </span><span style="font-size:12pt">space.
By changing the address space used for allocas, </span><span
style="font-size:12pt">we can change our generic pointer
type to be LLVM's generic </span><span
style="font-size:12pt">pointer type which does have similar
properties.</span>
<div><br>
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<div>The proposal here is to add a -A field to the datalayout
string which will specify the address space for allocas.
IRBuilder::CreateAlloca and company gain a DataLayout
argument, and some intrinsics that currently don't support
address spaces need to support them.</div>
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<div>This has been implemented out of tree before before
for CHERI.<br>
<span style="font-size:12pt"></span>
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<p>This has also been proposed before but for different
reasons: <a moz-do-not-send="true"
href="http://lists.llvm.org/pipermail/llvm-dev/2015-August/089706.html"
class="OWAAutoLink" id="LPlnk407520"
previewremoved="true">http://lists.llvm.org/pipermail/llvm-dev/2015-August/089706.html</a></p>
<p><br>
</p>
<p>My current proposal is more focused than the previous
proposal. Instead of allowing specifying address spaces
on individual allocas, this restricts it to one chosen
address space specified in the datalayout.</p>
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This sounds fine to me.<br>
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Seems reasonable to me as well. Stating that explicitly since I was
one of the folks hesitant on the last proposal. This one appears to
have a more straight-forward justification. Given we have two
possible uses, even better.<br>
<blockquote cite="mid:d5ac12b5-1fbf-1d60-99c2-bd4e66b18102@anl.gov"
type="cite"> <br>
-Hal<br>
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<p> I think the work to support different address spaces
per-alloca is a strict superset of this proposal, so if
people are interested in that I think that is a separate
step beyond this.</p>
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<p>-Matt</p>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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