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<div class="moz-cite-prefix">On 3/16/2017 12:41 PM, Hal Finkel
wrote:<br>
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<div class="moz-cite-prefix">On 03/16/2017 01:54 PM, Friedman, Eli
via llvm-dev wrote:<br>
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<div class="moz-cite-prefix">On 3/16/2017 10:51 AM, 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>
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font-family:Calibri,Arial,Helvetica,sans-serif"> <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>
</div>
<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>
<div><br>
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<br>
I think we should also include the address-space of each alloca
explicitly in textual IR. Having the type depend on the
datalayout would be confusing, because you'd need to look at the
datalayout to figure out which address-space the alloca in a
snippet of IR uses.<br>
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<br>
Are we talking about this syntax?<br>
<br>
%i = alloca f32, align 4, addrspace 5<br>
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<br>
Something like that, yes, I think.<br>
<br>
-Eli<br>
<pre class="moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
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