<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Jan 7, 2015, at 3:10 PM, Philip Reames <<a href="mailto:listmail@philipreames.com" class="">listmail@philipreames.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div class="moz-cite-prefix">On 01/07/2015 12:05 PM, Matt Arsenault
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<div class="">On Jan 7, 2015, at 2:55 PM, Philip Reames <<a moz-do-not-send="true" href="mailto:listmail@philipreames.com" class="">listmail@philipreames.com</a>>
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<div class="moz-cite-prefix">On 01/07/2015 11:52 AM, Matt
Arsenault wrote:<br class="">
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<div class="">On Jan 7, 2015, at 2:25 PM, Owen
Anderson <<a moz-do-not-send="true" href="mailto:resistor@mac.com" class="">resistor@mac.com</a>>
wrote:</div>
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<div class="">I'm not aware of any such
restriction, and I know of several LLVM based
systems that use address space 1 for something
other than that.<br class="">
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-Owen</div>
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<div class="">Yes, this would be a problem for us. We
use 1 for a normal address space where 0 is invalid.
However, we also have a problem where some other
address spaces do want 0 to be a valid address,
which just sort of don’t work correctly now.</div>
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If you have an example with a null in a non-0 address
space being mishandled, please file a bug. We'll fix them
as we find them. <br class="">
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<div class="">I think the problems aren’t so much that accessing 0
doesn’t work (although I imagine there are problems with
that), but expectations of comparison with null. The main
problem I’m aware of is comparisons with null pointers. The
first global object in addrspace(3) will have the address of
0, so if a user does if (x != NULL), it will not behave as
expected. For C I think this is supposed to be fixed by
changing the value of NULL to -1, but I don’t think that is
currently supported. That is also complicated because the null
value is different for different address spaces, and I think
the actual null pointer value must be 0 for C++. It doesn’t
really turn up often in real code so I don’t think anybody has
really spent time thinking about how to properly solve this.</div>
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Just to make sure I'm interpreting this right: the problem is
essentially that we hard code "null" to mean address 0 in all
address spaces? If we allowed the numeric value of null to be
configurable per address space, would that resolve the issue at the
LLVM IR level?<br class=""></div></div></blockquote><div><br class=""></div><div>Yes, it would. I’ve always imagined this to a be a large undertaking though</div><br class=""><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class="">
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Solving the frontend/language spec problem seems out of scope for
LLVM, though probably not for clang. Can you point me to a usage of
C++ with non-zero address spaces? I'd be curious to know what's
happening in this space. <br class=""></div></div></blockquote><div><br class=""></div><div>There’s an AMD static C++ extension language, and a khronos draft for an OpenCL C++ kernel language which applies the same sort of restrictions and address spaces to C++ as OpenCL C has. Last time I looked at this I remember that C allows a non-zero null pointer value, but 0 must be implicitly converted to the correct null pointer value and the NULL macro will expand to this integer value. I don’t think the OpenCL C spec touches the issue of different NULL values for different address spaces, but does explicitly allow different sized pointers for each. I am less clear on what C++ requires, but C++11 4.10 says "A null pointer constant is an integral constant expression (5.19) prvalue of integer type that evaluates to zero or a prvalue of type std::nullptr_t."</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class="">
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On Jan 7, 2015, at 1:18 PM, Philip Reames <<a moz-do-not-send="true" href="mailto:listmail@philipreames.com" class="">listmail@philipreames.com</a>>
wrote:<br class="">
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On the review for <a moz-do-not-send="true" class="moz-txt-link-freetext" href="http://reviews.llvm.org/D6808">http://reviews.llvm.org/D6808</a>,
<a moz-do-not-send="true" href="http://reviews.llvm.org/p/majnemer/" class=" phui-handle phui-link-person">majnemer</a>
commented that:<br class="">
<span class="transaction-comment" data-sigil="transaction-comment" data-meta="14_7">"Address space 1 has a
special meaning in LLVM, it's identical to
address space 0 except for the fact that
"null" may be dereferenced. You might want
to consider a different address space."<br class="">
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This is the first I've heard of this and I
can't find any documentation about it
being reserved, either in general, or
specifically for x86. Can anyone clarify?<br class="">
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The only address spaces with special
meanings I know of are:<br class="">
- 0 (the normal address space, null is not
dereferencable)<br class="">
- 256 - TLS, GS relative addressing<br class="">
- 257 - FS relative addressing<br class="">
<br class="">
Philip<br class="">
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