<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 2:10 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 12:17 PM, Pete Cooper
wrote:<br class="">
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<div class="">On Jan 7, 2015, at 12:05 PM, Matt Arsenault <<a moz-do-not-send="true" href="mailto:arsenm2@gmail.com" class="">arsenm2@gmail.com</a>> wrote:</div>
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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>>
wrote:</div>
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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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Actually, we had a similar discussion a while ago about this: <a moz-do-not-send="true" href="http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064624.html" class="">http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064624.html</a></div>
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<div class="">In the link I gave, I proposed using global metadata to
describe address spaces. Its useful, for example, to know that
an address space is always to constant memory, i.e., the CL
model.</div>
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<div class="">I think later in the conversation we also thought about
defining the relationships between address spaces in a similar
method to tbaa on types. Then you could do address space AA.</div>
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I'm a bit hesitant* to do this with metadata. At least to start
with, these seem like backend specific properties. Why not
introduce some hooks into Target or Subtarget with the appropriate
queries?<br class="">
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* Reasons for hesitancy:<br class="">
- Not sure these are purely optimizations - is dropping always
legal?<br class=""></div></div></blockquote>It would be global metadata so can’t be dropped (or just isn’t right now so its ok anyway)<br class=""><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class="">
- How do we merge such things in LTO? <br class=""></div></div></blockquote>Thats a good point.<br class=""><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class="">
- Forward serialization? It might be better to define the
properties better than design a reasonable scheme.<br class=""></div></div></blockquote>As is that.</div><div><br class=""></div><div>I think at the time I proposed metadata we didn’t have TTI or anything else similar. I would be happy to say that things like null ptr deref are defined only for address space 0, and all other address spaces can only be optimised if TTI supports it. This means no TTI would default to not optimizing anything other than address space 0 which I think is good. </div><div><br class=""></div><div>You could also move all of the checks to TTI and define that NoTTI gives an answer for address space 0 and ignores all others. Then you can just query TTI everywhere instead of special casing address space 0 everywhere.</div><div><br class=""></div><div>Pete<br class=""><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class="">
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<div class="">Pete</div>
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<div class="">-Matt</div>
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<div class="">-Matt</div>
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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="">
<br class="">
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="">
<br class="">
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="">
</span> </div>
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