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<div class="moz-cite-prefix">On 01/07/2015 02:15 PM, Pete Cooper
wrote:<br>
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<div class="">On Jan 7, 2015, at 2:10 PM, Philip Reames <<a
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href="mailto:listmail@philipreames.com" class="">listmail@philipreames.com</a>>
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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="">
<br class="">
-Owen</div>
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<div class=""><br class="">
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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>
<div class=""><br class="">
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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="">
<br class="">
* Reasons for hesitancy:<br class="">
- Not sure these are purely optimizations - is dropping
always legal?<br class="">
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It would be global metadata so can’t be dropped (or just isn’t
right now so its ok anyway)<br class="">
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<div class="">
<div bgcolor="#FFFFFF" text="#000000" class=""> - How do we
merge such things in LTO? <br class="">
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Thats a good point.<br class="">
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<div bgcolor="#FFFFFF" text="#000000" class=""> - Forward
serialization? It might be better to define the
properties better than design a reasonable scheme.<br
class="">
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As is that.</div>
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<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>
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<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>
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Either approach sounds fine; I have no opinion. Volunteers? :)<br>
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<div>Pete<br 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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