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<div class="moz-cite-prefix">On 4/19/2018 11:48 AM, Manoj Gupta via
llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAH=QcshhbvkNM-h2Cf_oYG=bo67RfiJzea-1eYAhBJadSEqo7Q@mail.gmail.com">
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Wed, Apr 18, 2018 at 12:54 PM Tim Northover <<a
href="mailto:t.p.northover@gmail.com" moz-do-not-send="true">t.p.northover@gmail.com</a>>
wrote:<br>
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0px 0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex"><span
style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">On
Wed, Apr 18, 2018 at 12:02 PM Friedman, Eli <<a
href="mailto:efriedma@codeaurora.org"
moz-do-not-send="true">efriedma@codeaurora.org</a>>
wrote:</span> </blockquote>
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0px 0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">> Despite the name, the
flag actually has rather straightforward semantics<br>
> from the compiler's perspective. From the gcc docs for<br>
> -fdelete-null-pointer-checks: "Assume that programs
cannot safely<br>
> dereference null pointers, and that no code or data
element resides at<br>
> address zero." (-fno-delete-null-pointer-checks is the
opposite.)<br>
<br>
Ah, now that's quite a bit more palatable. I really should
have read<br>
the docs before spouting "my favourite rant #1". Then my main
concern<br>
is that we end up with a sufficiently clear (and documented)<br>
definition that we're not promising more than we intend. I get
very<br>
grumpy if I can't tell someone with UB that they're Doing It
Wrong.<br>
<br>
Of the two options, I still think the second is a non-starter.<br>
Something between the two might be a datalayout flag
specifying<br>
addrspace(0) behaviour. It's pretty easy to argue that it'd be
good if<br>
code used some kind of<br>
"DataLayout::isPointerIntrinsicallyInvalid(Value *)" for this
kind of<br>
thing anyway (rename or relocate at will).<br>
<br>
And the name really is terrible, we should change it if at all
feasible</blockquote>
<br>
<br>
<div class="gmail_quote">
<div dir="ltr">On Wed, Apr 18, 2018 at 1:46 PM Jon
Chesterfield via llvm-dev <<a
href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
wrote:<br>
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0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
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<div>I'm working with an embedded architecture that could,
in some situations, run faster if code or data could be
located at address zero. I don't know whether this
applies to other embedded chips.</div>
<br>
<div class="gmail_extra">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0px 0px
0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
Despite the name, the flag actually has rather
straightforward semantics <br>
from the compiler's perspective. From the gcc docs
for <br>
-fdelete-null-pointer-checks: "Assume that programs
cannot safely <br>
dereference null pointers, and that no code or data
element resides at <br>
address zero." (-fno-delete-null-pointer-checks is
the opposite.)<br>
<br>
-Eli<br>
<br>
</blockquote>
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</div>
</div>
</blockquote>
<div><br>
</div>
<div> Thanks Tim and Eli,</div>
<div>I should have put the GCC description for the flag in the
email.</div>
<div><br>
</div>
<div>Regarding the suggestion to DataLayout, It is an
interesting idea. I like it in particular since it will
avoid creating a new llvm optimization flag and is auto
embedded in IR.</div>
<div><br>
</div>
<div>Given that, how do we want to proceed, do we want to add
yet another field to the DataLayout string?</div>
</div>
</div>
</blockquote>
<br>
Modifying the datalayout is not a good idea; it doesn't interact
with LTO correctly, and the frontend and the backend generally need
to agree on the datalayout.<br>
<br>
You could maybe use a module flag, if the address-space thing
doesn't work for some reason, but we don't like to introduce more of
those if we can avoid it.<br>
<br>
-Eli<br>
<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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