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<div class="moz-cite-prefix">On 08/17/2018 08:46 AM, Troy Johnson
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CY4PR11MB1397062B18744566C67323CAC63D0@CY4PR11MB1397.namprd11.prod.outlook.com">
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Thanks to both of you for clarifying.<br>
<br>
Bekket, local restrict is pretty common in my experience, at least
with our users. It would be good for it to work as expected.<br>
<br>
Hal, I appreciate the difficulty in doing the based-on analysis,
but it needs to be understood that a C compiler was never really
expected to do that analysis! Being able to do it would be great,
of course, but it is quite daunting and was not the original
intent of restrict. I have in fact worked with the original
author of the restrict keyword and proposed WG14/N2260 to clarify
its usage and intent.<br>
<br>
Basically, the based-on terminology was the least bad way anyone
thought of at the time to describe the semantics. The belief was
that users would supply restrict on multiple pointers of the same
type whenever none of them aliased, then use those pointers
directly. Doing the based-on analysis is similar to what's
required to usefully interpret a single restrict-qualified pointer
in isolation, and that's considered unreasonably hard.<br>
</blockquote>
<br>
You're proposing that there is some common cases, dealing
specifically with multiple restrict pointers, where the based-on
relationship is clear, and handling those using the existing
metadata. I'm not sure how limiting this would be, but certainly
could be worth doing. We should discuss this in more detail.<br>
<br>
<blockquote type="cite"
cite="mid:CY4PR11MB1397062B18744566C67323CAC63D0@CY4PR11MB1397.namprd11.prod.outlook.com"><br>
So I think this problem is quite solvable in the common (and
recommended) case of the user specifying restrict liberally and
accessing data directly via those pointers.<br>
<br>
The use case that I normally see is a bunch of local restricted
pointers initialized once and indexed by some increasing integer.
The compiler would honor restrict if the user outlined the code
into a separate function and the pointers were parameters
instead. Sometimes that is the recommended workaround, but it
shouldn't be necessary.<br>
</blockquote>
<br>
I agree. I'd like this workaround not to be necessary.<br>
<br>
<blockquote type="cite"
cite="mid:CY4PR11MB1397062B18744566C67323CAC63D0@CY4PR11MB1397.namprd11.prod.outlook.com"><br>
I'm happy to discuss futher with whomever works on the related
parts of Clang and LLVM. I am not familiar with who that might
be.<br>
</blockquote>
<br>
I did most of the work associated with the llvm.noalias metadata and
the proposed llvm.noalias patches. Others have been investigating
this as well. I'm happy to discuss this with you (and, unless
there's some reason to do otherwise, we can discuss this here, for
LLVM issues, or on cfe-dev, for issues of how Clang translates C
into LLVM).<br>
<br>
Thanks again,<br>
Hal<br>
<br>
<blockquote type="cite"
cite="mid:CY4PR11MB1397062B18744566C67323CAC63D0@CY4PR11MB1397.namprd11.prod.outlook.com"><br>
-Troy
<hr style="display:inline-block;width:98%" tabindex="-1">
<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
color="#000000" face="Calibri, sans-serif"><b>From:</b> Hal
Finkel <a class="moz-txt-link-rfc2396E" href="mailto:hfinkel@anl.gov"><hfinkel@anl.gov></a><br>
<b>Sent:</b> Thursday, August 16, 2018 10:31:52 PM<br>
<b>To:</b> Bekket McClane; Troy Johnson<br>
<b>Cc:</b> <a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [llvm-dev] alias.scope and local
restricted C pointers</font>
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<div class="x_moz-cite-prefix">On 08/16/2018 07:52 PM, Bekket
McClane via llvm-dev wrote:<br>
</div>
<blockquote type="cite"><br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Aug 16, 2018, at 4:41 PM, Troy Johnson
via llvm-dev <<a
href="mailto:llvm-dev@lists.llvm.org" class=""
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
wrote:</div>
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<div class="" style="margin:0in 0in 0.0001pt;
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Concerning slide 16 of<span
class="x_Apple-converted-space"> </span><a
href="https://llvm.org/devmtg/2017-02-04/Restrict-Qualified-Pointers-in-LLVM.pdf"
class="" style="color:rgb(149,79,114);
text-decoration:underline" moz-do-not-send="true">https://llvm.org/devmtg/2017-02-04/Restrict-Qualified-Pointers-in-LLVM.pdf</a></div>
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Specifically “Currently, LLVM only supports restrict
on function arguments, although we have a way to
preserve that information if the function is
inlined.”</div>
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Is that statement still accurate? </div>
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<div><br class="">
</div>
<div>Yes, correct (actually I was just working on restrict,
no_alias and alias.scope attributes). The inliner also
propagates them correctly. </div>
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It would seem that<span
class="x_Apple-converted-space"> </span><a
href="https://llvm.org/docs/LangRef.html#noalias-and-alias-scope-metadata"
class="" style="color:rgb(149,79,114);
text-decoration:underline" moz-do-not-send="true">https://llvm.org/docs/LangRef.html#noalias-and-alias-scope-metadata</a><span
class="x_Apple-converted-space"> </span>should be
sufficiently general to honor C’s restrict qualifier
on local pointers,</div>
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but it does not appear that Clang uses this part of
LLVM’s IR for that purpose today and thus local
restricts are ignored.</div>
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<div><br class="">
</div>
<div>I think that’s correct, but I haven’t come out with any
scenarios regarding local variables/memory that can _not_
be solved by AA. As BasicAA is able to solved most of the
local cases, including malloc and some memory intrinsics.</div>
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</blockquote>
<br>
int *restrict x = some_external_function();<br>
int *restrict y = some_other_external_function();<br>
<br>
This is one of the fundamental use cases for restrict and
BasicAA has nothing to offer in this regard. In other words,
it's a mechanism for encoding an interface contract.<br>
<br>
-Hal<br>
<br>
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Best</div>
<div>Bekket<br class="">
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Thanks,</div>
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Troy</div>
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<pre class="x_moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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