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<p>I think because it is prohibited by the standard. According to
OpenMP standard the new copies of the variables must be of the
same type and the same size. If the type has the different size on
the device, we become not compatible with the standard.</p>
<p>I agree with Eric Christofer. From my point of view, it just
breaks the existing ABI. Instead of breaking the existing ABI, I
think, it would better to introduce a new, portable, ABI. The
classic ABI could be used to get a little bit more performance,
while the new one could be used to get the compatibility between
pointer-size incompatible targets. In this ABI all variables must
be passed by reference. I think it will solve all the problems.</p>
With the proposed change there would be a difference between the
code compiled for (1) 64 bit host+64 bit device and (2) 64 bit host
+64|32 bit device. You won't be able to make it work properly the
program linked from the (2)nd host object + the (1)st device object
because of the different ABIs. I think it is better to explicitly
specify that we're going to use some special ABI rather than doing
such tricky and dangerous things as in this proposal.
<pre class="moz-signature" cols="72">-------------
Best regards,
Alexey Bataev</pre>
<div class="moz-cite-prefix">26.04.2018 20:14, Hal Finkel via
llvm-dev пишет:<br>
</div>
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<div class="moz-cite-prefix">On 04/26/2018 07:03 PM,
Narayanaswamy, Ravi wrote:<br>
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<div class="WordSection1">
<p class="MsoNormal"><span style="color:#1F497D">Hi Hal,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> We are not
trying to address issues where the object mapped are of
different sizes between host and target with different
ABI. <br>
</span></p>
</div>
</blockquote>
<br>
Why are you not trying to address that issue?<br>
<br>
-Hal<br>
<br>
<blockquote type="cite"
cite="mid:BCFA27423AEC4E41B6CBD7B6A8C696B69B39C7AC@ORSMSX109.amr.corp.intel.com">
<div class="WordSection1">
<p class="MsoNormal"><span style="color:#1F497D"> The issue is
when the objects are of same size like double which is
8bytes on both 32bit and 64bit platform. If a double is
used in a first_private on a target clause, the 64 bit
side will pass it as value whereas on the 32bit side since
the value does not fit in the argument it will be passed
as pointer to a double. There will be a mis-match at the
call site and entry site on this value.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> The main
reason for this change is that when we do backend
outlining for target pragmas the targets information needs
to be communicated to the backend to generate the tables
with the right names. Generate LLVM IR for passing this
information is one mechanism and other is passing the
command option to the backend. For the later each pass
which needs this info will have to change.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Ravi<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"
style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
<a name="_____replyseparator" moz-do-not-send="true"></a><b><span
style="color:windowtext">From:</span></b><span
style="color:windowtext"> Hal Finkel [<a
href="mailto:hfinkel@anl.gov" moz-do-not-send="true">mailto:hfinkel@anl.gov</a>]
<br>
<b>Sent:</b> Wednesday, April 25, 2018 5:50 PM<br>
<b>To:</b> Lin, Jin <<a
href="mailto:jin.lin@intel.com"
moz-do-not-send="true">jin.lin@intel.com</a>>;
Friedman, Eli <<a
href="mailto:efriedma@codeaurora.org"
moz-do-not-send="true">efriedma@codeaurora.org</a>>;
'<a class="moz-txt-link-abbreviated"
href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>'
<<a href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>><br>
<b>Subject:</b> Re: [llvm-dev] [LLVM][RFC]
Representing the target device information in the LLVM
IR<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p>Hi, Jin,<span style="font-size:12.0pt;line-height:105%"><o:p></o:p></span></p>
<p>Can you please back up a bit and talk about the programming
environment in which this problem manifests?<o:p></o:p></p>
<p>If I have a host and a target with different ABIs, then it
seems we have lots of problems. For one thing, the layouts
of structures are different, the sizes of some integer types
are different, the sizes of pointers are different, and so
on. It seems like a solution in this space should address,
somehow, this general translation problem. Fixing this
particular problem with the dispatch function's parameters
feels like only the tip of the iceberg. What if I'm passing
a pointer to some structure, or a pointer to other pointers,
etc.?<o:p></o:p></p>
<p>I understand that OpenMP v5 is expected to have some custom
"mappers" to handle deep copying and translation. Is this
related to the design space here?<o:p></o:p></p>
<p>Thanks again,<o:p></o:p></p>
<p>Hal<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">On 04/25/2018 07:22 PM, Lin, Jin via
llvm-dev wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span style="color:#1F497D">For the
firstprivate clause, the compiler generates code to pass
it by value or by reference to the outlined function.
The reason the first private scalars is generally passed
by value is for the performance reason.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">For this
particular case, the compiler cannot generate code to
pass the double @gg by value under i386-pc-linux-gnu
since the value is 64 bit while the architecture is
32bit. </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">For the
host compilation, the compiler generates the code to
pass the data as well as the outlined function name to
the OMP runtime. </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">For the
target compilation, the compiler generates the outlined
function so that it can be called by the OMP runtime. </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">So, the
compiler is required to generate a single call on the
host to support all the targets. All the target versions
must have the same interface. So the common interface of
the outline function should be used. For this particular
example, the variable @gcc should be passed by reference
under x86_64-mic.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Please let
me know if you have more questions.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Jin</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<div>
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1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"
style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
<b><span style="color:windowtext">From:</span></b><span
style="color:windowtext"> Friedman, Eli [<a
href="mailto:efriedma@codeaurora.org"
moz-do-not-send="true">mailto:efriedma@codeaurora.org</a>]
<br>
<b>Sent:</b> Wednesday, April 25, 2018 4:14 PM<br>
<b>To:</b> Lin, Jin <a
href="mailto:jin.lin@intel.com"
moz-do-not-send="true"><jin.lin@intel.com></a>;
'<a href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>'
<a href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true"><llvm-dev@lists.llvm.org></a><br>
<b>Subject:</b> Re: [llvm-dev] [LLVM][RFC]
Representing the target device information in the
LLVM IR</span><o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"> <o:p></o:p></p>
<div>
<p class="MsoNormal">On 4/25/2018 3:48 PM, Lin, Jin wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span style="color:#0070C0">Given a
global variable @gg, the compiler has to generate code
on the host to specify whether it is passed by value
or passed by reference. In the following example, if
the compiler generates the code for passing by value,
the outlined function on the target </span><span
style="font-family:"color:#0070C0;border:none
windowtext 1.0pt;padding:0in">i386-pc-linux-gnu</span><span
style="color:#0070C0"> cannot get the correct value
since it assumes the variable @gg is passed by
reference. </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#0070C0"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#0070C0">Here is
the corresponding IR on the host side.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:red"> %0 = load
double, double* @gg, align 8, !tbaa !3</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:red"> %1 =
bitcast double %0 to i64</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> …</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#0070C0"> %12 =
getelementptr inbounds [4 x i8*], [4 x i8*]*
%.offload_baseptrs, i32 0, i32 2</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#0070C0"> %13 =
bitcast i8** %12 to i64*</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:red"> store i64
%1, i64* %13, align 8</span><o:p></o:p></p>
</blockquote>
<p class="MsoNormal"
style="margin-bottom:12.0pt;line-height:normal"><span
style="font-size:12.0pt;font-family:"Times New
Roman ,serif",serif"><br>
Could you describe the overall process of calling an
offloaded function in a bit more detail? How do you
describe the ABI of the called function to the OpenMP
runtime?<br>
<br>
I suspect you shouldn't be trying to store things which
aren't pointers into offload_baseptrs.<br>
<br>
-Eli<br>
<br>
<br>
</span><o:p></o:p></p>
<pre>-- <o:p></o:p></pre>
<pre>Employee of Qualcomm Innovation Center, Inc.<o:p></o:p></pre>
<pre>Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project<o:p></o:p></pre>
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style="margin-bottom:12.0pt;line-height:normal"><span
style="font-size:12.0pt;font-family:"Times New
Roman",serif"><br>
<br>
<o:p></o:p></span></p>
<pre>_______________________________________________<o:p></o:p></pre>
<pre>LLVM Developers mailing list<o:p></o:p></pre>
<pre><a href="mailto:llvm-dev@lists.llvm.org" moz-do-not-send="true">llvm-dev@lists.llvm.org</a><o:p></o:p></pre>
<pre><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" moz-do-not-send="true">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></pre>
</blockquote>
<p class="MsoNormal"
style="margin-bottom:12.0pt;line-height:normal"><span
style="font-size:12.0pt;font-family:"Times New
Roman",serif"><o:p> </o:p></span></p>
<pre>-- <o:p></o:p></pre>
<pre>Hal Finkel<o:p></o:p></pre>
<pre>Lead, Compiler Technology and Programming Languages<o:p></o:p></pre>
<pre>Leadership Computing Facility<o:p></o:p></pre>
<pre>Argonne National Laboratory<o:p></o:p></pre>
</div>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
</blockquote>
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