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<div class="moz-cite-prefix">On 10/7/19 4:57 PM, Artem Belevich wrote:<br>
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<div dir="ltr" class="gmail_attr">On Mon, Oct 7, 2019 at 1:22 PM Siva Chandra <<a href="mailto:sivachandra@google.com" moz-do-not-send="true">sivachandra@google.com</a>> wrote:<br>
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Hello Hal,<br>
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You had asked me a question about nvtpx on<br>
<a href="https://reviews.llvm.org/D67867" rel="noreferrer" target="_blank" moz-do-not-send="true">https://reviews.llvm.org/D67867</a>. I did some homework on that and below<br>
is what I have learnt.<br>
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For CUDA/nvptx, a libc in general might be irrelevant. However, I<br>
learned from Art (copied in this email) that there is a desire to have<br>
a single library of math functions that clang can rely on for the GPU.<br>
So, even if a libc in general could be irrelevant, a subset of the<br>
libc might indeed become relevant for GPUs.<br>
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We want llvm-libc to expose a thin layer of C symbols over the<br>
underlying C++ implementation library. My patch<br>
(<a href="https://reviews.llvm.org/rL373764" rel="noreferrer" target="_blank" moz-do-not-send="true">https://reviews.llvm.org/rL373764</a>) showcased one way of doing this<br>
for ELF using the section attribute followed by a post-processing<br>
step. We might have to take a different approach for nvptx because ELF<br>
like sections and tooling might not be feasible/available (as there is<br>
no linking phase during GPU-side comiplation for NVIDIA GPUs). Art<br>
explained to me that device code undergoes whole program analysis by<br>
LLVM. Hence, we can provide an explicit C wrapper layer over the C++<br>
implementation library. If source level wrappers are not desirable, we<br>
can consider using IR level aliases (will we have to deal with mangled<br>
names??). This gives the benefit that, while it looks like a normal C<br>
function call from the user's point of view, the whole program<br>
analysis performed by LLVM will eliminate the additional wrapper call<br>
preventing any performance hits.<br>
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<div class="gmail_default" style="font-family:verdana,sans-serif">We're currently using the
<a href="https://github.com/llvm-mirror/clang/blob/master/lib/Headers/__clang_cuda_device_functions.h" moz-do-not-send="true">
wrappers in Clang headers</a>, so this proposal should not make things worse.</div>
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<div class="gmail_default" style="font-family:verdana,sans-serif">The #1 on my wish list for the standard library is to have libm available to clang/llvm as bitcode library, which would make it possible to re-enable lowering to various library calls in LLVM
when we target NVPTX and, possibly, avoid rather precarious dependency on the binary libdevice bitcode blob which comes with CUDA SDK.</div>
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<div class="gmail_default" style="font-family:verdana,sans-serif">AMDGPU folks are also using
<a href="https://github.com/RadeonOpenCompute/ROCm-Device-Libs/tree/master/ocml/src" moz-do-not-send="true">
bitcode libraries</a>, so providing standard math library as bitcode may benefit them, too.</div>
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<p>Thanks, Siva, Art. +1 to this. Across GPUs from several vendors, and other such platforms, I think that this will be very valuable. It's also not just math functions, although the math functions are likely the performance-sensitive cases, but there are a
lot of libc functions that we would like to have available to ease transitioning code to accelerators. For example, snprintf, qsort.</p>
<p> -Hal<br>
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<div class="gmail_default" style="font-family:verdana,sans-serif">--Artem</div>
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<br>
Thanks,<br>
Siva Chandra<br>
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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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