<div dir="ltr"><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Sep 23, 2020 at 9:48 PM Philip Reames via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>No active work on my side, but I have given the topic of threaded
interpreters (which is what I think you're wanting to produce) a
good amount of thought. <br>
</p>
<p>I'm really not sure that switch is the right canonical form. The
main reason being that having a loop over a large switch is very
likely to encourage code motion which is generally profitable, but
harmful in this particular context.</p>
<p>I had been thinking down the lines of representing the intepreter
as a family of mutually recursive functions with a calling
convention optimized for this case and using a musttail call
through a lookup table for the dispatch. </p></div></blockquote><div><br></div><div>I believe the Wasm3 project (<a href="https://github.com/wasm3/wasm3">https://github.com/wasm3/wasm3</a>) which is a WebAssembly interpreter is using this dispatch technique described by Philip.</div><div>I don't know how exactly it is guaranteed that the indirect calls are converted to tail calls (maybe it's not). But the performance is quite impressive.</div><div><br></div><div>// Paweł<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div>
<p>I've played with the notion of extending clang with a custom
attribute for guaranteed tail calls. I think this is pretty much
the only extension needed to be able to natively write out a
threaded interpreter as a set of mutually recursive functions.</p>
<p>This is all thought experiment from my side; I haven't had time
to sit down and actually prototype any of this.</p>
<p>Philip<br>
</p>
<div>On 9/23/20 7:33 AM, Phipps, Alan via
llvm-dev wrote:<br>
</div>
<blockquote type="cite">
<div>
<p class="MsoNormal">It is my understanding that the
implementation for jump-threading in LLVM is not presently
able to effectively optimize code containing a state-machine
implemented using a loop + switch. This is the case, for
example, with the Coremark benchmark function
core_state_transition(). Bug 42313 was filed to address this
in 2019:<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal"><a href="https://bugs.llvm.org/show_bug.cgi?id=42313" target="_blank">https://bugs.llvm.org/show_bug.cgi?id=42313</a><u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">It appears that GCC improved support for
jump threading in 2015 along the same lines:<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal"><a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54742" target="_blank">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54742</a><u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Is anyone aware of any plan to do improve
LLVM jump-threading along the same lines for LLVM?<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Thanks!<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Alan Phipps<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<br>
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