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<div class="moz-cite-prefix">On 02/15/2016 04:57 PM, Andrew Trick
wrote:<br>
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<blockquote
cite="mid:980E4DAB-9A80-434B-B27B-8FDDCBD84408@apple.com"
type="cite">
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<div class="">On Feb 15, 2016, at 4:25 PM, Philip Reames <<a
moz-do-not-send="true"
href="mailto:listmail@philipreames.com" class=""><a class="moz-txt-link-abbreviated" href="mailto:listmail@philipreames.com">listmail@philipreames.com</a></a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
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http-equiv="Content-Type" class="">
<div text="#000000" bgcolor="#FFFFFF" class=""> After
reading <a moz-do-not-send="true"
href="https://webkit.org/blog/5852/introducing-the-b3-jit-compiler/"
class="">https://webkit.org/blog/5852/introducing-the-b3-jit-compiler/</a>.,
I jotted down a couple of thoughts of my own here:
<a moz-do-not-send="true"
href="http://www.philipreames.com/Blog/2016/02/15/quick-thoughts-on-webkits-b3/"
class="">http://www.philipreames.com/Blog/2016/02/15/quick-thoughts-on-webkits-b3/</a><br
class="">
</div>
</div>
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<div><br class="">
</div>
Thanks for sharing. I think it’s worth noting that what you are
doing would be considered 5th tier for WebKit, since you already
had a decent optimizing backend without LLVM. </div>
</blockquote>
So, serious, but naive question: what are the other tiers for? My
mental model is generally:<br>
tier 0 - interpreter or splat compiler -- (to deal with run once
code)<br>
tier 1 - a fast (above all else) but decent compiler which gets the
obvious stuff -- (does most of the compilation by methods)<br>
tier 2 - a good, but fast, compiler which generates good quality
code without burning too much time -- (specifically for the hotish
stuff)<br>
tier 3 - "a compile time does not matter, get this hot method"
compiler, decidedly optional -- (compiles only *really* hot stuff)<br>
<br>
(Profiling is handled by tier 0, and tier 1, in the above.)<br>
<br>
It really sounds to me like FTL is positioned somewhere between tier
1 and tier 2 in the above. Is that about right?<br>
<blockquote
cite="mid:980E4DAB-9A80-434B-B27B-8FDDCBD84408@apple.com"
type="cite">
<div>You also have more room for background compilation threads
and aren’t benchmarking on a MacBook Air.</div>
</blockquote>
True! Both definitely matter.<br>
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cite="mid:980E4DAB-9A80-434B-B27B-8FDDCBD84408@apple.com"
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<div><br class="">
</div>
<div>Andy</div>
<div><br class="">
<blockquote type="cite" class="">
<div class="">
<div text="#000000" bgcolor="#FFFFFF" class=""> <br
class="">
Philip<br class="">
<br class="">
<div class="moz-cite-prefix">On 02/15/2016 03:12 PM,
Andrew Trick via llvm-dev wrote:<br class="">
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cite="mid:7D3EEF0B-8563-4E26-8CEE-B38C3B6CB2EA@apple.com"
type="cite" class="">
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<div class="">On Feb 9, 2016, at 9:55 AM, Rafael
Espíndola via llvm-dev <<a
moz-do-not-send="true"
href="mailto:llvm-dev@lists.llvm.org" class=""><a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a></a>>
wrote:</div>
<div class="">
<p dir="ltr" class=""> ><br class="">
> JavaScriptCore's [FTL JIT](<a
moz-do-not-send="true"
href="https://trac.webkit.org/wiki/FTLJIT"
class=""><a class="moz-txt-link-freetext" href="https://trac.webkit.org/wiki/FTLJIT">https://trac.webkit.org/wiki/FTLJIT</a></a>)
is moving away<br class="">
> from using LLVM as its backend, towards [B3
(Bare Bones<br class="">
> Backend)](<a moz-do-not-send="true"
href="https://webkit.org/docs/b3/" class="">https://webkit.org/docs/b3/</a>).
This includes its own [SSA<br class="">
> IR](<a moz-do-not-send="true"
href="https://webkit.org/docs/b3/intermediate-representation.html"
class="">https://webkit.org/docs/b3/intermediate-</a><a
moz-do-not-send="true"
href="https://webkit.org/docs/b3/intermediate-representation.html"
class="">representation.html</a>),<br class="">
> optimisations, and instruction selection
backend.</p>
<p dir="ltr" class="">In the end, what was the
main motivation for creating a new IR?</p>
</div>
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</div>
<div class="">I can't speak to the motivation of the
WebKit team. Those are outlined in <a
moz-do-not-send="true"
href="https://webkit.org/blog/5852/introducing-the-b3-jit-compiler/"
class=""><a class="moz-txt-link-freetext" href="https://webkit.org/blog/5852/introducing-the-b3-jit-compiler/">https://webkit.org/blog/5852/introducing-the-b3-jit-compiler/</a></a>.</div>
<div class="">I'll give you my personal perspective on
using LLVM for JITs, which may be interesting to the
LLVM community.</div>
<div class=""><br class="">
</div>
<div class="">Most of the payoff for high level
languages comes from the language-specific optimizer.
It was simpler for JavaScriptCore to perform loop
optimization at that level, so it doesn't even make
use of LLVM's most powerful optimizations,
particularly SCEV based optimization. There is a
relatively small, finite amount of low-level
optimization that is going to be important for
JavaScript benchmarks (most of InstCombine is not
relevant).</div>
<div class=""><br class="">
</div>
<div class="">SelectionDAG ISEL's compile time makes it
a very poor choice for a JIT. We never put the effort
into making x86 FastISEL competitive for WebKit's
needs. The focus now is on Global ISEL, but that won't
be ready for a while.</div>
<div class=""><br class="">
</div>
<div class="">Even when LLVM's compile time problems are
largely solved, and I believe they can be, there will
always be systemic compile time and memory overhead
from design decisions that achieve generality,
flexibility, and layering. These are software
engineering tradeoffs.</div>
<div class=""><br class="">
</div>
<div class="">It is possible to design an extremely
lightweight SSA IR that works well in a carefully
controlled, fixed optimization pipeline. You then
benefit from basic SSA optimizations, which are not
hard to write. You end up working with an IR of
arrays, where identifiers are indicies into the array.
It's a different way of writing passes, but very
efficient. It's probably worth it for WebKit, but not
LLVM.</div>
<div class=""><br class="">
</div>
<div class="">LLVM's patchpoints and stackmaps features
are critical for managed runtimes. However, directly
supporting these features in a custom IR is simply
more convenient. It takes more time to make design
changes to LLVM IR vs. a custom IR. For example, LLVM
does not yet support TBAA on calls, which would be
very useful for optimizating around patchpoints and
runtime calls.</div>
<div class=""><br class="">
</div>
<div class="">Prior to FTL, JavaScriptCore had no
dependence on the LLVM project. Maintaining a
dependence on an external project naturally has
integration overhead.</div>
<div class=""><br class="">
</div>
<div class="">So, while LLVM is not the perfect JIT IR,
it is very useful for JIT developers who want a quick
solution for low-level optimization and retargetable
codegen. WebKit FTL was a great example of using it to
bootstrap a higher tier JIT.</div>
<div class=""><br class="">
</div>
<div class="">To that end, I think it is important for
LLVM to have a well-supported -Ojit pipeline (compile
fast) with the right set of passes for higher-level
languages (e.g. Tail Duplication).</div>
<div class=""><br class="">
</div>
<div class="">-Andy</div>
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