<html><body><div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: #000000"><div><br></div><div><br></div><hr id="zwchr" data-marker="__DIVIDER__"><div data-marker="__HEADERS__"><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><b>From: </b>"jan lahoda" <jan.lahoda@oracle.com><br><b>To: </b>"Remi Forax" <forax@univ-mlv.fr><br><b>Cc: </b>"Jan Lahoda" <jlahoda@openjdk.org>, "Brian Goetz" <brian.goetz@oracle.com>, "compiler-dev" <compiler-dev@openjdk.org><br><b>Sent: </b>Friday, August 5, 2022 11:54:05 PM<br><b>Subject: </b>Re: [External] : Re: RFR: 8291769: Translation of switch with record patterns could be improved<br></blockquote></div><div data-marker="__QUOTED_TEXT__"><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><p>On 05. 08. 22 22:52, <a class="moz-txt-link-abbreviated" href="mailto:forax@univ-mlv.fr" target="_blank">forax@univ-mlv.fr</a> wrote:<br>
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<blockquote cite="mid:1724934379.19079517.1659732755546.JavaMail.zimbra@u-pem.fr">
<pre class="moz-quote-pre">[...]
</pre>
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<pre class="moz-quote-pre">In any case, I'm a bit afraid that we could enhance a translation patch
forever, always adding new cases that can be optimized. So, I think we
need to draw a line somewhere where the patch provides a benefit while
still being testable and reviewable. With a possible separate
improvements, if they prove to be useful. It is possible I haven't drawn
the line far enough, but looking at the patch, it seems complex enough.
</pre>
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<pre class="moz-quote-pre">I think the issue is that a list of pattern is not the right intermediary
structure to try to optimize the pattern matching, a decision tree is a better
intermediary representation (this is how pattern matching is usually optimized,
i've not invented something new :) )
Here is an example of such decision tree:
<a class="moz-txt-link-freetext" href="https://urldefense.com/v3/__https://github.com/forax/switch-pattern-combinator__;!!ACWV5N9M2RV99hQ!JevAQE8IkRf4n-BMKA2sMTNFfJ5sAyJ1xopR2O9rq_45-xqaHOgirecR8E4zIGB_RgMb27G3tlcHEuGADjk$" target="_blank">https://urldefense.com/v3/__https://github.com/forax/switch-pattern-combinator__;!!ACWV5N9M2RV99hQ!JevAQE8IkRf4n-BMKA2sMTNFfJ5sAyJ1xopR2O9rq_45-xqaHOgirecR8E4zIGB_RgMb27G3tlcHEuGADjk$</a>
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<pre class="moz-quote-pre">I am not really opposed to having an intermediate structure, but I think
the overall cost of the structure is likely to be significant, and it
should provide significantly better code to offset that.
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<pre class="moz-quote-pre">I agree.
</pre>
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<pre class="moz-quote-pre">Looking at the page, I see:
The advantage of a decision tree is that if two patterns have a common
prefix, the code for the common prefix is generated only once.
I wonder if there's a *conceptual* difference between what this PR is
doing and what your code is doing. The PR here is trying to take common
prefixes (of viable cases) and generate code for them only once. I don't
think that not optimizing
case A(String s) -> {}
case A a -> {}
is a conceptual difference, but if this is something that is seen as
critical, I am fairly sure I can implement that without creating a brand
new structure and code to convert the AST into it and back.
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<pre class="moz-quote-pre">You need the intermediary structure when the common prefix are not subsequent
case A(B b) ->
case B(A a) ->
case A(C c) ->
Here the first and the last case shares a common prefix. </pre>
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<p><br>
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<p>Sorry, but I don't see why I can't do this particular thing
without a specialized intermediate representation. </p></blockquote><div><br></div><div>The patterns are a partial orders, because we can optimize the ones with a common prefix, we need a sort, a decision tree is equivalent to a sort.</div><div>There are several advantages to use a decision tree instead of a sort,<br data-mce-bogus="1"></div><div>- it's easier to visualize<br data-mce-bogus="1"></div><div>- it mimics the the usual hand-written code (you don't repeat the instanceofs, you enclose them)</div><div>- i believe that for the code generation, doing local decisions on each node is good enough, we do not need to propagate information in between nodes.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><p>A bigger
question, I think, is when it is safe to do so, and when not.</p></blockquote><div><br></div><div>yes, not all re-ordering are safe, we need to keep some constraints. Fortunately, the ordering problems only arise when there are guards.<br data-mce-bogus="1"></div><div>The idea is that for each node of the decision tree, you should generate the code in that order, first for the record pattern, then the code for the guards then the code for the type pattern, in that order.<br data-mce-bogus="1"></div><div>If the guards are recorded in the same order as in the pattern matching (this is equivalent to say that the sort should be stable in term of guards), we should be ok.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div>As you said, the current spec allows to freely mix type patterns and type pattern + guard pattern with the same type, this has to be fixed at the spec level.<br data-mce-bogus="1"></div><div>I think it's reasonable to introduce an order between the type pattern + guard pattern and the type pattern on the same type, because it's more readable and because we do not want users to care to much about the order of the patterns to get good performance. You should get good performance by default, and not have to crawle StackOverflow to learn the arcane of finding the "right order" for patterns.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div>Rémi<div><br></div></div></div></body></html>