<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>"Brian Goetz" <brian.goetz@oracle.com><br><b>To: </b>"Kevin Bourrillion" <kevinb@google.com><br><b>Cc: </b>"Remi Forax" <forax@univ-mlv.fr>, "daniel smith" <daniel.smith@oracle.com>, "valhalla-spec-experts" <valhalla-spec-experts@openjdk.java.net><br><b>Sent: </b>Wednesday, February 8, 2023 10:10:14 PM<br><b>Subject: </b>Re: Nullness markers to enable flattening<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;"><br>
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<div>To get my stance (that you already know) onto the list:
for a nullable type, null is immeasurably *better* than just
a "sensible default"; it is *no default at all*. Information
is simply missing, never guessed at. Null is a wonderful
thing (it's only a null-oblivious type system that makes us
think it isn't).</div>
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Yes, though we have to be careful to not push this argument too far
-- if we do, it leads back to "B1 is all I need", and we can all go
home. <br>
<br>
The reason B1 is not sufficient is that, absent representational
heroics (such as using type-specific slack bits to encode null), it
basically forces an indirect representation, losing the flatness and
density that Valhalla aims to give us. And the same is true for B2,
but less so. B2 gets half the benefit; by giving up identity, it
gets flattening on the stack (calling convention optimization.)
That's good, but if that's all we got, it wouldn't be so great. <br>
<br>
B3 is what it is because the VM has a very strong bias towards
initializing to zero. If the zero-bits are part of your domain and
an acceptable default, then great, you have the option to fully
flatten. But if the zero bits are not, then we have no efficient
and safe VM representation for non-nullable B2; either we choose
indirection (giving up performance) or we use Q types and risk the
zero leaking (giving up safety). B1 doesn't have this problem
because what leaks when erasure gets fooled is null, which fails
fast when you try to use it, rather than leaking a value that looks
like it might be a valid value, but which the constructor would
never have generated.<br>
<br>
So this is why Valhalla distinguishes B3 -- because its the kind of
value that we can give the full-flat treatment to. And while there
may be more use cases that are suitable for B2 than B3, there are
likely more *instances* of the B3 values -- because we will allocate
huge matrices of Complex or HalfFloat. <br>
<br>
So we can't say "B2 is all we'll need", because B3 is where the
performance that motivated Valhalla comes from. <br>
<br>
</blockquote><div><br></div><div>I would say, Valhalla has two objectives, providing a more compact memory representation aka B3 is one, having a better escape analysis aka B2 is another.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div>Now, B2 and B3 are from the VM POV, it does not have to leak too much in the user model, we can have value class and zero-default value class at the user level.<br data-mce-bogus="1"></div><div>I kind a like the progression "value class", "zero-default value class", "atomic zero-default value class" that intuitively goes toward better performance at the expense of a non-usual semantics. <br data-mce-bogus="1"></div><div><br></div><div>Rémi<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div></div></div></body></html>