Switch on long: sure.
Switch on float/double: why?
As someone who puts nontrivial effort into trying to get developers in
my company to /stop/ ever depending on exact equality of floats and
doubles, the only effect of this change will be to give me one
additional thing to tell people not to do.
We already have ==, !=, equals, assertEquals, and using as a key in
that list, so in a sense, "what's one more?". But -- were any actual
advantages to doing this mentioned in this thread? I don't see them.
It seems like the thread skipped right over that part?
If I had to guess, I'm guessing it might have something to do with the
idea that Float/Double will automatically get supported by
pattern-matching and there's nothing we can do about that. Is it
something like that?
On Wed, Dec 13, 2017 at 5:44 PM, Paul Sandoz <[email protected]
<mailto:[email protected]>> wrote:
Recently i was mildly annoyed to discover that Float/DoubleBuffer
provide another variant of equality different to that of
Float.equals/Arrays.equals and ==, specifically:
* This method considers two float elements {@code a} and {@code b}
* to be equal if
* {@code (a == b) || (Float.isNaN(a) && Float.isNaN(b))}.
* The values {@code -0.0} and {@code +0.0} are considered to be
* equal, unlike {@link Float#equals(Object)}.
The vectorized implementations for float/double
comparison/equality leverage the equivalent of floatToRawIntBits
and on a mismatch have to check if it was caused by NaNs and if
continue the search. The equivalent vectorized implementation for
buffers (in progress) needs to do the same for NaNs and +0/-0.
I can imagine bit-wise comparison is problematic since IIUC the
actually bit pattern of a NaN value can, in a platform specific
manner, change depending on how it’s operated on.
Paul.
> On 13 Dec 2017, at 16:51, John Rose <[email protected]
<mailto:[email protected]>> wrote:
>
> Joe's points make perfect sense to me.
>
> Because of distinct problems with float, double, and reference
operand
> types, the "==" operator of Java is a poor equivalence relation,
so just
> referring the semantics of switch to op== is IMO a false start
for defining
> switch. Switch-on-string has already broken with that false
start, in the
> case of references, using Object.equals. A coherent way to
break from
> op== on floats is to, also, refer to the closest possible
Object.equals
> method, that on Float (and Double). Joe's proposal in fact
appeals to
> the same standards, that of floatToIntBits.
>
>
https://docs.oracle.com/javase/7/docs/api/java/lang/Float.html#equals(java.lang.Object)
<https://docs.oracle.com/javase/7/docs/api/java/lang/Float.html#equals%28java.lang.Object%29>
>
> The most fine-grained equality relation that can be defined
across all
> types does not have an API point, but it can be called
"substitutability".
> For references substitutability is simply acmp, or
op==(Object,Object).
> For floats, substitutability is approximated by equality on
floatToIntBits,
> but defined rigorously by equality on floatToRawIntBits, which
preserves
> distinctions among NaNs. Since those distinctions can be
observed by
> code, two distinct NaNs cannot be said to be substitutable for each
> other.
>
> Joe's comparison, and that of Float.equals, is slightly more
coarse-grained
> of an equivalence relation, because all the NaNs are grouped
into a single
> cluster. I wish the designer of Float.equals had not stopped
arbitrarily at
> NaN folding, and used floatToRawIntBits. But, given that
history, I think
> when switch supports floats and doubles, it will use Joe's
comparison.
>
> As Remi points out, suitable third-party extractors (or value
type wrappers)
> can provide other relations besides Joe's default, either
distinguishing
> NaNs or lumping zeroes. Perhaps even rejecting NaNs, since they
aren't
> equal to themselves, supposedly.
>
> But we only get to set the default once. So perhaps we should delay
> supporting floats directly, until we can put all three or four float
> matching predicates in front of us and decide which is the default.
>
> I see no corresponding reason to delay longs. Instead, I see a
pressing
> need to figure out the correct relation between switch (x) {
case (byte)1; }
> where x might be a long or Long. I don't see a way to delay
that decision.
>
> Backing up a bit, I prefer to evaluate match semantics in terms
of assignment
> detection, rather than ad hoc equality predicates. If the story
is only ad
> hoc, "if this type then this predicate" I am sure it will have
more nasty
> corners than it needs. If it has an overarching principle, then
I am sure
> it will have nasty corners (as with +0 and NaNs), but only a minimum
> of them. And the overarching principle I prefer for match is to
ask the
> following polymorphic question: "Could a value just like this case
> expression have been assigned to that switch variable?" This, IMO,
> unwinds a lot of otherwise ad hoc special pleading. It does require
> some ad hoc definition of what "just like this" means, but the
rest falls
> out of prior JLS semantics. Including the vexed questions which
will
> be occurring to you, above, about Long vs. long vs. byte.
>
> — John
>
> On Dec 12, 2017, at 1:52 PM, Remi Forax <[email protected]
<mailto:[email protected]>> wrote:
>>
>> While we could do that, use bits representation for float and
double, this is typically the kind of things that a user can also
do with a record (a value type record ?) and a deconstructor, so
in my opinion, we should not rush to implement this kind of switch
given that we will soon provide a general mechanism to implement
them outside of the JDK.
>>
>> Rémi
>>
>> De: "Brian Goetz" <[email protected]
<mailto:[email protected]>>
>> À: "amber-spec-experts" <[email protected]
<mailto:[email protected]>>
>> Envoyé: Lundi 11 Décembre 2017 22:25:34
>> Objet: Switching on float/double/long
>> A target of opportunity for the new switch JEP is to fill out
the set of types that traditional switches can operate on --
specifically float, double, and long. The reason that we don't
support these now is mostly an accident of history; the
`tableswitch` and `lookupswitch` opcodes are int-based, so the
compiler doesn't have a convenient target for translating these.
As you've seen from the recent notes on switch translation, we're
working towards using indy more broadly as a translation target
for most switch constructs. This makes it far easier to bust the
limitations on switch argument types, and so this has been listed
as a target of opportunity in the JEP (for both statement and
expression switches.)
>>
>> Our resident floating-point expert, Joe Darcy, offers the
following additional thoughts on the subject:
>>
>> -- Begin forwarded message
>>
>> Per a recent request from Brian, I've written a few thoughts
about switching on floating-point values.
>>
>> To address some common misunderstandings of floating-point,
while it is often recommended to *not* compare floating-point
values for equality, it is perfectly well-defined to do such
comparisons, it just might not do what you want
>>
>> For example, instead of
>>
>> // Infinite loop since sum stored in d never exactly equals
1.0, doh!
>> while(d != 1.0)\u000B
>> d += 0.1;
>>
>> use either
>>
>> // Counted loop
>> for(int i = 0; i < 10; i++)\u000B
>> d += 0.1;
>>
>> or
>>
>> // Stop when numerical threshold is met
>> while(d <= 1.0)\u000B
>> d += 0.1;
>>
>> depending on the semantics the loop is trying to capture.
>>
>> I've attached a slide from my JVMLS talk this year to help
illustrate the semantic modeling going in in IEEE 754
floating-point. Each of the 232 possible bit patterns of a float
is some floating-point value, likewise for the 264 possible bit
patterns of a double. However, from a Java language or JVM
perspective, there are not 232 or 264 distinct values we need or
want to distinguish in most cases. In particular, we almost always
want to treat all bit patterns which encode a NaN as a single
conceptual NaN. Another wrinkle concerns zero: IEEE 754 has both a
positive zero and a negative zero. Why are there *two* zeros?
Because there are two infinities. The signed infinities and
distinguished by divide (1.0/0.0 => +infinity, 1.0/-0.0 =>
-infinity) and by various library functions.
>>
>> So we want to:
>>
>> * Allow every distinct finite nonzero floating-point value
to be the case of a switch.
>> * Allow -0.0 and +0.0 to be treated separately.
>> * Allow -infinity and +infinity to be treated separately.
>> * Collapse all NaN representation as a single value.
>>
>> For the "Rounding" mapping in the diagram which goes from the
extended real numbers to floating-point data, there is a nonempty
segment of the real number line which maps to a given
representable floating-point number. For example, besides the
string "1.0" mapping exactly to the reprentable floating-point
value 1.0, there is a region slightly small than 1
(0.99999999999999999999...) which will round up to 1.0 and a
region slightly larger than 1 (1.000000000000000001...) which will
round down to 1 from decimal -> binary conversion. This would need
to be factored into any distinctiveness requirements for the
different arms of the switch. In other words
>>
>> case 1.000000000000000001:
>> ....
>> case 0.99999999999999999999
>> ...
>>
>> would need to be rejected just as
>>
>> case 0:
>> ....
>> case 00:
>>
>> is rejected.
>>
>> In terms of JDK 9 structures and operations, the following
transformation of a float switch has what I think are reasonable
semantics:
>>
>> Replace each float case label y in the source with an int
label resulting from floatToIntBits(y). Note that floatToIntBits
is used for the mapping rather than floatToRawIntBits since we
want NaNs to be grouped together.
>>
>> Instead of switching on float value x, switch on
floatToIntBits(x).
>>
>> HTH,
>>
>> -Joe
>
--
Kevin Bourrillion | Java Librarian | Google, Inc. |[email protected]
<mailto:[email protected]>
