Switch translation, part 1

[Brian Goetz]

This doc covers some groundwork on rationalizing existing translation of 
switch as we move towards pattern-enabled switches.


# Switch Translation, Part 1
#### Maurizio Cimadamore and Brian Goetz
#### December 2017

This document examines the current translation of `switch` constructs by 
`javac`, and proposes a more general translation for switching on 
primitives, boxes, strings, and enums, with the goals of:

  - Unify the treatment of `switch` variants, simplifying the compiler 
implementation and reducing the static footprint of generated code;
  - Move responsibility for target classification from compile time to 
run time, allowing us to more freely update the logic without updating 
the compiler;
  - Lay the groundwork for patterns in `switch`.

Part 2 of this document will focus on the challenges of translating 
pattern `switch`.

## Current translation

Switches on `int` (and the smaller integer primitives) are translated in 
one of two ways.  If the labels are relatively dense, we translate an 
`int` switch to a `tableswitch`; if they are sparse, we translate to a 
`lookupswitch`.  The current heuristic appears to be that we use a 
`tableswitch` if it results in a smaller bytecode than a `lookupswitch` 
(which uses twice as many bytes per entry), which is a reasonable 
heuristic.

#### Switches on boxes

Switches on primitive boxes are currently implemented as if they were 
primitive switches, unconditionally unboxing the target before entry 
(possibly throwing NPE).

#### Switches on strings

Switches on strings are implemented as a two-step process, exploiting 
the fact that strings cache their `hashCode()` and that hash codes are 
reasonably spread out. Given a switch on strings like the one below:

     switch (s) {
         case "Hello": ...
         case "World": ...
         default: ...
     }

The compiler desugar this into two separate switches, where the first 
switch maps the input strings into a range of numbers [0..1], as shown 
below, which can then be used in a subsequent plain switch on ints.  The 
generated code unconditionally calls `hashCode()`, again possibly 
throwing NPE.

     int index=-1;
     switch (s.hashCode()) {
         case 12345: if (!s.equals("Hello")) break; index = 1; break;
         case 6789: if (!s.equals("World")) break; index = 0; break;
         default: index = -1;
     }
     switch (index) {
         case 0: ...
         case 1: ...
         default: ...
     }

If there are hash collisions between the strings, the first switch must 
try all possible matching strings.

#### Switches on enums

Switches on `enum` constants exploit the fact that enums have (usually 
dense) integral ordinal values.  Unfortunately, because an ordinal value 
can change between compilation time and runtime, we cannot rely on this 
mapping directly, but instead need to do an extra layer of mapping.  
Given a switch like:

     switch(color) {
         case RED: ...
         case GREEN: ...
     }

The compiler numbers the cases starting a 1 (as with string switch), and 
creates a synthetic class that maps the runtime values of the enum 
ordinals to the statically numbered cases:

     class Outer$0 {
         synthetic final int[] $EnumMap$Color = new 
int[Color.values().length];
         static {
             try { $EnumMap$Color[RED.ordinal()] = 1; } catch 
(NoSuchFieldError ex) {}
             try { $EnumMap$Color[GREEN.ordinal()] = 2; } catch 
(NoSuchFieldError ex) {}
         }
     }

Then, the switch is translated as follows:

     switch(Outer$0.$EnumMap$Color[color.ordinal()]) {
         case 1: stmt1;
         case 2: stmt2
     }

In other words, we construct an array whose size is the cardinality of 
the enum, and then the element at position *i* of such array will 
contain the case index corresponding to the enum constant with whose 
ordinal is *i*.

## A more general scheme

The handling of strings and enums give us a hint of how to create a more 
regular scheme; for `switch` targets more complex than `int`, we lower 
the `switch` to an `int` switch with consecutive `case` labels, and use 
a separate process to map the target into the range of synthetic case 
labels.

Now that we have `invokedynamic` in our toolbox, we can reduce all of 
the non-`int` cases to a single form, where we number the cases with 
consecutive integers, and perform case selection via an 
`invokedynamic`-based classifier function, whose static argument list 
receives a description of the actual targets, and which returns an `int` 
identifying what `case` to select.

This approach has several advantages:
  - Reduced compiler complexity -- all switches follow a common pattern;
  - Reduced static code size;
  - The classification function can select from a wide range of 
strategies (linear search, binary search, building a `HashMap`, 
constructing a perfect hash function, etc), which can vary over time or 
from situation to situation;
  - We are free to improve the strategy or select an alternate strategy 
(say, to optimize for startup time) without having to recompile the code;
  - Hopefully at least, if not more, JIT-friendly than the existing 
translation.

We can also use this approach in preference to `lookupswitch` for 
non-dense `int` switches, as well as use it to extend `switch` to handle 
`long`, `float`, and `double` targets (which were surely excluded in 
part because the JVM didn't provide a convenient translation target for 
these types.)

#### Bootstrap design

When designing the `invokedynamic` bootstraps to support this 
translation, we face the classic lumping-vs-splitting decision. For now, 
we'll bias towards splitting.  In the following example, 
`BOOTSTRAP_PREAMBLE` indicates the usual leading arguments for an indy 
bootstrap.  We assume the compiler has numbered the case values densely 
from 0..N, and the bootstrap will return [0,n) for success, or N for "no 
match".

A strawman design might be:

     // Numeric switches for P, accepts invocation as P -> I or Box(P) -> I
     CallSite intSwitch(BOOTSTRAP_PREAMBLE, int... caseValues)

     // Switch for String, invocation descriptor is String -> I
     CallSite stringSwitch(BOOTSTRAP_PREAMBLE, String... caseValues)

     // Switch for Enum, invocation descriptor is E -> I
     CallSite enumSwitch(BOOTSTRAP_PREAMBLE, Class<Enum<E extends 
Enum<E>>> clazz,
                         String... caseNames)

It might be possible to encode all of these into a single bootstrap, but 
given that the compiler already treats each type slightly differently, 
it seems there is little value in this sort of lumping for non-pattern 
switches.

The `enumSwitch` bootstrap as proposed uses `String` values to describe 
the enum constants, rather than encoding the enum constants directly via 
condy.  This allows us to be more robust to enums disappearing after 
compilation.

This strategy is also dependent on having broken the limitation on 253 
bootstrap arguments in indy/condy.

#### Extending to other primitive types

This approach extends naturally to other primitive types (long, double, 
float), by the addition of some more bootstraps (which need to deal with 
the additional complexities of infinity, NaN, etc):

     CallSite longSwitch(BOOTSTRAP_PREAMBLE, long... caseValues)
     CallSite floatSwitch(BOOTSTRAP_PREAMBLE, float... caseValues)
     CallSite doubleSwitch(BOOTSTRAP_PREAMBLE, double... caseValues)

#### Extending to null

The scheme as proposed above does not explicitly handle nulls, which is 
a feature we'd like to have in `switch`.  There are a few ways we could 
add null handling into the API:

  - Split entry points into null-friendly or null-hostile switches;
  - Find a way to encode nulls in the array of case values (which can be 
done with condy);
  - Always treat null as a possible input and a distinguished output, 
and have the compiler ensure the switch can handle this distinguished 
output.

The last strategy is appealing and straightforward; assign a sentinel 
value (-1) to `null`, and always return this sentinel when the input is 
null.  The compiler ensures that some case handles `null`, and if no 
case handles `null` then it inserts an implicit

     case -1: throw new NullPointerException();

into the generated code.

#### General example

If we have a string switch:

     switch (x) {
         case "Foo": m(); break;
         case "Bar": n(); // fall through
         case "Baz": r(); break;
         default: p();
     }

we translate into:

     int t = indy[bsm=stringSwitch["Foo", "Bar", "Baz"]](x)
     switch (t) {
         case -1: throw new NullPointerException();  // implicit null case
         case 0: m(); break;
         case 1: n(); // fall through
         case 2: r(); break;
         case 3: p();                                // default case
     }

All switches, with the exception of `int` switches (and maybe not even 
non-dense `int` switches), follow this exact pattern.  If the target 
type is not a reference type, the `null` case is not needed.

## Patterns in narrow-target switches

When we add patterns to the language, we may encounter switches whose 
targets are tightly typed (e.g., `String` or `int`) but still use some 
patterns in their expression.  For switches whose target type is a 
primitive, primitive box, `String`, or `enum`, we'd like to use the 
optimized translation strategy outlined here, but the following kinds of 
patterns might still show up in a switch on, say, `Integer`:

     case var x:
     case _:
     case Integer x:
     case Integer(var x):

The first three (if supported at all) can be translated away by the 
source compiler, as they are semantically equivalent to `default`.  If 
any nontrivial patterns are present (including deconstruction patterns), 
we will need to translate as a pattern switch scheme -- details coming 
in Part 2.  (While the language may not distinguish between "legacy" and 
"pattern" switches -- in that all switches are pattern switches -- we'd 
like to avoid giving up obvious optimizations if we can.)

## Looking ahead -- patterns

A key motivation for reexamining switch translation is the impending 
arrival of patterns in switch.  We expect switch translation for the 
pattern case to follow a similar structure -- lower to an `int` switch 
and use an indy-based classifier to select an index.  However, there are 
a few additional complexities.  One is that pattern cases may have 
guards, which means we need to be able to re-enter the bootstrap with an 
indication to "continue matching from case N", in the event of a failed 
guard.

Translating pattern switches is more complicated because there are more 
options for how to divide the work between the statically generated code 
and the switch classifier, and different choices have different 
performance side-effects (are binding variables "boxed" into a tuple to 
be returned, or do they need to be redundantly calculated).  These will 
be explored in Part 2.






-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/amber-spec-experts/attachments/20171207/163c99c4/attachment-0001.html>