Switch translation

Tagir Valeev amaembo at gmail.com
Sun Apr 8 12:09:46 UTC 2018


> A possible spoiler here is fallthrough; if case A falls into case B, then
cases A and B have to be moved as a group.  (This is another reason to
consider limiting fallthrough.)

I don't think it's a big problem. If we first just need to determine an
index to be passed to the tableswitch, then only the final tableswitch will
have a fallthrough, while the index determination procedure never need a
fallthrough. Thus during the index determination we are free to reorder
branches along with the index values.

With best regards,
Tagir Valeev.

6 апр. 2018 г. 22:52 пользователь "Brian Goetz" <brian.goetz at oracle.com>

The following outlines our story for translating improved switches,
including both the switch improvements coming as part of JEP 325, and
follow-on work to add pattern matching to switches.  Much of this has been
discussed already over the last year, but here it is in one place.

# Switch Translation
#### Maurizio Cimadamore and Brian Goetz
#### April 2018

## Part 1 -- constant switches

This part 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

## 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

#### 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

    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
        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
 - 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

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>>>
                        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

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

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.

This strategy is implemented in the `switch` branch of the amber
repository; see `java.lang.runtime.SwitchBootstraps` in that branch for
(rough!) implementations of the bootstraps.

## Patterns in narrow-target switches

When we add patterns, 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 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 may need to translate as a
pattern switch scheme -- see 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.)

# Part 2 -- type test patterns and guards

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. (Even if
the language doesn't support guards directly, the obvious implementation
strategy for nested patterns is to desugar them into guards.)

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).

## Type-test patterns

Type-test patterns are notable because their applicability predicate is
purely based on the type system, meaning that the compiler can directly
reason about it both statically (using flow analysis, optimizing away
dynamic type tests) and dynamically (with `instanceof`.)  A switch
involving type-tests:

    switch (x) {
        case String s: ...
        case Integer i: ...
        case Long l: ...

can (among other strategies) be translated into a chain of `if-else` using
`instanceof` and casts:

    if (x instanceof String) { String s = (String) x; ... }
    else if (x instanceof Integer) { Integer i = (Integer) x; ... }
    else if (x instanceof Long) { Long l = (Long) x; ... }

#### Guards

The `if-else` desugaring can also naturally handle guards:

    switch (x) {
        case String s
            where (s.length() > 0): ...
        case Integer i
            where (i > 0): ...
        case Long l
            where (l > 0L): ...

can be translated to:

    if (x instanceof String
        && ((String) x).length() > 0) { String s = (String) x; ... }
    else if (x instanceof Integer
             && ((Integer) x) > 0) { Integer i = (Integer) x; ... }
    else if (x instanceof Long
             && ((Long) x) > 0L) { Long l = (Long) x; ... }

#### Performance concerns

The translation to `if-else` chains is simple (for switches without
fallthrough), but is harder for the VM to optimize, because we've used a
more general control flow mechanism.  If the target is an empty `String`,
which means we'd pass the first `instanceof` but fail the guard,
class-hierarchy analysis could tell us that it can't possibly be an
`Integer` or a `Long`, and so there's no need to perform those tests. But
generating code that takes advantage of this information is more complex.

In the extreme case, where a switch consists entirely of type test patterns
for final classes, this could be performed as an O(1) operation by
hashing.  And this is a common case involving switches over alternatives in
a sum (sealed) type.  (We shouldn't rely on finality at compile time, as
this can change between compile and run time, but we should take advantage
of this at run time if we can.)

Finally, the straightforward static translation may miss opportunities for
optimization.  For example:

    switch (x) {
        case Point p
            where p.x > 0 && p.y > 0: A
        case Point p
            where p.x > 0 && p.y == 0: B

Here, not only would we potentially test the target twice to see if it is a
`Point`, but we then further extract the `x` component twice and perform
the `p.x > 0` test twice.

#### Optimization opportunities

The compiler can eliminate some redundant calculations through
straightforward techniques.  The previous switch can be transformed to:

    switch (x) {
        case Point p:
            if (((Point) p).x > 0 && ((Point) p).y > 0) { A }
            else if (((Point) p).x > 0 && ((Point) p).y > 0) { B }

to eliminate the redundant `instanceof` (and admits further CSE

#### Clause reordering

The above example was easy to transform because the two `case Point`
clauses were adjacent.  But what if they are not?  In some cases, it is
safe to reorder them.  For types `T` and `U`, it is safe to reorder `case
T` and `case U` if the two types have no intersection; that there can be no
types that are subtypes of them both.  This is true when `T` and `U` are
classes and neither extends the other, or when one is a final class and the
other is an interface that the class does not implement.

The compiler could then reorder case clauses so that all the ones whose
first test is `case Point` are adjacent, and then coalesce them all into a
single arm of the `if-else` chain.

A possible spoiler here is fallthrough; if case A falls into case B, then
cases A and B have to be moved as a group.  (This is another reason to
consider limiting fallthrough.)

A bigger possible spoiler here is separate compilation.  If at compile
time, we see that `T` and `U` are disjoint types, do we want to bake that
assumption into the compilation, or do we have to re-check that assumption
at runtime?

#### Summary of if-else translation

While the if-else translation at first looks pretty bad, we are able to
extract a fair amount of redundancy through well-understood compiler
transformations.  If an N-way switch has only M distinct types in it, in
most cases we can reduce the cost from _O(N)_ to _O(M)_.  Sometimes _M ==
N_, so this doesn't help, but sometimes _M << N_ (and sometimes `N` is
small, in which case _O(N)_ is fine.)

Reordering clauses involves some risk; specifically, that the class
hierarchy will change between compile and run time.  It seems eminently
safe to reorder `String` and `Integer`, but more questionable to reorder an
arbitrary class `Foo` with `Runnable`, even if `Foo` doesn't implement
`Runnable` now, because it might easily be changed to do so later.  Ideally
we'd like to perform class-hierarchy optimizations using the runtime
hierarchy, not the compile-time hierarchy.

## Type classifiers

The technique outlined in _Part 1_, where we lower the complex switch to a
dense `int` switch, and use an indy-based classifier to select an index, is
applicable here as well.  First let's consider a switch consisting only of
unguarded type-test patterns, optionally with a default clause.

We'll start with an `indy` bootstrap whose static argument are `Class`
constants corresponding to each arm of the switch, whose dynamic argument
is the switch target, and whose return value is a case number (or
distinguished sentinels for "no match" and `null`.)  We can easily
implement such a bootstrap with a linear search, but can also do better; if
some subset of the classes are `final`, we can choose between these more
quickly (such as via binary search on `hashCode()`, hash function, or hash
table), and we need perform only a single operation to test all of those at
once. Dynamic techniques (such as a building a hash map of previously seen
target types), which `indy` is well-suited to, can asymptotically approach
_O(1)_ even when the classes involved are not final.

So we can lower:

    switch (x) {
        case T t: A
        case U u: B
        case V v: C


    int y = indy[bootstrap=typeSwitch(T.class, U.class, V.class)](x)
    switch (y) {
        case 0: A
        case 1: B
        case 2: C

This has the advantages that the generated code is very similar to the
source code, we can (in some cases) get _O(1)_ dispatch performance, and we
can handle fallthrough with no additional complexity.

#### Guards

There are two approaches we could take to add support for guards into the
process; we could try to teach the bootstrap about guards (and would have
to pass locals that appear in guard expressions as additional arguments to
the classifier), or we could leave guards to the generated bytecode.  The
latter seems far more attractive, but requires some tweaks to the bootstrap
arguments and to the shape of the generated code.

If the classifier says "you have matched case #3", but then we fail the
guard for #3, we want to go back into the classifier and start again at
#4.  (Sometimes the classifier can also use this information ("start over
at #4") to optimize away unnecessary tests.)

We add a second argument (where to start) to the classifier invocation
signature, and wrap the switch in a loop, lowering:

    switch (target) {
        case T t where (e1): A
        case T t where (e2): B
        case U u where (e3): C


    int index = -1; // start at the top
    while (true) {
        index = indy[...](target, index)
        switch (index) {
            case 0: if (!e1) continue; A
            case 1: if (!e2) continue; B
            case 2: if (!e3) continue; C
            default: break;

For cases where the same type test is repeated in consecutive positions (at
N and N+1), we can have the static compiler coalesce them as above, or we
could have the bootstrap maintain a table so that if you re-enter the
bootstrap where the previous answer was N, then it can immediately return
N+1.  Similarly, if N and N+1 are known to be mutually exclusive types
(like `String` and `Integer`), on reentering the classifier with N, we can
skip right to N+2 since if we matched `String`, we cannot match `Integer`.
Lookup tables for such optimizations can be built at callsite linkage time.

#### Mixing constants and type tests

This approach also extends to tests that are a mix of constant patterns and
type-test patterns, such as:

    switch (x) {
        case "Foo": ...
        case 0L: ...
        case Integer i:

We can extend the bootstrap protocol to accept constants as well as types,
and it is a straightforward optimization to combine both type matching and
constant matching in a single pass.

## Nested patterns

Nested patterns are essentially guards; even if we don't expose guards in
the language, we can desugar

    case Point(0, var x):

into the equivalent of

    case Point(var a, var x) && a matches 0:

using the same translation story as above -- use the classifier to select a
candidate case arm based on the top-type of the pattern, and then do
additional checks in the generated bytecode, and if the checks fail,
continue and re-enter the classifier starting at the next case.

#### Explicit continue

An alternative to exposing guards is to expose an explicit `continue`
statement in switch, which would have the effect of "keep matching at the
next case."  Then guards could be expressed imperatively as:

    case P:
        if (!guard)
    case Q: ...

More information about the amber-spec-observers mailing list