Consider code like:
void test(Object o) {
switch (o) {
case X1 -> {}
case X2 -> {}
...(about 100 cases)
```
javac will compile the switch into a switch whose selector is an indy
invocation to `SwitchBootstraps.typeSwitch`, with static arguments being the
types in the cases.
`SwitchBootstraps.typeSwitch` will then create a chain of `MethodHandle`s
performing `instanceof` checks between the switch's selector and the given case
type. The problem is that when the number of cases is high enough, (more than
~40-50), the chain gets too long, and the tests won't inline anymore. This then
leads to a very bad performance, when compared to manually written
if-instanceof-else-if-instanceof- chain.
The proposal herein is to use bytecode (written using the ClassFile
API/library) instead of the `MethodHandle`s chain. The overall performance of
this seems to be similar to the manually written
if-instanceof-else-if-instanceof- chain.
Using the benchmark from the bug, and this patch, I am getting:
MyBenchmark.testIfElse100 thrpt 5 521826.326 ± 7510.042 ops/s
MyBenchmark.testSwitch100 thrpt 5 505440.170 ± 3757.178 ops/s
The most tricky part of this new way to generate the tests is handling of
non-type case labels, and in particular cases with enum constant labels. The
resolution of enum constants is deferred as much as possible, by using an
indirection through the `ResolvedEnumLabels`.
Further improvements may be possible, esp. for some specific cases (like all
cases having a type, and the type being a final class).
-------------
Commit messages:
- Avoiding generating dead code - review feedback.
- Cleanup.
- Attempting to speeding SwitchBootstraps.
Changes: https://git.openjdk.org/jdk/pull/16489/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16489&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8319220
Stats: 302 lines in 2 files changed: 187 ins; 89 del; 26 mod
Patch: https://git.openjdk.org/jdk/pull/16489.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/16489/head:pull/16489
PR: https://git.openjdk.org/jdk/pull/16489
