RFR: JDK-8237528: Inefficient compilation of Pattern Matching for instanceof

[Remi Forax]

----- Mail original -----
> De: "jan lahoda" <jan.lahoda at oracle.com>
> À: "compiler-dev" <compiler-dev at openjdk.java.net>
> Envoyé: Jeudi 23 Janvier 2020 18:10:36
> Objet: RFR: JDK-8237528: Inefficient compilation of Pattern Matching for instanceof

> Hi,
> 
> When javac compiles:
> o instanceof String s
> 
> it desugars this to:
> 
> (let /*synthetic*/ final Object s$temp = o in s$temp instanceof String
> && (s = (String)s$temp) == (String)s$temp)
> 
> The "(s = (String)s$temp) == (String)s$temp)" is there to set the
> variable "s" to the correct, and still return/have a boolean value. But
> this produces several unnecessary instructions for the comparison -
> second cast and unnecessary test/jump. This can be changed to:
> 
> (let /*synthetic*/ final Object s$temp = o in s$temp instanceof String
> && (let s = (String)s$temp; in true))
> 
> I.e. the (s = (String)s$temp) == (String)s$temp) is replaced with
> 
> (let s = (String)s$temp; in true)
> 
> This is a let expression that will execute the assignment as a statement
> and then return true without any tests. This eliminates the unnecessary
> cast and test/jump, producing a smaller code (see bytecode before and
> after below).
> 

Nice !
You can shave two more bytes (the first two instructions) by detecting that the left part of instanceof is already a local variable, so you don't need the first let.

> Proposed patch:
> http://cr.openjdk.java.net/~jlahoda/8237528/webrev.00/index.html
> 
> JBS:
> https://bugs.openjdk.java.net/browse/JDK-8237528
> 
> How does this look?
> 

Rémi

> Jan