String concatenation tweaks
forax at univ-mlv.fr
Fri Apr 17 17:04:58 UTC 2015
On 04/17/2015 06:51 PM, Aleksey Shipilev wrote:
> On 03/12/2015 11:34 AM, Aleksey Shipilev wrote:
>> I am not really fond of doing the optimizations on javac level: asking
>> users to recompile their programs for better performance and/or fixing
>> the (probable) javac bugs is arguably against what users expect.
>> But in this case, changing the bytecode shape before hitting the JIT
>> compiler seems to be the sanest route. JIT compilers have to maintain
>> more strong invariants than most users let on, see e.g.:
> All right, let me propose a radical alternative here. Asking users to
> recompile their Java code for performance once is probably okay, if we
> describe the performance boosts. But asking for the second, third,
> (N+1)-th time would probably make our lives harder -- people would
> refuse to migrate, and we will be stuck supporting multiple possible
> code shapes in VM.
> Given we have arguments about the exact specifics how to generate string
> concat on javac side, chances are we would not get it right from the
> start; and we would need to make adjustments in future. This is
> especially scary if we want to introduce special string builders that
> would have to be leaked into bytecode ABIs.
> If only there was a way to declare the intent in Java code, and then
> delay the exact specifics how that intent is fulfilled (e.g. what code
> is generated) until the JVM runs... wait, that's invokedynamic!
> So, what if we make a radical ABI change once and for all: implement all
> concatenations via (for example) the signature-polymorphic
> java.lang.StringConcat.concat(Object... methods), and let JDK to figure
> out how to do this exactly at runtime?
you don't need a signature polymorphic method, indy is enough,
Object a = ...
int b = ..
a + " foo " + b
can be compiled to:
aload 0 // a
iload 1 // b
java.lang.invoke.StringConcat [0, "foo", 1]
note that in that case, Java can also support the interpolation syntax:
"$a foo $b"
because the translation is exactly the same :)
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