guardIfConstant, the constant sniffer combinator

[John Rose]

This is very similar to a "growable switch" combinator,
which would call the provider for each distinct selector
value.  A key difference is that non-constant values
go through the fallback, where a "growable switch"
doesn't need a fallback, since the provider MH is
always free to just return a standard fallback as
its result (as is the case with your combinator).

I wonder if the concepts can be combined somehow?

Here's a test question:  When the JIT, after heroic
effort, discovers that an argument is constant, can
it run the provider (in the compiler thread???) to
determine a custom handler for the newly discovered
branch of constant code?  The answer is probably,
"sorry, no", although the JIT might set up an uncommon
trap (and/or an execution counter) that can revisit
the question at some point.

Here's a close point of correspondence between the
idea of a growable switch and your combinator:  Both
need a memory.  Both want to remember the appearance
of constants, so a later optimization phase can use the
full historical knowledge.

— John

> On Apr 10, 2019, at 2:47 PM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> The problem is the following,
> with the java compiler intrinsic of amber, String.format() is optimized using invokedynamic in the case the format (the first argument) is constant (and some other conditions on the format), this is great, perf are like 25x in simple benchmarks, and that all because in a lot of code, the format is not constant for the Java compiler. 
> 
> By example,
>  class Logger {
>    public static void log(String format, String message) {
>      System.err.println(String.format(format, message));
>    }
>  }
>  ...
>  logger.log("%s", "hello");
> 
> The format is not a constant inside Logger::log for the Java compiler but when the code is JITed, due to inlining, logger.log("hello") calls String.format() with a constant.
> 
> 
> I propose a way to fix that, by providing a method handle combiner (guardIfConstant) that detects if an argument is a constant and do something different if it's a constant or not.
> It's a little more complex than that, we don't only want to have a separate path if the argument is a constant, we also want to be able to build a method handle tree depending on the value of that constant.
> 
>  MethodHandle guardIfConstant(int argument, MethodHandle targetProvider, MethodHandle fallback)
> 
> the semantics: if the nth argument is a constant, the target provider is called with that argument and the return value, a method handle, is called with all the arguments, otherwise the fallback is called.
> 
> in term of method type:
>  - the method type of the return value of guardIfConstant is the same as fallback
>  - the method type of targetProvider returns a MethodHandle and takes a single parameter which is the nth parameter type of the fallback method type,
>    the returned method handle as to have the same method type as the fallback.
> 
> Rémi
> 
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