RFR: 8245756: Reduce bootstrap cost of StringConcatFactory prependers

[Remi Forax]

Hi Claes,
do you have evaluated the improvement (1) in separation of the others ?

(2) or (3) are clearly improvement but (1) is a trade of because you are adding a second lookup into another table and make prepender() less likely to be inlined.

Rémi

----- Mail original -----
> De: "Claes Redestad" <claes.redestad at oracle.com>
> À: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> Envoyé: Lundi 25 Mai 2020 22:23:23
> Objet: RFR: 8245756: Reduce bootstrap cost of StringConcatFactory prependers

> Hi,
> 
> let's improve the StringConcatFactory a bit!
> 
> - prependers where both prefix and suffix is null are common in typical
> expressions; caching these specifically is profitable
> 
> - current prepender argument ordering is unfortunate: String prefix,
> <arg>, String suffix. This means we have to do two back-to-back
> insertArguments rather than one if they were grouped together
> 
> - add a bridge to the String mix function in StringConcatHelper to avoid
> method handle invokes in the bootstrap path.
> 
> Webrev: http://cr.openjdk.java.net/~redestad/8245756/open.00/
> Bug:    https://bugs.openjdk.java.net/browse/JDK-8245756
> 
> All together I see a substantial improvement on a number of
> concatenation bootstrap tests. On one stress test[1] I get these numbers
> before:
> 
>      4,257,229,918 instructions # 1.10 insn per cycle ( +- 0.84% )
>        847,230,764 branches # 504.826 M/sec ( +- 0.87% )
>         34,600,787 branch-misses # 4.08% of all branches ( +- 0.91% )
>        0.319545331 seconds time elapsed ( +- 1.18% )
> 
> And after:
> 
>      3,692,867,129 instructions # 1.10 insn per cycle ( +- 0.73% )
>        734,054,515 branches # 498.753 M/sec ( +- 0.74% )
>         30,699,692 branch-misses # 4.18% of all branches ( +- 0.75% )
>        0.295842958 seconds time elapsed ( +- 1.10% )
> 
> So ~13% reduction on this test.
> 
> Testing: tier1+2
> 
> Thanks!
> 
> /Claes
> 
> [1]
> http://cr.openjdk.java.net/~redestad/scratch/erase_scf_types.00/ObjStringCombos.java