RFR (S) 8148787: StringConcatFactory exactness check produces bad bytecode when a non-arg concat is requested

Aleksey Shipilev aleksey.shipilev at oracle.com
Tue Feb 2 07:00:57 UTC 2016

Anyone? This looks like a trivial fix.


On 02/01/2016 10:47 PM, Aleksey Shipilev wrote:
> Hi,
> Please review the fix for a corner case in StringConcatFactory exactness
> check, which produces invalid bytecode:
>   https://bugs.openjdk.java.net/browse/JDK-8148787
> Note that this happens when all three things align:
>   a) BSM is called directly, as Java method -- javac would never produce
> such a String concat shape;
>   b) BC_SB_SIZED_EXACT strategy is used, so exactness check can be applied;
>   c) -Djava.lang.invoke.stringConcat.debug=true is set, forcing
> exactness check to run;
> The issue is with exactness debug check using a temporary local variable
> when the local variable table is small (like it is in non-arg case). The
> code can be reformulated without using temporary variables, with a
> creative use of "swap" instruction.
> Ironically, the bug is within the debugging code, so we don't care about
> its performance at all:
>   http://cr.openjdk.java.net/~shade/8148787/webrev.01/
> Cheers,
> -Aleksey

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