lsl-luau discrepancy with mono: register exhaustion with concatenation & possible performance issue of list string casting

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Dal 4/11/2026.

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Descrizione

While trying to stress test / benchmark a comparison for (string) list and llDumpList2String, I came across a register exhaustion for a script in lsl-luau that works in mono.

I'm reporting both as one canny because I'm unsure if these are related.

A) First, for the benchmark:

In LSO:


=== list-to-string benchmark: 100 iterations x 300 items ===

(string)list       total = 1040.432007 ms   mean = 10.404320 ms

llDumpList2String  total = 1896.921143 ms   mean = 18.969212 ms

ratio (string/dump) = 0.548485x

=== DONE ===

In Mono:


=== list-to-string benchmark: 100 iterations x 300 items ===

(string)list       total = 226.023000 ms   mean = 2.260230 ms

llDumpList2String  total = 316.186000 ms   mean = 3.161860 ms

ratio (string/dump) = 0.714842x

=== DONE ===

In LSL-LUAU:


=== list-to-string benchmark: 100 iterations x 300 items ===

(string)list       total = 2287.902000 ms   mean = 22.879020 ms

llDumpList2String  total = 589.087000 ms   mean = 5.890871 ms

ratio (string/dump) = 3.883810x

=== DONE ===

===  list-to-string benchmark: 100 reps x 500 items ===

(string)list       total = 4536.675000 ms   mean = 45.366750 ms

llDumpList2String  total = 1022.474000 ms   mean = 10.224740 ms

ratio (string/dump) = 4.436957x

=== DONE ===

Something about how casting a list to a string is way worse in lsl-luau. My thought was that it's likely something that could be optimized if llDumpList2String is about 4x faster already.

B) Second, a test script of the discrepancy is the following


default

{

    state_entry()

    {

        integer i = 1;

        string s =

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i +

            (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i + (string)i ;

        llOwnerSay(s);

    }

}

It prints "1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111" in mono, but in lsl-luau it fails to compile with "Out of registers when trying to allocate 1 registers: exceeded limit 255"

I doubt there are many real use cases where this would have an impact after switching off mono, but it is still a notable difference.

https://secondlife.canny.io/admin/feedback/slua-alpha/p/lsl-luau-discrepancy-with-mono-register-exhaustion-with-concatenation-possible-p

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