There are other issues with that auto-allocation. I tested all 3 backends on very large queries (hundreds of KBs) per query. Performance of all of them (+LLVM, but -sljit) was abysmal - the compiler overhead was in seconds to tens(!) of seconds. They have some non-linear components in their optimization algorithms. While sljit was scaling linearly and almost as fast as for smaller queries. So yes, it gives higher run-time performance but the cost of that performance grows non-linearly with code size and complexity. While you still can have good performance with manual allocations. I also don't believe you can make AsmJit 2x faster without sacrificing that auto-allocation algorithm.

SLJIT is a bit smarter than just to use hardcoded registers. It's multi-platform anyway, so it uses registers when they are available on the target platform, if not it will use memory, that's why performance can differ between Windows and Linux on x64 for example - different number of available registers.

Good point about SIMD opportunities though - it's something other 2 JITs lack.