AEaengelke·Sep 30, 2025
TPDE co-author here. Nice work, this was easier than expected; so we'll have better upstream ORC support soon [1].
The benchmark is suboptimal in multiple ways:
- Multi-threading makes things just slower. When enabling multi-threading, LLJIT clones every module into a new context before compilation, which is much more expensive than compilation. There's also no way to disable this. This causes a ~1.5x (LLVM)/~6.5x (TPDE) slowdown (very rough measurement on my laptop).
- The benchmark compares against the optimizing LLVM back-end, not the unoptimizing back-end (which would be a fairer comparison) (Code: JTMB.setCodeGenOptLevel(CodeGenOptLevel::None);). Additionally, enabling FastISel helps (command line -fast-isel; setting the TargetOption EnableFastISel seems to have no effect). This gives LLVM a 1.6x speedup.
- The benchmark is not reall
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y representative, as it causes FastISel fallbacks to SelectionDAG in some very large basic blocks -- i24 occurs rather rarely in real-world code. This is the reason why the speedup from the unoptimizing LLVM back-end is so low. Replacing i24 with i16 gives LLVM another 2.2x speedup. (Hint: to get information on FastISel fallbacks, enable FastISel and pass the command line options "-fast-isel-report-on-fallback -pass-remarks-missed=sdagisel" to LLVM. This is really valuable when optimizing for compile times.)
So we get ~140ms (TPDE), ~730ms (LLVM -O0), or 5.2x improvement. This is nowhere near the 10-20x speedup that TPDE typically achieves. Why? The new bottleneck is JITLink, which is featureful but slow -- profiling indicates that it consumes ~55% of the TPDE "compile time" (so the net compile time speedup is ~10x). TPDE therefore ships its own JIT mapper, which has fewer features but is much faster.
LLVM is really powerful, and despite being not particularly fast, the JIT API makes it extremely difficult to make it not extra-slow, even for LLVM experts.
[1]: https://github.com/tpde2/tpde/commit/29bcf1841c572fcdc75dd61...