Rust ir Projects

qbe-rs

15 45 4.6 Rust

QBE IR in natural Rust data structures

Project mention: A whirlwind tour of the LLVM optimizer | news.ycombinator.com | 2023-05-12

You might be underestimating the accuracy of the CPU models LLVM uses.
For x86, the same data the code generator uses drives llvm-mca[1], which given a loop body can tell you the throughput, latency, and microarchitectural bottlenecks (decoding, ports, dependencies, store forwarding, etc.)—if not always precisely, then still not worse then IACA, the tool written at Intel by people who presumably knew how the CPUs work, unlike LLVM contributors and the rest of us who can only guess and measure. This separately for Haswell, Sandy Bridge, Skylake, etc.; not “x86”.
Now, is this the best model you can get? Not exactly[2], but it’s close enough to not matter. Do we often need machine code to be optimized to that level of detail? Perhaps not[3], and with that in mind you can shave at least a factor of ten off LLVM’s considerable bulk at the cost of 20—30% of performance[4,5]. But if you do want those as well, it seems that the complexity of LLVM is a fair price, or has the right order of magnitude at least.
(Frontend not included, C++ frontend required to bootstrap sold separately, at a similar markup compared to a C-only frontend with somewhat worse ergonomics.)
[1] https://llvm.org/docs/CommandGuide/llvm-mca.html
[2] https://www.uops.info/
[3] https://briancallahan.net/blog/20211010.html
[4] https://c9x.me/compile/
[5] https://drewdevault.com/talks/qbe.html
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NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020). The latest post mention was on 2023-05-12.