1Date: Wed, 31 Jan 2001 12:04:33 -0600 2From: Vikram S. Adve <vadve@cs.uiuc.edu> 3To: Chris Lattner <lattner@cs.uiuc.edu> 4Subject: another thought 5 6I have a budding idea about making LLVM a little more ambitious: a 7customizable runtime system that can be used to implement language-specific 8virtual machines for many different languages. E.g., a C vm, a C++ vm, a 9Java vm, a Lisp vm, .. 10 11The idea would be that LLVM would provide a standard set of runtime features 12(some low-level like standard assembly instructions with code generation and 13static and runtime optimization; some higher-level like type-safety and 14perhaps a garbage collection library). Each language vm would select the 15runtime features needed for that language, extending or customizing them as 16needed. Most of the machine-dependent code-generation and optimization 17features as well as low-level machine-independent optimizations (like PRE) 18could be provided by LLVM and should be sufficient for any language, 19simplifying the language compiler. (This would also help interoperability 20between languages.) Also, some or most of the higher-level 21machine-independent features like type-safety and access safety should be 22reusable by different languages, with minor extensions. The language 23compiler could then focus on language-specific analyses and optimizations. 24 25The risk is that this sounds like a universal IR -- something that the 26compiler community has tried and failed to develop for decades, and is 27universally skeptical about. No matter what we say, we won't be able to 28convince anyone that we have a universal IR that will work. We need to 29think about whether LLVM is different or if has something novel that might 30convince people. E.g., the idea of providing a package of separable 31features that different languages select from. Also, using SSA with or 32without type-safety as the intermediate representation. 33 34One interesting starting point would be to discuss how a JVM would be 35implemented on top of LLVM a bit more. That might give us clues on how to 36structure LLVM to support one or more language VMs. 37 38--Vikram 39 40