(PLDI’17) • JVM bytecode verification takes more than 150 pages to describe (WebAssembly fits on one page) • It took a decade of research to hash out the details of correct JVM verification • JVM, CIL, Android Dalvik, allow bytecode to create irreducible loops and unbalanced locking structures, features which usually cause optimizing JITs to give up “
a collection of modular and reusable compiler and toolchain technologies. “Despite its name, LLVM has little to do with traditional virtual machines. “The name "LLVM" itself is not an acronym; it is the full name of the project. llvm.org
(PLDI’17) Disadvantages with LLVM bitcode in particular are that 1. it is not entirely stable, 2. it has undefined behavior which had to be corrected in PNaCl, 3. it was found to be less compact than a stack machine, 4. it essentially requires every consumer to either include LLVM or reimplement a fairly complex LLVM IR decoder/verifier, and 5. the LLVM backend is notoriously slow. “
(PLDI’17) Other compiler IRs have similar, sometimes worse, properties. In general, compiler IRs are better suited to optimization and transformation, and not as compact, verifiable code formats. “
WebAssembly is • an abstraction over modern hardware • making it language-, hardware-, and platform-independent, • with use cases beyond just the Web. Bringing the Web up to Speed with WebAssembly (PLDI’17) “
be considered a structured stack machine; • a machine where most computations use a stack of values, • but control flow is expressed in structured constructs such as blocks, ifs, and loops. • virtual instruction set architecture (virtual ISA) https:/ /github.com/WebAssembly/design/blob/main/Rationale.md
(PLDI’17) • JVM, CIL, Android Dalvik, allow bytecode to create irreducible loops and unbalanced locking structures, features which usually cause optimizing JITs to give up • In contrast, the structured control flow of WebAssembly makes validation and compilation fast and simple • and paves the way for structured locking and exception constructs in the future “
console object, you'll need to pass this in from JavaScript (import "console" "log" (func $log (param i32))) (func (i32.const 0) ;; change to positive number (true) if you want to run the if block (call 0)) (func (param i32) local.get 0 (if (then i32.const 1 call $log ;; should log '1' ) (else i32.const 0 call $log ;; should log '0' ))) (start 1) ;; run the first function automatically ) https://developer.mozilla.org/en-US/docs/WebAssembly/Reference/Control_flow/if...else
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![@evacchi @mastodon.social WebAssembly [...] offers • compact representation, • efficient](https://files.speakerdeck.com/presentations/6f7d3541e8ed402f9382a9b1896de3d4/slide_42.jpg){kind=link}
