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I get the impression WASM is really a very clunky representation, far from what any greenfield project would ever have arrived at. That is to say, its decisions

I get the impression WASM is really a very clunky representation, far from what any greenfield project would ever have arrived at. That is to say, its decisions aren't just tradeoffs that people disagree about, they're simply inappropriate for what it's trying to do. More than once I've encountered comments and blog-posts lamenting its fundamentals, e.g. [0]. [0] http://troubles.md/wasm-is-not-a-stack-machine/ http://troubles.md/wasm-is-not-a-stack-machine/

tedmielczarek 7y ago
I don't think that's true. It was designed for its purpose by people that knew what they were doing based on experience gathered from implementing previous incremental steps including emscripten's original JavaScript output and asm.js (a formalization of some techniques emscripten employed). The design rationale is right there in the spec repository: https://github.com/WebAssembly/design/blob/master/Rationale.md https://github.com/WebAssembly/design/blob/master/Rationale.... It was built to be a better target than asm.js for compiled languages to run in JavaScript VMs and it seems to have succeeded on that front. That it's not a perfect fully-general bytecode format doesn't seem like a real knock against it, that's a much harder problem to solve. In any event it seems to be getting a lot more traction than any previous entries in the space, which is exciting!
klodolph 7y ago
The criticisms in the linked article are simply not grounded in fact, and the article was obviously written by someone without any expertise in how modern compilers work. There are also a number of basic errors in the article. Liveness information simply doesn’t belong in the bytecode. SSA is trivial to recreate from local mutable variables (it would make a good homework assignment for someone in an undergrad “intro to compilers” class). WebAssembly is obviously a register machine. > With this, it becomes possible to get rid of locals entirely. Both factually incorrect and pointless. There is no tangible benefit in getting rid of locals entirely. You are merely changing out one representation (register machine) for a different one (stack machine). There are valid criticisms of WASM, but the linked article doesn’t have any. The weird part of WASM is the control structures. The rest of it is a fairly sensible, actually rather nice register machine. You can see that older bytecode systems like the JVM are stack machines, but newer ones tend to be register machines. This isn’t because people are getting stupid, it’s because there are legitimate reasons to prefer register machines, and on the balance of things, my observations are that people with experience in the field tend to prefer register machines.
- MaxBarraclough 7y ago
  Interesting, thanks for the analysis. > You can see that older bytecode systems like the JVM are stack machines, but newer ones tend to be register machines. Like LLVM. Is there a good reason for WebAssembly not being SSA?
  - klodolph 7y ago
    There’s no good reason for WebAssembly to be SSA. That is a good enough reason for it not to be. The purpose of SSA is to make it easier to write optimizations. However, you wouldn’t be optimizing WASM anyway, you would necessarily translate it first into some kind of IR suitable for optimization passes. Might as well convert to SSA at that point, rather than bloat the bytecode by exposing implementation details of the target. Remember that WASM’s purpose is to be portable and safe. Making it more complicated just in order to make sophisticated back-ends slightly faster is a net loss. Using SSA would also make naïve/simple backends slower.
- drew-y 7y ago
  > The weird part of WASM is the control structures. Out of curiosity, what do you find weird about the control structure portion? I just wrote a basic language that compiles to wasm and found the built in control structures made my life easier.
  - klodolph 7y ago
    Open any compilers book, WASM will look alien. Control flow analysis is normally done through basic blocks. If you make a toy language and don’t do control flow analysis you won’t notice what’s so weird about WASM. If you take an existing compiler and port it to WASM, you’ll cry, then gather yourself together and read how the relooper algorithm works.
    - afiori 7y ago
      The reason wasm is like this is that with "normal" control flow chrome would have had to implement itself the relooper algorithm internally before the MVP release. Overall the main negative point seems to be that compilers toward wasm are more complex, but under the constraints of the web moving complexity from the client looks like a valuable positive point. Two other negative points would possibly be performance and code size (which is the reason for a if-then-else specific instruction for example) about which I know very little. Do you think it would have made a difference here?

tedmielczarek 7y ago

I don't think that's true. It was designed for its purpose by people that knew what they were doing based on experience gathered from implementing previous incremental steps including emscripten's original JavaScript output and asm.js (a formalization of some techniques emscripten employed). The design rationale is right there in the spec repository: https://github.com/WebAssembly/design/blob/master/Rationale.md https://github.com/WebAssembly/design/blob/master/Rationale.... It was built to be a better target than asm.js for compiled languages to run in JavaScript VMs and it seems to have succeeded on that front. That it's not a perfect fully-general bytecode format doesn't seem like a real knock against it, that's a much harder problem to solve. In any event it seems to be getting a lot more traction than any previous entries in the space, which is exciting!

klodolph 7y ago

The criticisms in the linked article are simply not grounded in fact, and the article was obviously written by someone without any expertise in how modern compilers work. There are also a number of basic errors in the article. Liveness information simply doesn’t belong in the bytecode. SSA is trivial to recreate from local mutable variables (it would make a good homework assignment for someone in an undergrad “intro to compilers” class). WebAssembly is obviously a register machine. > With this, it becomes possible to get rid of locals entirely. Both factually incorrect and pointless. There is no tangible benefit in getting rid of locals entirely. You are merely changing out one representation (register machine) for a different one (stack machine). There are valid criticisms of WASM, but the linked article doesn’t have any. The weird part of WASM is the control structures. The rest of it is a fairly sensible, actually rather nice register machine. You can see that older bytecode systems like the JVM are stack machines, but newer ones tend to be register machines. This isn’t because people are getting stupid, it’s because there are legitimate reasons to prefer register machines, and on the balance of things, my observations are that people with experience in the field tend to prefer register machines.

MaxBarraclough 7y ago
Interesting, thanks for the analysis. > You can see that older bytecode systems like the JVM are stack machines, but newer ones tend to be register machines. Like LLVM. Is there a good reason for WebAssembly not being SSA?
- klodolph 7y ago
  There’s no good reason for WebAssembly to be SSA. That is a good enough reason for it not to be. The purpose of SSA is to make it easier to write optimizations. However, you wouldn’t be optimizing WASM anyway, you would necessarily translate it first into some kind of IR suitable for optimization passes. Might as well convert to SSA at that point, rather than bloat the bytecode by exposing implementation details of the target. Remember that WASM’s purpose is to be portable and safe. Making it more complicated just in order to make sophisticated back-ends slightly faster is a net loss. Using SSA would also make naïve/simple backends slower.
drew-y 7y ago
> The weird part of WASM is the control structures. Out of curiosity, what do you find weird about the control structure portion? I just wrote a basic language that compiles to wasm and found the built in control structures made my life easier.
- klodolph 7y ago
  Open any compilers book, WASM will look alien. Control flow analysis is normally done through basic blocks. If you make a toy language and don’t do control flow analysis you won’t notice what’s so weird about WASM. If you take an existing compiler and port it to WASM, you’ll cry, then gather yourself together and read how the relooper algorithm works.
  - afiori 7y ago
    The reason wasm is like this is that with "normal" control flow chrome would have had to implement itself the relooper algorithm internally before the MVP release. Overall the main negative point seems to be that compilers toward wasm are more complex, but under the constraints of the web moving complexity from the client looks like a valuable positive point. Two other negative points would possibly be performance and code size (which is the reason for a if-then-else specific instruction for example) about which I know very little. Do you think it would have made a difference here?