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55 votes and no comments. Looks like a lot of people are interested in WASM but no one has anything to say about it.

by dexwiz 4y ago

rob74 4y ago
There are some comments now... I guess a lot of people (like me) are interested in WASM, but most don't have enough knowledge to say anything "authoritative" about it. When I read "It is a stack machine, no registers", I remembered this other article http://troubles.md/wasm-is-not-a-stack-machine/ http://troubles.md/wasm-is-not-a-stack-machine/ and thought "Ok now, these can't both be right at the same time, right?". But apparently they can - as another comment here states, "WebAssembly is not exactly a stack machine but a stack machine encoding of an instruction tree". To that I say, if I can use the stack as I'm used to from other systems, I don't really care if it's a "real" stack machine. But I'm not really sure of that either...
- lifthrasiir 4y ago
  I've updated my comment to add more clarification, but let me elaborate a bit more here. If you have the following code: int powi(int n, int m) { int s = 1; while (--m >= 0) s *= n; return s; } The compiled WebAssembly code would look like this (the actual textual format): (func $powi (param $0 i32) (param $1 i32) (result i32) (local $2 i32) (set_local $2 (i32.const 1)) (block $break (loop $continue (br_if $break (i32.ge_s (tee_local $1 (i32.sub (get_local $1) (i32.const 1))) (i32.const 0))) (set_local $2 (i32.mul (get_local $2) (get_local $0))) (br $continue) ) ) (get_local $2) ) Note the clear resemblance between the original code and the compiled binary. The main difference is two branch labels, because WebAssembly distinguishes forward and backward jumps by block types (`block` vs. `loop`). The binary retains all parameters and local variables. When we say WebAssembly is a stack machine, it means that this gets serialized like this: (The textual format allows both forms, while the binary format is a direct translation of the latter.) (func $powi (param $0 i32) (param $1 i32) (result i32) (local $2 i32) i32.const 1 set_local $2 block $break loop $continue get_local $1 i32.const 1 i32.sub tee_local $1 i32.const 0 i32.ge_s br_if $break get_local $2 get_local $0 i32.mul set_local $2 br $continue end end get_local $2 ) In principle you can execute this just like a stack machine with variables, but optimizing implementations will translate this into machine codes, which wouldn't look much like either forms. So the stack machine representation doesn't imply the stack machine execution.
  - whizzter 4y ago
    Iirc WASM was initially explicitly designed to be translate-able to Asm.JS (that in turn translated to JS), basically they continued the way Asm.JS had retained backwards compatiblity with JS to either force other browsers to play along in implementing the standard or just look bad in benchmarks. (I think they've relaxed on back-compat now but it was a tool for getting everyone on board and once that had happened it was easier to break it). This is why WASM kind-of-is a stack machine. Basically the opcode sequences should be possible to easily re-translate back into a syntax tree for a back-compat JS generator. This is also why you have these block/loop branches.. it basically is translateable to labelled while loops and if blocks.

55 votes and no comments. Looks like a lot of people are interested in WASM but no one has anything to say about it.

by dexwiz 4y ago

55 votes and no comments. Looks like a lot of people are interested in WASM but no one has anything to say about it.

There are some comments now... I guess a lot of people (like me) are interested in WASM, but most don't have enough knowledge to say anything "authoritative" about it. When I read "It is a stack machine, no registers", I remembered this other article http://troubles.md/wasm-is-not-a-stack-machine/ http://troubles.md/wasm-is-not-a-stack-machine/ and thought "Ok now, these can't both be right at the same time, right?". But apparently they can - as another comment here states, "WebAssembly is not exactly a stack machine but a stack machine encoding of an instruction tree". To that I say, if I can use the stack as I'm used to from other systems, I don't really care if it's a "real" stack machine. But I'm not really sure of that either...

lifthrasiir 4y ago
I've updated my comment to add more clarification, but let me elaborate a bit more here. If you have the following code: int powi(int n, int m) { int s = 1; while (--m >= 0) s *= n; return s; } The compiled WebAssembly code would look like this (the actual textual format): (func $powi (param $0 i32) (param $1 i32) (result i32) (local $2 i32) (set_local $2 (i32.const 1)) (block $break (loop $continue (br_if $break (i32.ge_s (tee_local $1 (i32.sub (get_local $1) (i32.const 1))) (i32.const 0))) (set_local $2 (i32.mul (get_local $2) (get_local $0))) (br $continue) ) ) (get_local $2) ) Note the clear resemblance between the original code and the compiled binary. The main difference is two branch labels, because WebAssembly distinguishes forward and backward jumps by block types (`block` vs. `loop`). The binary retains all parameters and local variables. When we say WebAssembly is a stack machine, it means that this gets serialized like this: (The textual format allows both forms, while the binary format is a direct translation of the latter.) (func $powi (param $0 i32) (param $1 i32) (result i32) (local $2 i32) i32.const 1 set_local $2 block $break loop $continue get_local $1 i32.const 1 i32.sub tee_local $1 i32.const 0 i32.ge_s br_if $break get_local $2 get_local $0 i32.mul set_local $2 br $continue end end get_local $2 ) In principle you can execute this just like a stack machine with variables, but optimizing implementations will translate this into machine codes, which wouldn't look much like either forms. So the stack machine representation doesn't imply the stack machine execution.
- whizzter 4y ago
  Iirc WASM was initially explicitly designed to be translate-able to Asm.JS (that in turn translated to JS), basically they continued the way Asm.JS had retained backwards compatiblity with JS to either force other browsers to play along in implementing the standard or just look bad in benchmarks. (I think they've relaxed on back-compat now but it was a tool for getting everyone on board and once that had happened it was easier to break it). This is why WASM kind-of-is a stack machine. Basically the opcode sequences should be possible to easily re-translate back into a syntax tree for a back-compat JS generator. This is also why you have these block/loop branches.. it basically is translateable to labelled while loops and if blocks.