array.length set to lower value should remove items properly

[vird]

array.length manipulations allowed in JS/TS so I tried this

class Struct {
  a:i32;
}
export function report(): i32 {
  let res = 0;
  let arr:Struct[] = [];
  
  for(let i=0;i<10;i++) {
    arr.push(new Struct());
  }
  arr.length = 0; // will throw here
  
  return res;
}

It produces Uncaught Error: abort: Index out of range at ~lib/rt.ts:22:27

Also setting length to higher values should allocate more memory, but fill with 0 (because on array deallocation it would try free pointer to garbage)

[MaxGraey]

Can't reproduce this
https://webassembly.studio/?f=toghxldo0tl

[MaxGraey]

And that's always works properly and often used. Could you recheck?

[vird]

Reproduced with following steps
1 create index.ts

class Struct {
  a:i32;
}
export function report(): i32 {
  let res = 0;
  let arr:Struct[] = [];
  
  for(let i=0;i<10000;i++) {
    arr.push(new Struct());
  }
  arr.length = 0;
  
  return res;
}

2 ./node_modules/.bin/asc ./index.ts -b ./index.wasm -t ./index.wat --validate --use abort=
used ./node_modules/.bin/asc --version 0.7.0
node -v v11.15.0
3 wasm_launch.js

 fs = require('fs');
 cont = fs.readFileSync('index.wasm');
 WebAssembly.instantiate(cont, {imports:{}})
   .then(module=>{
     module.instance.exports.report()
   })

result

(node:28807) UnhandledPromiseRejectionWarning: RuntimeError: unreachable
    at wasm-function[18]:17
    at wasm-function[24]:104
    at wasm-function[25]:13
    at wasm-function[33]:38
    at wasm-function[34]:75
    at WebAssembly.instantiate.then.module (/data/nodejs/experiments/as_hotreload_env/wasm_launch.js:5:29)

https://gist.github.com/vird/e0fb47a79696fd87a54b920fa58d02ac

[MaxGraey]

That's strange. I reproduced this only locally with --runtime full but not with runtime --half. In WebAssembly Studio even with --runtime full this not reproduced.

Output with (asc --debug):

  RuntimeError: memory access out of bounds
    at ~lib/rt/pure/decrement (wasm-function[25]:0x1886)
    at ~lib/rt/pure/__release (wasm-function[26]:0x195b)
    at ~lib/array/Array<index/Struct>#set:length (wasm-function[34]:0x1dbe)
    at index/report (wasm-function[36]:0x1e2e)
    at ./.../reproduce-as-bug/run.js:10:27