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Redundant global read

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#4,274 5 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
38/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
wasm

Research direction

Start by locating Binaryen's optimization pass that handles global.set and global.get, then inspect existing tests for global transformations. Add coverage for simple and complex values as shown in the issue, and consider the work done when the redundant read is removed while the examples retain their behavior.

Written by the indexing model from the issue text.

Description

In situations when global reads immediately after write we could avoid this read and use argument of global.set. For example this:

(module
  (global $g (mut i32) (i32.const 0))
  (func $test (param $0 i32) (result i32)
    (global.set $g (local.get $0))
    (global.get $g)
  )
)

can be optimized as:

(module
  (global $g (mut i32) (i32.const 0))
  (func $test (param $0 i32) (result i32)
    (global.set $g (local.get $0))
    (local.get $0)
  )
)

if arg is simple like local.get or i32.const
and something like this if arg more complex:

(module
  (global $g (mut i32) (i32.const 0))
  (func $test (param $0 i32) (result i32)
    (global.set $g (local.tee $tmp (...complex expr...)))
    (local.get $tmp)
  )
)
Dominant language
WebAssembly
Stars
8.6k
Forks
885
Avg merge
2d 4h
Merged PRs (30d)
77

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Open the contributing guide

First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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