Invalid optimization to select with --ignore-implicit-traps

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#3,934 opened on Jun 16, 2021 9 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: Bug
Clarity: Clearly specified
Activity status: Stale
Tech stack: wasm
Domain: compilers

Research direction

Reproduce the issue with the provided WAT module and -all -O1 --ignore-implicit-traps, then inspect the optimization that turns the conditional struct.get into a select. Done means the optimizer no longer makes potentially trapping code execute unconditionally, while preserving the valid optimized behavior.

Written by the indexing model from the issue text.

Description

Optimizing this program with -all -O1 --ignore-implicit-traps:

(module
 (type $box (struct (field (mut i32)) (field (mut i64))))
 (export "f" (func $f))
 (func $f (param $0 (ref null $box)) (result i64)
  (block $label$1
   (block $label$2
    (br_if $label$2
     (ref.is_null
      (local.get $0)
     )
    )
    (br_if $label$1
     (i64.gt_s
      (struct.get $box 1
       (local.get $0)
      )
      (i64.const 8)
     )
    )
   )
   (return
    (i64.const 8)
   )
  )
  (struct.get $box 1
   (local.get $0)
  )
 )
)

yields

(module
 (type $box (struct (field (mut i32)) (field (mut i64))))
 (type $ref?|$box|_=>_i64 (func (param (ref null $box)) (result i64)))
 (export "f" (func $f))
 (func $f (param $0 (ref null $box)) (result i64)
  (if
   (select
    (ref.is_null
     (local.get $0)
    )
    (i32.const 1)
    (i64.gt_s
     (struct.get $box 1
      (local.get $0)
     )
     (i64.const 8)
    )
   )
   (return
    (i64.const 8)
   )
  )
  (struct.get $box 1
   (local.get $0)
  )
 )
)

Even when assuming that no traps are executed, it is invalid in general to change potentially trapping code from being conditionally executed to being unconditionally executed. This happens with the first struct.get here, as it is placed as the second arm of the select.

Dominant language: WebAssembly
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