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LocalCSE: Migrate from invalidates to orderedBefore #8744

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LocalCSE: Migrate from invalidates to orderedBefore #8744

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43b0811
LICM: Migrate from invalidates to orderedBefore
tlively May 20, 2026
99b67b9
LocalCSE: Migrate from invalidates to orderedBefore
tlively May 20, 2026
273ca1f
explanatory comment
tlively May 21, 2026
0cd1a70
Merge branch 'main' into local-cse-ordered-effects
tlively May 22, 2026
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4 changes: 3 additions & 1 deletion src/passes/LocalCSE.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -497,7 +497,9 @@ struct Checker
continue;
}
auto& originalInfo = kv.second;
if (effects.invalidates(originalInfo.effects)) {
// Check whether curr must remain before COPY. We use ORIGINAL's effects
// in the check because we know they are the same as COPY's effects.
if (effects.orderedBefore(originalInfo.effects)) {
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@kripken kripken May 21, 2026

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We have [original, curr] as the order of expressions. Shouldn't we invalidate like this?

Suggested change
if (effects.orderedBefore(originalInfo.effects)) {
if (!effects.orderedAfter(originalInfo.effects)) {

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@kripken kripken May 21, 2026

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i.e. effects/curr begin after. Should we not ask if they must remain after, as before/after is not symmetric?

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@tlively tlively May 21, 2026

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No, this is correct-as is. Say we have A; B; A', where A and A' is the same, so CSE is trying to optimize out A' and use the result of A instead. This is semantically the same as reordering A' to be before B (or alternatively B after A'): A; A'; B, so we want to invalidate the optimization if B ordered-before A', preventing the move. You're right that originalInfo.effects is calculated from A, but A' is the same expression, so here we're using it as A''s effects. So checking B ordered-before A' is the same as effects.orderedBefore(originalInfo.effects).

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@kripken kripken May 21, 2026

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I see, thanks. Maybe add a comment? Specifically the bit about using the effects of original to model those of the thing being optimized away, which is later, seems non-obvious.

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@tlively tlively May 21, 2026

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I've added a comment clarifying that originalInfo.effects is standing in for COPY's effects.

invalidated.push_back(original);
}
}
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58 changes: 58 additions & 0 deletions test/lit/passes/local-cse-atomics.wast
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@@ -0,0 +1,58 @@
;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
;; RUN: wasm-opt -all --local-cse -S -o - %s | filecheck %s

(module
;; CHECK: (type $struct (shared (struct (field (mut i32)))))
(type $struct (shared (struct (field (mut i32)))))

;; CHECK: (memory $mem 1 1 shared)
(memory $mem 1 1 shared)

;; Test 1: Allowed reordering (GC read reused across Wasm release store)
;; CHECK: (func $allowed (type $1) (param $x (ref $struct)) (result i32)
;; CHECK-NEXT: (local $y i32)
;; CHECK-NEXT: (local $2 i32)
;; CHECK-NEXT: (local.set $y
;; CHECK-NEXT: (local.tee $2
;; CHECK-NEXT: (struct.get $struct 0
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.atomic.store acqrel
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: (local.get $2)
;; CHECK-NEXT: )
(func $allowed (param $x (ref $struct)) (result i32)
(local $y i32)
(local.set $y (struct.get $struct 0 (local.get $x)))
(i32.atomic.store acqrel (i32.const 0) (i32.const 42))
(struct.get $struct 0 (local.get $x))
)

;; Test 2: Disallowed reordering (GC read NOT reused across Wasm acquire load)
;; CHECK: (func $disallowed (type $1) (param $x (ref $struct)) (result i32)
;; CHECK-NEXT: (local $y i32)
;; CHECK-NEXT: (local.set $y
;; CHECK-NEXT: (struct.get $struct 0
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (i32.atomic.load acqrel
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (struct.get $struct 0
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $disallowed (param $x (ref $struct)) (result i32)
(local $y i32)
(local.set $y (struct.get $struct 0 (local.get $x)))
(drop (i32.atomic.load acqrel (i32.const 0)))
(struct.get $struct 0 (local.get $x))
)
)
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