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OptimizeInstructions: Improve areConsecutiveInputsEqualAndFoldable #8355

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OptimizeInstructions: Improve areConsecutiveInputsEqualAndFoldable #8355

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kripken Feb 20, 2026
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kripken Feb 20, 2026
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34 changes: 8 additions & 26 deletions src/passes/OptimizeInstructions.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2826,31 +2826,9 @@ struct OptimizeInstructions
return !Properties::isGenerative(left);
}

// Similar to areConsecutiveInputsEqual() but also checks if we can remove
// them (but we do not assume the caller will always remove them).
bool areConsecutiveInputsEqualAndRemovable(Expression* left,
Expression* right) {
// First, check for side effects. If there are any, then we can't even
// assume things like local.get's of the same index being identical. (It is
// also ok to have removable side effects here, see the function
// description.)
auto& passOptions = getPassOptions();
if (EffectAnalyzer(passOptions, *getModule(), left)
.hasUnremovableSideEffects() ||
EffectAnalyzer(passOptions, *getModule(), right)
.hasUnremovableSideEffects()) {
return false;
}

return areConsecutiveInputsEqual(left, right);
}

// Check if two consecutive inputs to an instruction are equal and can also be
// folded into the first of the two (but we do not assume the caller will
// always fold them). This is similar to areConsecutiveInputsEqualAndRemovable
// but also identifies reads from the same local variable when the first of
// them is a "tee" operation and the second is a get (in which case, it is
// fine to remove the get, but not the tee).
// always fold them).
//
// The inputs here must be consecutive, but it is also ok to have code with no
// side effects at all in the middle. For example, a Const in between is ok.
Expand All @@ -2862,9 +2840,13 @@ struct OptimizeInstructions
return true;
}

// stronger property than we need - we can not only fold
// them but remove them entirely.
return areConsecutiveInputsEqualAndRemovable(left, right);
// To fold the right side into the left, it must have no effects.
if (EffectAnalyzer(getPassOptions(), *getModule(), right)
.hasUnremovableSideEffects()) {
return false;
}

return areConsecutiveInputsEqual(left, right);
}

// Canonicalizing the order of a symmetric binary helps us
Expand Down
20 changes: 9 additions & 11 deletions test/lit/passes/optimize-instructions-mvp.wast
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Expand Up @@ -16141,17 +16141,13 @@
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (select
;; CHECK-NEXT: (block (result i32)
;; CHECK-NEXT: (call $send-i32
;; CHECK-NEXT: (i32.const 1337)
;; CHECK-NEXT: )
;; CHECK-NEXT: (local.tee $x
;; CHECK-NEXT: (local.get $param)
;; CHECK-NEXT: )
;; CHECK-NEXT: (block (result i32)
;; CHECK-NEXT: (call $send-i32
;; CHECK-NEXT: (i32.const 1337)
;; CHECK-NEXT: )
;; CHECK-NEXT: (local.tee $x
;; CHECK-NEXT: (local.get $param)
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (drop
Expand Down Expand Up @@ -16179,7 +16175,9 @@
)
)
)
;; Side effect on the ifTrue - same outcome, we cannot optimize yet.
;; Side effect on the ifTrue - we handle this case, as we can easily see
;; that those effects are not a problem. We keep those effects around, of
;; course.
(drop
(select
(block (result i32)
Expand Down
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