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Avoid invalid exact casts in RemoveUnusedBrs (#7522)
RemoveUnusedBrs can introduce new casts when optimizing out br_on_cast and br_on_cast_fail instructions. This happens when the pass finds a more precise type for the fallthrough value reaching the casting branch, allowing it to improve the cast target type. When the branch is subsequently optimized out, this precise type information is recovered with an inserted cast. Update the pass to avoid creating invalid casts to exact types when custom descriptors is not enabled.
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-32
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2 files changed

+101
-32
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‎src/passes/RemoveUnusedBrs.cpp

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Original file line numberDiff line numberDiff line change
@@ -906,6 +906,12 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
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// further optimizations after this, and those optimizations might even
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// benefit from this improvement.
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auto glb = Type::getGreatestLowerBound(curr->castType, refType);
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if (!getModule()->features.hasCustomDescriptors() && glb.isExact() &&
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!curr->castType.isExact()) {
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// When custom descriptors is not enabled, nontrivial exact casts are
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// not allowed.
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glb = glb.with(Inexact);
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}
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if (curr->op == BrOnCastFail) {
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// BrOnCastFail sends the input type, with adjusted nullability. The
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// input heap type makes sense for the branch target, and we will not

‎test/lit/passes/remove-unused-brs-exact.wast

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@@ -1,42 +1,105 @@
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;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
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;; RUN: wasm-opt %s -all --remove-unused-brs -S -o - | filecheck %s
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;; Check that we optimize the cast correctly when the fallthrough has exact
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;; type. In particular, we should not insert a ref.as_non_null, which would
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;; trap.
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;; type, whether or not custom descriptors is enabled.
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;; RUN: wasm-opt %s -all --remove-unused-brs -S -o - | filecheck %s
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;; RUN: wasm-opt %s -all --disable-custom-descriptors --remove-unused-brs -S -o - | filecheck %s --check-prefix=NO_CD
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(module
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;; CHECK: (type $struct (struct))
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(type $struct (struct))
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;; CHECK: (func $br_on_cast_fail (type $1) (param $0 (ref null (exact $struct)))
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;; CHECK-NEXT: (local $1 (ref null $struct))
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (block $block
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (ref.cast (ref null (exact $struct))
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;; CHECK-NEXT: (local.tee $1
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;; CHECK-NEXT: (local.get $0)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: (return)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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(func $br_on_cast_fail (param (ref null (exact $struct)))
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(local $1 (ref null $struct))
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(drop
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(block $block (result (ref $struct))
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;; CHECK: (type $foo (struct))
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;; NO_CD: (type $foo (struct))
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(type $foo (struct))
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;; CHECK: (func $br_on_cast (type $1) (param $0 (ref (exact $foo)))
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;; CHECK-NEXT: (local $inexact (ref $foo))
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (block $block (result (ref (exact $foo)))
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (br $block
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;; CHECK-NEXT: (ref.cast (ref (exact $foo))
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;; CHECK-NEXT: (local.tee $inexact
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;; CHECK-NEXT: (local.get $0)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: (local.get $0)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; NO_CD: (func $br_on_cast (type $1) (param $0 (ref (exact $foo)))
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;; NO_CD-NEXT: (local $inexact (ref $foo))
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;; NO_CD-NEXT: (drop
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;; NO_CD-NEXT: (block $block (result (ref $foo))
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;; NO_CD-NEXT: (drop
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;; NO_CD-NEXT: (br $block
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;; NO_CD-NEXT: (local.tee $inexact
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;; NO_CD-NEXT: (local.get $0)
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: (local.get $0)
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: )
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(func $br_on_cast (param (ref (exact $foo)))
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(local $inexact (ref $foo))
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(drop
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(block (result (ref $foo))
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(drop
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(br_on_cast 0 anyref (ref $foo)
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(local.tee $inexact
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(local.get 0)
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)
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)
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)
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(local.get 0)
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)
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)
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)
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;; CHECK: (func $br_on_cast_fail (type $1) (param $0 (ref (exact $foo)))
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;; CHECK-NEXT: (local $inexact (ref $foo))
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (block $block (result (ref (exact $foo)))
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (ref.cast (ref (exact $foo))
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;; CHECK-NEXT: (local.tee $inexact
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;; CHECK-NEXT: (local.get $0)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: (local.get $0)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; NO_CD: (func $br_on_cast_fail (type $1) (param $0 (ref (exact $foo)))
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;; NO_CD-NEXT: (local $inexact (ref $foo))
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;; NO_CD-NEXT: (drop
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;; NO_CD-NEXT: (block $block (result (ref (exact $foo)))
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;; NO_CD-NEXT: (drop
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;; NO_CD-NEXT: (local.tee $inexact
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;; NO_CD-NEXT: (local.get $0)
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: (local.get $0)
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: )
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;; NO_CD-NEXT: )
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(func $br_on_cast_fail (param (ref (exact $foo)))
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(local $inexact (ref $foo))
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(drop
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(br_on_cast_fail $block (ref null $struct) (ref null $struct)
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(local.tee $1
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(local.get 0)
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(block (result (ref $foo))
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(drop
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(br_on_cast_fail 0 anyref (ref $foo)
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(local.tee $inexact
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(local.get 0)
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)
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)
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)
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(local.get 0)
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)
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)
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)
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(return)
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)
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)
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)
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)

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