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[NFC] Clarify why RemoveUnusedModuleElements does not need descriptor-specific tracking #7880

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kripken merged 2 commits into from
Sep 4, 2025
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[NFC] Clarify why RemoveUnusedModuleElements does not need descriptor-specific tracking #7880

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0f73d76
clarify
kripken Sep 4, 2025
3e7bb52
rephrase
kripken Sep 4, 2025
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23 changes: 21 additions & 2 deletions src/passes/RemoveUnusedModuleElements.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -561,8 +561,27 @@ struct Analyzer {

auto* new_ = curr->cast<StructNew>();

// Use the descriptor right now, normally. (We only have special
// optimization for struct.new operands, below.)
// Use the descriptor right now, normally. We only have special
// optimization for struct.new operands, below, because this is not needed
// for descriptors: descriptors always have a struct "in the middle", so
// optimizing normal struct fields is enough. That is, imagine we have a
// struct with a descriptor:
//
// (struct.new $A
// (ref.func $c)
// (struct.new $A.desc
// (ref.func $d)
// )
// )
//
// The struct has a ref.func on it, and the descriptor does as well. Say we
// never field 0 from $A, then we can avoid marking $c as reached; this is
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// the usual struct optimization we do, below. Now, say we never read the
// descriptor, then we also never read field 0 from $A.desc, that is, the
// usual struct optimization on the descriptor class is enough for us to
// avoid marking $d as reached. Put another way, a descriptor must be a
// struct; if it could be a function, then we'd need to optimize descriptors
// as we do normal fields.
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@tlively tlively Sep 4, 2025

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Maybe lead with this? I think this clarifies things more than saying descriptors have a struct "in the middle."

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@kripken kripken Sep 4, 2025

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Sure, rewritten.

if (new_->desc) {
use(new_->desc);
}
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70 changes: 70 additions & 0 deletions test/lit/passes/remove-unused-module-elements-refs-descriptors.wast
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Expand Up @@ -127,3 +127,73 @@
(elem $no-trap-get anyref (item (struct.new $struct (global.get $desc))))
)

(module
;; CHECK: (type $void (func))
(type $void (func))

(rec
;; CHECK: (rec
;; CHECK-NEXT: (type $vtable (sub (descriptor $vtable.desc (struct (field (ref $void))))))
(type $vtable (sub (descriptor $vtable.desc (struct (field (ref $void))))))
;; CHECK: (type $vtable.desc (sub (describes $vtable (struct (field (ref $void))))))
(type $vtable.desc (sub (describes $vtable (struct (field (ref $void))))))
)

;; CHECK: (global $vtable (ref $vtable) (struct.new $vtable
;; CHECK-NEXT: (ref.func $a)
;; CHECK-NEXT: (struct.new $vtable.desc
;; CHECK-NEXT: (ref.func $b)
;; CHECK-NEXT: )
;; CHECK-NEXT: ))
(global $vtable (ref $vtable) (struct.new $vtable
(ref.func $a)
(struct.new $vtable.desc
(ref.func $b)
)
))

;; CHECK: (export "export" (func $export))

;; CHECK: (func $export (type $void)
;; CHECK-NEXT: (call_ref $void
;; CHECK-NEXT: (struct.get $vtable 0
;; CHECK-NEXT: (global.get $vtable)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $export (export "export")
;; Read $a and call it. $b, in the descriptor, should not be callable.
(call_ref $void
(struct.get $vtable 0
(global.get $vtable)
)
)
)

;; CHECK: (func $a (type $void)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $a (type $void)
;; This is reached from above.
(drop (i32.const 42))
)

;; CHECK: (func $b (type $void)
;; CHECK-NEXT: (unreachable)
;; CHECK-NEXT: )
(func $b (type $void)
;; This is not reached: We never read the descriptor, so we never read field 0
;; in it, leaving this as dead (in closed world). That it itself seems to read
;; the descriptor should not confuse us.
(call_ref $void
(struct.get $vtable.desc 0
(ref.get_desc $vtable
(global.get $vtable)
)
)
)
)
)

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