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[Custom Descriptors] Propagate subtyping to describees in Unsubtyping #7927

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Merged
merged 3 commits into from
Sep 29, 2025
Merged

[Custom Descriptors] Propagate subtyping to describees in Unsubtyping #7927

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fb093b2
[NFC] Add debug tracing to Unsubtyping
tlively Sep 26, 2025
1931b55
[Custom Descriptors] Propagate subtyping to describees in Unsubtyping
tlively Sep 26, 2025
a3a0a30
Merge branch 'main' into unsubypting-propagate-left
tlively Sep 29, 2025
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43 changes: 5 additions & 38 deletions src/passes/Unsubtyping.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -525,7 +525,6 @@ struct Unsubtyping : Pass {
}
if (HeapType::isSubType(*oldSuper, super)) {
// sub <: oldSuper <: super
processDescribed(sub, *oldSuper, super);
noteSubtype(*oldSuper, super);
// We already handled sub <: oldSuper, so we're done.
return;
Expand All @@ -535,7 +534,6 @@ struct Unsubtyping : Pass {
// super will already be in the same tree when we process them below, so
// when we process casts we will know that we only need to process up to
// oldSuper.
processDescribed(sub, super, *oldSuper);
process(super, *oldSuper);
}

Expand All @@ -547,42 +545,6 @@ struct Unsubtyping : Pass {
processCasts(sub, super, oldSuper);
}

void processDescribed(HeapType sub, HeapType mid, HeapType super) {
// We are establishing sub <: mid <: super. If super describes the immediate
// supertype of the type sub describes, then once we insert mid between them
// we would have this:
//
// A -> super
// ^ ^
// | mid
// | ^
// C -> sub
//
// This violates the requirement that the descriptor of C's immediate
// supertype must be the immediate supertype of C's descriptor. To fix it,
// we have to find the type B that mid describes and insert it between A and
// C:
//
// A -> super
// ^ ^
// B -> mid
// ^ ^
// C -> sub
//
// We do this eagerly before we establish sub <: mid <: super so that if
// establishing that subtyping requires recursively establishing other
// subtypings, we can depend on the invariant that the described types are
// always set up correctly beforehand.
auto subDescribed = sub.getDescribedType();
auto superDescribed = super.getDescribedType();
if (subDescribed && superDescribed &&
types.getSupertype(*subDescribed) == superDescribed) {
auto midDescribed = mid.getDescribedType();
assert(midDescribed);
process(*subDescribed, *midDescribed);
}
}

void processDefinitions(HeapType sub, HeapType super) {
if (super.isBasic()) {
return;
Expand Down Expand Up @@ -618,6 +580,11 @@ struct Unsubtyping : Pass {
noteSubtype(*desc, *superDesc);
}
}
if (auto desc = sub.getDescribedType()) {
if (auto superDesc = super.getDescribedType()) {
noteSubtype(*desc, *superDesc);
}
}
}

void
Expand Down
78 changes: 75 additions & 3 deletions test/lit/passes/unsubtyping-desc.wast
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Original file line number Diff line number Diff line change
Expand Up @@ -49,14 +49,13 @@
(type $A (sub (descriptor $A.desc (struct))))
;; CHECK: (type $A.desc (sub (describes $A (struct))))
(type $A.desc (sub (describes $A (struct))))
;; CHECK: (type $B (sub (descriptor $B.desc (struct))))
;; CHECK: (type $B (sub $A (descriptor $B.desc (struct))))
(type $B (sub $A (descriptor $B.desc (struct))))
;; CHECK: (type $B.desc (sub $A.desc (describes $B (struct))))
(type $B.desc (sub $A.desc (describes $B (struct))))
)

;; Now we directly require B.desc <: A.desc. This does *not* imply B <: A, so
;; we can optimize $B (but not $B.desc).
;; Now we require B.desc <: A.desc, which similarly implies B <: A.
;; CHECK: (global $B.desc (ref null $B.desc) (ref.null none))
(global $B.desc (ref null $B.desc) (ref.null none))
;; CHECK: (global $A.desc (ref null $A.desc) (global.get $B.desc))
Expand Down Expand Up @@ -134,3 +133,76 @@
;; CHECK: (global $bot-mid-desc (ref null $mid.desc) (struct.new_default $bot.desc))
(global $bot-mid-desc (ref null $mid.desc) (struct.new $bot.desc))
)

(module
(rec
;; CHECK: (rec
;; CHECK-NEXT: (type $top (sub (descriptor $top.desc (struct))))
(type $top (sub (descriptor $top.desc (struct))))
;; CHECK: (type $mid (sub $top (descriptor $mid.desc (struct))))
(type $mid (sub $top (descriptor $mid.desc (struct))))
;; CHECK: (type $bot (sub $mid (descriptor $bot.desc (struct))))
(type $bot (sub $mid (descriptor $bot.desc (struct))))
;; CHECK: (type $top.desc (sub (describes $top (struct))))
(type $top.desc (sub (describes $top (struct))))
;; CHECK: (type $mid.desc (sub $top.desc (describes $mid (struct))))
(type $mid.desc (sub $top.desc (describes $mid (struct))))
;; CHECK: (type $bot.desc (sub $mid.desc (describes $bot (struct))))
(type $bot.desc (sub $mid.desc (describes $bot (struct))))
)

;; Now go the other direction:
;;
;; top ---> top.desc
;; ^
;; mid -> mid.desc |(2)
;; ^ (1) |
;; bot ---> bot.desc
;;
;; bot.desc <: top.desc implies bot <: top, but we already have bot <: mid, so
;; that gives us bot <: mid <: top. This is only valid if we also have
;; bot.desc <: mid.desc <: top.desc.

;; CHECK: (global $bot-mid (ref null $mid) (struct.new_default $bot
;; CHECK-NEXT: (ref.null none)
;; CHECK-NEXT: ))
(global $bot-mid (ref null $mid) (struct.new $bot (ref.null none)))
;; CHECK: (global $bot-top-desc (ref null $top.desc) (struct.new_default $bot.desc))
(global $bot-top-desc (ref null $top.desc) (struct.new $bot.desc))
)

(module
(rec
;; CHECK: (rec
;; CHECK-NEXT: (type $top (sub (descriptor $top.desc (struct))))
(type $top (sub (descriptor $top.desc (struct))))
;; CHECK: (type $mid (sub $top (descriptor $mid.desc (struct))))
(type $mid (sub $top (descriptor $mid.desc (struct))))
;; CHECK: (type $bot (sub $mid (descriptor $bot.desc (struct))))
(type $bot (sub $mid (descriptor $bot.desc (struct))))
;; CHECK: (type $top.desc (sub (describes $top (struct))))
(type $top.desc (sub (describes $top (struct))))
;; CHECK: (type $mid.desc (sub $top.desc (describes $mid (struct))))
(type $mid.desc (sub $top.desc (describes $mid (struct))))
;; CHECK: (type $bot.desc (sub $mid.desc (describes $bot (struct))))
(type $bot.desc (sub $mid.desc (describes $bot (struct))))
)

;; Same as above, but the order of the initial subtypings is reversed.
;;
;; top ---> top.desc
;; ^
;; mid -> mid.desc |(1)
;; ^ (2) |
;; bot ---> bot.desc
;;
;; bot.desc <: top.desc implies bot <: top. When we add bot <: mid, that gives
;; us bot <: mid <: top. This is only valid if we also have
;; bot.desc <: mid.desc <: top.desc.
;; CHECK: (global $bot-top-desc (ref null $top.desc) (struct.new_default $bot.desc))
(global $bot-top-desc (ref null $top.desc) (struct.new $bot.desc))
;; CHECK: (global $bot-mid (ref null $mid) (struct.new_default $bot
;; CHECK-NEXT: (ref.null none)
;; CHECK-NEXT: ))
(global $bot-mid (ref null $mid) (struct.new $bot (ref.null none)))
)
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