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[Custom Descriptors] Ensure descriptor validity in Unsubtyping (#7743)
We already partially handled the interaction between descriptors and subtyping by ensuring that B <: A implies B.desc <: A.desc if B.desc and A.desc both exist. However, there was a subtler situation we did not handle: A -> A.desc ^ ^ | B.desc | ^ C -> C.desc This subtyping violates the constraint that the descriptor of C's immediate supertype must be the immediate supertype of C's descriptor. To fix this, find the described type B to insert between C and A before establishing C.desc <: B.desc <: A.desc.
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‎src/passes/Unsubtyping.cpp

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@@ -492,6 +492,7 @@ struct Unsubtyping : Pass {
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}
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if (HeapType::isSubType(*oldSuper, super)) {
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// sub <: oldSuper <: super
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processDescribed(sub, *oldSuper, super);
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noteSubtype(*oldSuper, super);
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// We already handled sub <: oldSuper, so we're done.
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return;
@@ -501,6 +502,7 @@ struct Unsubtyping : Pass {
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// super will already be in the same tree when we process them below, so
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// when we process casts we will know that we only need to process up to
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// oldSuper.
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processDescribed(sub, super, *oldSuper);
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process(super, *oldSuper);
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}
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@@ -512,6 +514,42 @@ struct Unsubtyping : Pass {
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processCasts(sub, super, oldSuper);
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}
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void processDescribed(HeapType sub, HeapType mid, HeapType super) {
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// We are establishing sub <: mid <: super. If super describes the immediate
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// supertype of the type sub describes, then once we insert mid between them
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// we would have this:
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//
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// A -> super
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// ^ ^
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// | mid
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// | ^
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// C -> sub
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//
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// This violates the requirement that the descriptor of C's immediate
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// supertype must be the immediate supertype of C's descriptor. To fix it,
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// we have to find the type B that mid describes and insert it between A and
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// C:
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//
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// A -> super
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// ^ ^
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// B -> mid
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// ^ ^
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// C -> sub
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//
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// We do this eagerly before we establish sub <: mid <: super so that if
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// establishing that subtyping requires recursively establishing other
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// subtypings, we can depend on the invariant that the described types are
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// always set up correctly beforehand.
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auto subDescribed = sub.getDescribedType();
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auto superDescribed = super.getDescribedType();
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if (subDescribed && superDescribed &&
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types.getSupertype(*subDescribed) == superDescribed) {
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auto midDescribed = mid.getDescribedType();
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assert(midDescribed);
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process(*subDescribed, *midDescribed);
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}
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}
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void processDefinitions(HeapType sub, HeapType super) {
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if (super.isBasic()) {
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return;

‎test/lit/passes/unsubtyping-desc.wast

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@@ -62,3 +62,75 @@
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;; CHECK: (global $A.desc (ref null $A.desc) (global.get $B.desc))
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(global $A.desc (ref null $A.desc) (global.get $B.desc))
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)
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(module
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(rec
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;; CHECK: (rec
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;; CHECK-NEXT: (type $top (sub (descriptor $top.desc (struct))))
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(type $top (sub (descriptor $top.desc (struct))))
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;; CHECK: (type $mid (sub $top (descriptor $mid.desc (struct))))
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(type $mid (sub $top (descriptor $mid.desc (struct))))
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;; CHECK: (type $bot (sub $mid (descriptor $bot.desc (struct))))
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(type $bot (sub $mid (descriptor $bot.desc (struct))))
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;; CHECK: (type $top.desc (sub (describes $top (struct))))
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(type $top.desc (sub (describes $top (struct))))
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;; CHECK: (type $mid.desc (sub $top.desc (describes $mid (struct))))
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(type $mid.desc (sub $top.desc (describes $mid (struct))))
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;; CHECK: (type $bot.desc (sub $mid.desc (describes $bot (struct))))
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(type $bot.desc (sub $mid.desc (describes $bot (struct))))
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)
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;; top -> top.desc
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;; ^
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;; |(2) mid -> mid.desc
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;; | ^ (1)
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;; bot -> bot.desc
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;;
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;; bot <: top implies bot.desc <: top.desc, but we already have
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;; bot.desc <: mid.desc, so that gives us bot.desc <: mid.desc <: top.desc.
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;; This is only valid if we also have bot <: mid <: top.
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;; CHECK: (global $bot-mid-desc (ref null $mid.desc) (struct.new_default $bot.desc))
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(global $bot-mid-desc (ref null $mid.desc) (struct.new $bot.desc))
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;; CHECK: (global $bot-top (ref null $top) (struct.new_default $bot
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;; CHECK-NEXT: (ref.null none)
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;; CHECK-NEXT: ))
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(global $bot-top (ref null $top) (struct.new $bot (ref.null none)))
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)
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(module
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(rec
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;; CHECK: (rec
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;; CHECK-NEXT: (type $top (sub (descriptor $top.desc (struct))))
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(type $top (sub (descriptor $top.desc (struct))))
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;; CHECK: (type $mid (sub $top (descriptor $mid.desc (struct))))
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(type $mid (sub $top (descriptor $mid.desc (struct))))
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;; CHECK: (type $bot (sub $mid (descriptor $bot.desc (struct))))
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(type $bot (sub $mid (descriptor $bot.desc (struct))))
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;; CHECK: (type $top.desc (sub (describes $top (struct))))
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(type $top.desc (sub (describes $top (struct))))
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;; CHECK: (type $mid.desc (sub $top.desc (describes $mid (struct))))
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(type $mid.desc (sub $top.desc (describes $mid (struct))))
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;; CHECK: (type $bot.desc (sub $mid.desc (describes $bot (struct))))
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(type $bot.desc (sub $mid.desc (describes $bot (struct))))
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)
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;; Same as above, but the order of the initial subtypings is reversed.
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;;
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;; top -> top.desc
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;; ^
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;; |(1) mid -> mid.desc
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;; | ^ (2)
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;; bot -> bot.desc
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;;
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;; bot <: top implies bot.desc <: top.desc. When we add bot.desc <: mid.desc,
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;; that gives us bot.desc <: mid.desc <: top.desc. This is only valid if we
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;; also have bot <: mid <: top.
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;; CHECK: (global $bot-top (ref null $top) (struct.new_default $bot
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;; CHECK-NEXT: (ref.null none)
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;; CHECK-NEXT: ))
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(global $bot-top (ref null $top) (struct.new $bot (ref.null none)))
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;; CHECK: (global $bot-mid-desc (ref null $mid.desc) (struct.new_default $bot.desc))
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(global $bot-mid-desc (ref null $mid.desc) (struct.new $bot.desc))
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)

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