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[Custom Descriptors] Do not optimize in TypeSSA (#7702)
Optimizing allocations of descriptor or described types in TypeSSA would be complicated. Creating a subtype of a described type would necesitate creating a subtype of its descriptor type as well, possibly in an arbitrarily long chain. For now, simply do not optimize these types. Add tests to exercise interesting corner cases that would come up if we did optimize, though. These tests also show that we successfully avoid crashing when optimizing modules with descriptor and described types.
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-3
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3 files changed

+232
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‎scripts/test/fuzzing.py

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@@ -128,6 +128,7 @@
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'simplify-locals-desc.wast',
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'optimize-instructions-desc.wast',
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'gto-desc.wast',
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'type-ssa-desc.wast',
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# TODO: fix split_wast() on tricky escaping situations like a string ending
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# in \\" (the " is not escaped - there is an escaped \ before it)
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'string-lifting-section.wast',

‎src/passes/TypeSSA.cpp

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@@ -460,11 +460,17 @@ struct TypeSSA : public Pass {
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return false;
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}
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if (!curr->type.getHeapType().isOpen()) {
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auto type = curr->type.getHeapType();
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if (!type.isOpen()) {
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// We can't create new subtypes of a final type anyway.
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return false;
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}
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// Do not attempt to optimize types with descriptors or describees yet.
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if (type.getDescribedType() || type.getDescriptorType()) {
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return false;
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}
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// Look for at least one interesting operand. We will consider each operand
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// against the declared type, that is, the type declared for where it is
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// stored. If it has more information than the declared type then it is
@@ -491,8 +497,6 @@ struct TypeSSA : public Pass {
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return false;
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};
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auto type = curr->type.getHeapType();
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if (auto* structNew = curr->dynCast<StructNew>()) {
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if (structNew->isWithDefault()) {
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// This starts with all default values - zeros and nulls - and that

‎test/lit/passes/type-ssa-desc.wast

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;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
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;; RUN: foreach %s %t wasm-opt --type-ssa -all -S -o - | filecheck %s
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(module
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(rec
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;; CHECK: (rec
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;; CHECK-NEXT: (type $struct (descriptor $desc (struct)))
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(type $struct (descriptor $desc (struct)))
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;; CHECK: (type $desc (sub (describes $struct (struct (field i32)))))
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(type $desc (sub (describes $struct (struct (field i32)))))
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)
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;; CHECK: (type $2 (func (result (ref $desc))))
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;; CHECK: (func $no-opt-desc (type $2) (result (ref $desc))
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;; CHECK-NEXT: (struct.new_default $desc)
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;; CHECK-NEXT: )
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(func $no-opt-desc (result (ref $desc))
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;; If we tried to optimize this allocation, we would have to create a new
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;; type for our new descriptor type to describe. This isn't very useful
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;; unless we can optimize a corresponding allocation of the described type,
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;; so do not optimize here.
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(struct.new_default $desc)
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)
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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 $struct (sub (descriptor $desc (struct (field i32)))))
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(type $struct (sub (descriptor $desc (struct (field i32)))))
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;; CHECK: (type $desc (describes $struct (struct)))
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(type $desc (sub final (describes $struct (struct))))
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)
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;; CHECK: (type $2 (func (result (ref $struct))))
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;; CHECK: (func $final-desc (type $2) (result (ref $struct))
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;; CHECK-NEXT: (struct.new_default $struct
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;; CHECK-NEXT: (struct.new_default $desc)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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(func $final-desc (result (ref $struct))
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;; We could optimize the allocation of the struct, but we would need to
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;; update the descriptor to be a corresponding new subtype. The descriptor
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;; is final, so this is not posssible and we cannot optimize.
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(struct.new_default $struct
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(struct.new $desc)
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)
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)
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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 $struct (sub (descriptor $desc (struct (field i32)))))
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(type $struct (sub (descriptor $desc (struct (field i32)))))
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;; CHECK: (type $desc (sub (describes $struct (struct))))
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(type $desc (sub (describes $struct (struct))))
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)
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;; CHECK: (type $2 (func (result (ref $struct))))
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;; CHECK: (func $opt (type $2) (result (ref $struct))
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;; CHECK-NEXT: (struct.new_default $struct
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;; CHECK-NEXT: (struct.new_default $desc)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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(func $opt (result (ref $struct))
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;; Now we could optimize, but we would still have to introduce a new
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;; descriptor subtype. We do not support this yet.
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(struct.new_default $struct
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(struct.new $desc)
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)
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)
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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 $A (sub (descriptor $B (struct (field i32)))))
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(type $A (sub (descriptor $B (struct (field i32)))))
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;; CHECK: (type $B (sub (describes $A (descriptor $C (struct)))))
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(type $B (sub (describes $A (descriptor $C (struct)))))
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;; CHECK: (type $C (sub (describes $B (descriptor $D (struct)))))
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(type $C (sub (describes $B (descriptor $D (struct)))))
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;; CHECK: (type $D (sub (describes $C (struct))))
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(type $D (sub (describes $C (struct))))
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)
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;; CHECK: (type $4 (func (result (ref $A))))
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;; CHECK: (func $opt-chain (type $4) (result (ref $A))
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;; CHECK-NEXT: (struct.new_default $A
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;; CHECK-NEXT: (struct.new_default $B
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;; CHECK-NEXT: (struct.new_default $C
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;; CHECK-NEXT: (struct.new_default $D)
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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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(func $opt-chain (result (ref $A))
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;; Now optimizing would require updating a whole chain of descriptors.
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(struct.new_default $A
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(struct.new $B
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(struct.new $C
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(struct.new $D)
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)
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)
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)
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)
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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 $struct (sub (descriptor $desc (struct (field i32)))))
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(type $struct (sub (descriptor $desc (struct (field i32)))))
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;; CHECK: (type $desc (sub (describes $struct (struct))))
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(type $desc (sub (describes $struct (struct))))
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)
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;; CHECK: (type $2 (func (result (ref $struct))))
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;; CHECK: (global $desc (ref (exact $desc)) (struct.new_default $desc))
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(global $desc (ref (exact $desc)) (struct.new $desc))
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;; CHECK: (func $global-desc (type $2) (result (ref $struct))
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;; CHECK-NEXT: (struct.new_default $struct
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;; CHECK-NEXT: (global.get $desc)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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(func $global-desc (result (ref $struct))
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;; Optimizing below would require creating a new global to hold the new
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;; descriptor subtype.
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(struct.new_default $struct
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(global.get $desc)
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)
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)
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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 $struct (sub (descriptor $desc (struct (field i32)))))
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(type $struct (sub (descriptor $desc (struct (field i32)))))
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;; CHECK: (type $desc (sub (describes $struct (struct))))
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(type $desc (sub (describes $struct (struct))))
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)
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;; CHECK: (type $2 (func (result i32)))
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;; CHECK: (global $desc (ref (exact $desc)) (struct.new_default $desc))
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(global $desc (ref (exact $desc)) (struct.new $desc))
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;; CHECK: (func $no-opt-desc-identity (type $2) (result i32)
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;; CHECK-NEXT: (ref.eq
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;; CHECK-NEXT: (ref.get_desc $struct
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;; CHECK-NEXT: (block (result (ref (exact $struct)))
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;; CHECK-NEXT: (struct.new_default $struct
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;; CHECK-NEXT: (global.get $desc)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: (global.get $desc)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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(func $no-opt-desc-identity (result i32)
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;; Same as above, but now we cannot optimize because the identity of the
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;; descriptors matters and we cannot replace them with different subtypes.
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(ref.eq
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(ref.get_desc $struct
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;; Use a block to stop the ref.get_desc from observing the exactness of
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;; the allocation, which would separately inhibit optimization.
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(block (result (ref $struct))
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(struct.new_default $struct
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(global.get $desc)
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)
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)
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)
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(global.get $desc)
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)
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)
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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 $A (sub (descriptor $B (struct (field i32)))))
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(type $A (sub (descriptor $B (struct (field i32)))))
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;; CHECK: (type $B (sub (describes $A (descriptor $C (struct)))))
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(type $B (sub (describes $A (descriptor $C (struct)))))
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;; CHECK: (type $C (sub (describes $B (descriptor $D (struct)))))
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(type $C (sub (describes $B (descriptor $D (struct)))))
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;; CHECK: (type $D (sub (describes $C (struct))))
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(type $D (sub (describes $C (struct))))
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)
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;; CHECK: (type $4 (func (result (ref $A))))
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;; CHECK: (global $D (ref (exact $D)) (struct.new_default $D))
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(global $D (ref (exact $D)) (struct.new $D))
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;; CHECK: (global $C (ref (exact $C)) (struct.new_default $C
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;; CHECK-NEXT: (global.get $D)
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;; CHECK-NEXT: ))
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(global $C (ref (exact $C)) (struct.new $C (global.get $D)))
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;; CHECK: (global $B (ref (exact $B)) (struct.new_default $B
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;; CHECK-NEXT: (global.get $C)
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;; CHECK-NEXT: ))
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(global $B (ref (exact $B)) (struct.new $B (global.get $C)))
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;; CHECK: (func $opt-global-chain (type $4) (result (ref $A))
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;; CHECK-NEXT: (struct.new_default $A
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;; CHECK-NEXT: (global.get $B)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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(func $opt-global-chain (result (ref $A))
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;; Same as above, but now we would have to copy a whole chain of descriptor
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;; globals.
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(struct.new_default $A
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(global.get $B)
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)
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)
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)

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