[Custom Descriptors] Optimize struct.new (#7691)

Optimize struct.new instructions that are known to trap due to having
null descriptors. Also optimize out non-null casts of descriptors, since
struct.new will implicitly perform that cast.

Author: tlively

scripts/test/fuzzing.py

@@ -126,6 +126,7 @@
     'precompute-desc.wast',
     'gc-desc.wast',
     'simplify-locals-desc.wast',
+    'optimize-instructions-desc.wast',
     # TODO: fix split_wast() on tricky escaping situations like a string ending
     #       in \\" (the " is not escaped - there is an escaped \ before it)
     'string-lifting-section.wast',

src/passes/OptimizeInstructions.cpp

@@ -1830,9 +1830,18 @@ struct OptimizeInstructions
   }
 
   void visitStructNew(StructNew* curr) {
+    if (curr->type == Type::unreachable) {
+      // Leave this for DCE.
+      return;
+    }
+    if (curr->desc) {
+      skipNonNullCast(curr->desc, curr);
+      trapOnNull(curr, curr->desc);
+    }
+
     // If values are provided, but they are all the default, then we can remove
     // them (in reachable code).
-    if (curr->type == Type::unreachable || curr->isWithDefault()) {
+    if (curr->isWithDefault()) {
       return;
     }
 

test/lit/passes/optimize-instructions-desc.wast

@@ -0,0 +1,53 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+
+;; RUN: wasm-opt %s -all --optimize-instructions -S -o - | filecheck %s
+
+(module
+  (rec
+    ;; CHECK:      (rec
+    ;; CHECK-NEXT:  (type $struct (descriptor $desc (struct)))
+    (type $struct (descriptor $desc (struct)))
+    ;; CHECK:       (type $desc (describes $struct (struct)))
+    (type $desc (describes $struct (struct)))
+  )
+
+  ;; CHECK:      (func $trap-null-desc (type $2) (result (ref (exact $struct)))
+  ;; CHECK-NEXT:  (unreachable)
+  ;; CHECK-NEXT: )
+  (func $trap-null-desc (result (ref (exact $struct)))
+    (struct.new $struct
+      (ref.null none)
+    )
+  )
+
+  ;; CHECK:      (func $trap-null-desc-fallthrough (type $2) (result (ref (exact $struct)))
+  ;; CHECK-NEXT:  (local $desc (ref null (exact $desc)))
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (local.tee $desc
+  ;; CHECK-NEXT:    (ref.null none)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (unreachable)
+  ;; CHECK-NEXT: )
+  (func $trap-null-desc-fallthrough (result (ref (exact $struct)))
+    (local $desc (ref null (exact $desc)))
+    (struct.new $struct
+      (local.tee $desc
+        (ref.null none)
+      )
+    )
+  )
+
+  ;; CHECK:      (func $nonnull-cast-desc (type $3) (param $desc (ref null (exact $desc))) (result (ref (exact $struct)))
+  ;; CHECK-NEXT:  (struct.new_default $struct
+  ;; CHECK-NEXT:   (local.get $desc)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $nonnull-cast-desc (param $desc (ref null (exact $desc))) (result (ref (exact $struct)))
+    (struct.new $struct
+      (ref.as_non_null
+        (local.get $desc)
+      )
+    )
+  )
+)