Fix assertion failure in type-refining-gufa (WebAssembly#7612)

`filterPackedDataReads` had an assertion that the read field existed,
but it was possible for this to not be the case when the reference being
read from was null. Avoid the assertion failure by checking for this
case and filtering the read results to `none` because a read from null
will never return.

Author: tlively

src/ir/possible-contents.cpp

@@ -2825,6 +2825,12 @@ void Flower::filterPackedDataReads(PossibleContents& contents,
     return;
   }
 
+  // If there is no struct or array to read, no value will ever be returned.
+  if (ref->type.isNull()) {
+    contents = PossibleContents::none();
+    return;
+  }
+
   // We are reading data here, so the reference must be a valid struct or
   // array, otherwise we would never have gotten here.
   assert(ref->type.isRef());

test/lit/passes/type-refining-gufa.wast

@@ -425,3 +425,54 @@
   (func $func (type $func)
   )
 )
+
+;; Regression test for an assertion failure on packed data reads from null
+;; references.
+(module
+ (type $i8 (struct (field i8)))
+
+ ;; NRML:      (type $0 (func (param (ref none)) (result i32)))
+
+ ;; NRML:      (table $0 1 funcref)
+ ;; GUFA:      (type $0 (func (param (ref none)) (result i32)))
+
+ ;; GUFA:      (table $0 1 funcref)
+ ;; O3O3:      (type $0 (func (param (ref none)) (result i32)))
+
+ ;; O3O3:      (table $0 1 funcref)
+ (table $0 1 funcref)
+ ;; NRML:      (elem $0 (i32.const 0) $test)
+ ;; GUFA:      (elem $0 (i32.const 0) $test)
+ ;; O3O3:      (elem $0 (i32.const 0) $test)
+ (elem $0 (i32.const 0) $test)
+
+ ;; NRML:      (export "table" (table $0))
+ ;; GUFA:      (export "table" (table $0))
+ ;; O3O3:      (export "table" (table $0))
+ (export "table" (table $0))
+
+ ;; NRML:      (func $test (type $0) (param $i8 (ref none)) (result i32)
+ ;; NRML-NEXT:  (block ;; (replaces unreachable StructGet we can't emit)
+ ;; NRML-NEXT:   (drop
+ ;; NRML-NEXT:    (local.get $i8)
+ ;; NRML-NEXT:   )
+ ;; NRML-NEXT:   (unreachable)
+ ;; NRML-NEXT:  )
+ ;; NRML-NEXT: )
+ ;; GUFA:      (func $test (type $0) (param $i8 (ref none)) (result i32)
+ ;; GUFA-NEXT:  (block ;; (replaces unreachable StructGet we can't emit)
+ ;; GUFA-NEXT:   (drop
+ ;; GUFA-NEXT:    (local.get $i8)
+ ;; GUFA-NEXT:   )
+ ;; GUFA-NEXT:   (unreachable)
+ ;; GUFA-NEXT:  )
+ ;; GUFA-NEXT: )
+ ;; O3O3:      (func $test (type $0) (param $0 (ref none)) (result i32)
+ ;; O3O3-NEXT:  (unreachable)
+ ;; O3O3-NEXT: )
+ (func $test (param $i8 (ref none)) (result i32)
+  (struct.get_s $i8 0
+   (local.get $i8)
+  )
+ )
+)