Use GLB in PossibleContents::intersect (#7550)

Use the standard utility rather than reimplementing type intersection.
The new code is simpler, shorter, and properly supports exactness,
avoiding an assertion failure in the added test case. The other
functional change is that when one of the intersected heap types is
bottom and the type GLB is a non-nullable reference to bottom, the
result of the intersection is `None` where it was previously a `Cone`.

Author: tlively

src/ir/possible-contents.cpp

@@ -182,47 +182,26 @@ void PossibleContents::intersect(const PossibleContents& other) {
   auto heapType = type.getHeapType();
   auto otherHeapType = otherType.getHeapType();
 
-  // If both inputs are nullable then the intersection is nullable as well.
-  auto nullability =
-    type.isNullable() && otherType.isNullable() ? Nullable : NonNullable;
+  // Intersect the types.
+  auto newType = Type::getGreatestLowerBound(type, otherType);
 
   auto setNoneOrNull = [&]() {
-    if (nullability == Nullable) {
+    if (newType.isNullable()) {
       value = Literal::makeNull(heapType);
     } else {
       value = None();
     }
   };
 
-  // If the heap types are not compatible then they are in separate hierarchies
-  // and there is no intersection, aside from possibly a null of the bottom
-  // type.
-  auto isSubType = HeapType::isSubType(heapType, otherHeapType);
-  auto otherIsSubType = HeapType::isSubType(otherHeapType, heapType);
-  if (!isSubType && !otherIsSubType) {
-    if (heapType.getBottom() == otherHeapType.getBottom()) {
-      setNoneOrNull();
-    } else {
-      value = None();
-    }
+  if (newType == Type::unreachable || newType.isNull()) {
+    setNoneOrNull();
     return;
   }
 
   // The heap types are compatible, so intersect the cones.
   auto depthFromRoot = heapType.getDepth();
   auto otherDepthFromRoot = otherHeapType.getDepth();
 
-  // To compute the new cone, find the new heap type for it, and to compute its
-  // depth, consider the adjustments to the existing depths that stem from the
-  // choice of new heap type.
-  HeapType newHeapType;
-
-  if (depthFromRoot < otherDepthFromRoot) {
-    newHeapType = otherHeapType;
-  } else {
-    newHeapType = heapType;
-  }
-
   // Note the global's information, if we started as a global. In that case, the
   // code below will refine our type but we can remain a global, which we will
   // accomplish by restoring our global status at the end.
@@ -231,8 +210,6 @@ void PossibleContents::intersect(const PossibleContents& other) {
     globalName = getGlobal();
   }
 
-  auto newType = Type(newHeapType, nullability);
-
   // By assumption |other| has full depth. Consider the other cone in |this|.
   if (hasFullCone()) {
     // Both are full cones, so the result is as well.
@@ -252,7 +229,7 @@ void PossibleContents::intersect(const PossibleContents& other) {
     // E.g. if |this| is a cone of depth 10, and |otherHeapType| is an immediate
     // subtype of |this|, then the new cone must be of depth 9.
     auto newDepth = getCone().depth;
-    if (newHeapType == otherHeapType) {
+    if (newType.getHeapType() == otherHeapType) {
       assert(depthFromRoot <= otherDepthFromRoot);
       auto reduction = otherDepthFromRoot - depthFromRoot;
       if (reduction > newDepth) {

test/lit/passes/gufa-cast-all-exact.wast

@@ -138,3 +138,39 @@
   )
  )
 )
+
+(module
+ ;; CHECK:      (type $foo (sub (struct (field i32))))
+ ;; NO_CD:      (type $foo (sub (struct (field i32))))
+ (type $foo (sub (struct (field i32))))
+
+ ;; CHECK:      (import "" "" (global $null-exact (ref null (exact $foo))))
+ ;; NO_CD:      (import "" "" (global $null-exact (ref null (exact $foo))))
+ (import "" "" (global $null-exact (ref null (exact $foo))))
+
+ ;; CHECK:      (func $as-non-null (type $1) (result i32)
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (ref.as_non_null
+ ;; CHECK-NEXT:    (global.get $null-exact)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (unreachable)
+ ;; CHECK-NEXT: )
+ ;; NO_CD:      (func $as-non-null (type $1) (result i32)
+ ;; NO_CD-NEXT:  (drop
+ ;; NO_CD-NEXT:   (ref.as_non_null
+ ;; NO_CD-NEXT:    (global.get $null-exact)
+ ;; NO_CD-NEXT:   )
+ ;; NO_CD-NEXT:  )
+ ;; NO_CD-NEXT:  (unreachable)
+ ;; NO_CD-NEXT: )
+ (func $as-non-null (result i32)
+  (struct.get $foo 0
+   ;; Regression test for an assertion failure when the intersection here
+   ;; dropped exactness as well as nullness.
+   (ref.as_non_null
+    (global.get $null-exact)
+   )
+  )
+ )
+)

test/lit/passes/gufa-refs.wast

@@ -6069,8 +6069,13 @@
   ;; CHECK:      (func $test-set-bottom (type $2)
   ;; CHECK-NEXT:  (block ;; (replaces unreachable ArraySet we can't emit)
   ;; CHECK-NEXT:   (drop
-  ;; CHECK-NEXT:    (ref.cast nullref
-  ;; CHECK-NEXT:     (global.get $global)
+  ;; CHECK-NEXT:    (block (result nullref)
+  ;; CHECK-NEXT:     (drop
+  ;; CHECK-NEXT:      (ref.cast nullref
+  ;; CHECK-NEXT:       (global.get $global)
+  ;; CHECK-NEXT:      )
+  ;; CHECK-NEXT:     )
+  ;; CHECK-NEXT:     (ref.null none)
   ;; CHECK-NEXT:    )
   ;; CHECK-NEXT:   )
   ;; CHECK-NEXT:   (drop