.

Author: tekknolagi

zjit/src/hir_type/mod.rs

@@ -453,11 +453,20 @@ impl Type {
         types::Empty
     }
 
+    /// Subtract `other` from `self`, preserving specialization if possible.
     pub fn subtract(&self, other: Type) -> Type {
+        if self.is_subtype(other) { return types::Empty; }
+        // If self's specialization is not a subtype of other's specialization (eg Int,Double or
+        // Top,Int or Int,Empty or Int[5],Int[4]), subtracting has no effect.
+        if !self.spec_is_subtype_of(other) {
+            return *self;
+        }
+        // Now the specializations overlap: self's specialization is a subtype of other's
+        // specialization (eg Int,Top or Int[4],Int[4], but NOT Empty,Int because that would mean
+        // self is a subtype of other). Keep self's specialization.
+        // Subtract the bits
         let bits = self.bits & !other.bits;
-        if bits == bits::Empty { return types::Empty; }
-        // TODO(max): See if we can preserve specialization
-        Type { bits, spec: Specialization::Any }
+        Type { bits, spec: self.spec }
     }
 
     pub fn could_be(&self, other: Type) -> bool {