cmd/compile: remove types.Compare map bucket special cases

runtime.hmap never directly refers to the bucket type (it uses an
unsafe.Pointer), thus it shouldn't be possible to have infinite
recursion here.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-linux-amd64-longtest-race,gotip-linux-amd64-longtest-aliastypeparams
Change-Id: I457885e48bbc352acedae1c0c389078f39ed9d65
Reviewed-on: https://go-review.googlesource.com/c/go/+/619037
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: prattmic

src/cmd/compile/internal/types/type.go

@@ -1190,35 +1190,35 @@ func (t *Type) cmp(x *Type) Cmp {
 
 	case TSTRUCT:
 		if buildcfg.Experiment.SwissMap {
+			// Is this a map group type?
 			if t.StructType().Map == nil {
 				if x.StructType().Map != nil {
 					return CMPlt // nil < non-nil
 				}
 				// to the fallthrough
 			} else if x.StructType().Map == nil {
 				return CMPgt // nil > non-nil
-			} else {
-				// TODO: I am confused by the purpose of the OldBucket stuff below.
-				return t.StructType().Map.cmp(x.StructType().Map)
-			} // If t != t.Map.SwissBucket, fall through to general case
+			}
+			// Both have non-nil Map, fallthrough to the general
+			// case. Note that the map type does not directly refer
+			// to the group type (it uses unsafe.Pointer). If it
+			// did, this would need special handling to avoid
+			// infinite recursion.
 		} else {
+			// Is this a map bucket type?
 			if t.StructType().Map == nil {
 				if x.StructType().Map != nil {
 					return CMPlt // nil < non-nil
 				}
 				// to the fallthrough
 			} else if x.StructType().Map == nil {
 				return CMPgt // nil > non-nil
-			} else if t.StructType().Map.MapType().OldBucket == t {
-				// Both have non-nil Map
-				// Special case for Maps which include a recursive type where the recursion is not broken with a named type
-				if x.StructType().Map.MapType().OldBucket != x {
-					return CMPlt // bucket maps are least
-				}
-				return t.StructType().Map.cmp(x.StructType().Map)
-			} else if x.StructType().Map.MapType().OldBucket == x {
-				return CMPgt // bucket maps are least
-			} // If t != t.Map.OldBucket, fall through to general case
+			}
+			// Both have non-nil Map, fallthrough to the general
+			// case. Note that the map type does not directly refer
+			// to the bucket type (it uses unsafe.Pointer). If it
+			// did, this would need special handling to avoid
+			// infinite recursion.
 		}
 
 		tfs := t.Fields()