internal/runtime: cleaner overflow checker

remove todo

Change-Id: I4b10d7a8c26bea9296b321f53abd0330f2afc35a
GitHub-Last-Rev: b939acc287
GitHub-Pull-Request: golang/go#74943
Reviewed-on: https://go-review.googlesource.com/c/go/+/694236
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/runtime/maps/table.go

@@ -8,6 +8,7 @@ package maps
 import (
 	"internal/abi"
 	"internal/goarch"
+	"internal/runtime/math"
 	"unsafe"
 )
 
@@ -127,8 +128,7 @@ func (t *table) maxGrowthLeft() uint16 {
 		// single-group tables, we could fill all slots.
 		return t.capacity - 1
 	} else {
-		if t.capacity*maxAvgGroupLoad < t.capacity {
-			// TODO(prattmic): Do something cleaner.
+		if t.capacity > math.MaxUint16/maxAvgGroupLoad {
 			panic("overflow")
 		}
 		return (t.capacity * maxAvgGroupLoad) / abi.MapGroupSlots

src/internal/runtime/math/math.go

@@ -7,6 +7,7 @@ package math
 import "internal/goarch"
 
 const (
+	MaxUint16  = ^uint16(0)
 	MaxUint32  = ^uint32(0)
 	MaxUint64  = ^uint64(0)
 	MaxUintptr = ^uintptr(0)