diff --git a/internal/engine/wazevo/backend/regalloc/regalloc.go b/internal/engine/wazevo/backend/regalloc/regalloc.go
index 6e11c085..731426fc 100644
--- a/internal/engine/wazevo/backend/regalloc/regalloc.go
+++ b/internal/engine/wazevo/backend/regalloc/regalloc.go
@@ -972,6 +972,12 @@ func (a *Allocator) fixMergeState(f Function, blk Block) {
 	bID := blk.ID()
 	blkSt := a.getOrAllocateBlockState(bID)
 	desiredOccupants := &blkSt.startRegs
+	var desiredOccupantsSet RegSet
+	for i, v := range desiredOccupants {
+		if v != VRegInvalid {
+			desiredOccupantsSet = desiredOccupantsSet.add(RealReg(i))
+		}
+	}
 
 	if wazevoapi.RegAllocLoggingEnabled {
 		fmt.Println("fixMergeState", blk.ID(), ":", desiredOccupants.format(a.regInfo))
@@ -993,12 +999,12 @@ func (a *Allocator) fixMergeState(f Function, blk Block) {
 		// Finds the free registers if any.
 		intTmp, floatTmp := VRegInvalid, VRegInvalid
 		if intFree := s.findAllocatable(
-			a.regInfo.AllocatableRegisters[RegTypeInt], desiredOccupants.set,
+			a.regInfo.AllocatableRegisters[RegTypeInt], desiredOccupantsSet,
 		); intFree != RealRegInvalid {
 			intTmp = FromRealReg(intFree, RegTypeInt)
 		}
 		if floatFree := s.findAllocatable(
-			a.regInfo.AllocatableRegisters[RegTypeFloat], desiredOccupants.set,
+			a.regInfo.AllocatableRegisters[RegTypeFloat], desiredOccupantsSet,
 		); floatFree != RealRegInvalid {
 			floatTmp = FromRealReg(floatFree, RegTypeFloat)
 		}
diff --git a/internal/engine/wazevo/backend/regalloc/regalloc_test.go b/internal/engine/wazevo/backend/regalloc/regalloc_test.go
index d0daf2f9..526d2792 100644
--- a/internal/engine/wazevo/backend/regalloc/regalloc_test.go
+++ b/internal/engine/wazevo/backend/regalloc/regalloc_test.go
@@ -366,7 +366,7 @@ func TestAllocator_livenessAnalysis_copy(t *testing.T) {
 
 func Test_findOrSpillAllocatable_prefersSpill(t *testing.T) {
 	t.Run("ok", func(t *testing.T) {
-		s := &state{}
+		s := &state{regsInUse: newRegInUseSet()}
 		s.regsInUse.add(RealReg(1), VReg(2222222))
 		got := s.findOrSpillAllocatable(&Allocator{}, []RealReg{3}, 0, 3)
 		require.Equal(t, RealReg(3), got)
diff --git a/internal/engine/wazevo/backend/regalloc/regset.go b/internal/engine/wazevo/backend/regalloc/regset.go
index e9bf6066..04a8e8f4 100644
--- a/internal/engine/wazevo/backend/regalloc/regset.go
+++ b/internal/engine/wazevo/backend/regalloc/regset.go
@@ -46,23 +46,24 @@ func (rs RegSet) Range(f func(allocatedRealReg RealReg)) {
 	}
 }
 
-type regInUseSet struct {
-	set RegSet
-	vrs [64]VReg
+type regInUseSet [64]VReg
+
+func newRegInUseSet() regInUseSet {
+	var ret regInUseSet
+	ret.reset()
+	return ret
 }
 
 func (rs *regInUseSet) reset() {
-	rs.set = 0
-	for i := range rs.vrs {
-		rs.vrs[i] = VRegInvalid
+	for i := range rs {
+		rs[i] = VRegInvalid
 	}
 }
 
 func (rs *regInUseSet) format(info *RegisterInfo) string { //nolint:unused
 	var ret []string
-	for i := 0; i < 64; i++ {
-		if rs.set&(1<<uint(i)) != 0 {
-			vr := rs.vrs[i]
+	for i, vr := range rs {
+		if vr != VRegInvalid {
 			ret = append(ret, fmt.Sprintf("(%s->v%d)", info.RealRegName(RealReg(i)), vr.ID()))
 		}
 	}
@@ -70,39 +71,28 @@ func (rs *regInUseSet) format(info *RegisterInfo) string { //nolint:unused
 }
 
 func (rs *regInUseSet) has(r RealReg) bool {
-	if r >= 64 {
-		return false
-	}
-	return rs.set&(1<<uint(r)) != 0
+	return r < 64 && rs[r] != VRegInvalid
 }
 
 func (rs *regInUseSet) get(r RealReg) VReg {
-	if r >= 64 {
-		return VRegInvalid
-	}
-	return rs.vrs[r]
+	return rs[r]
 }
 
 func (rs *regInUseSet) remove(r RealReg) {
-	if r >= 64 {
-		return
-	}
-	rs.set &= ^(1 << uint(r))
-	rs.vrs[r] = VRegInvalid
+	rs[r] = VRegInvalid
 }
 
 func (rs *regInUseSet) add(r RealReg, vr VReg) {
 	if r >= 64 {
 		return
 	}
-	rs.set |= 1 << uint(r)
-	rs.vrs[r] = vr
+	rs[r] = vr
 }
 
 func (rs *regInUseSet) range_(f func(allocatedRealReg RealReg, vr VReg)) {
-	for i := 0; i < 64; i++ {
-		if rs.set&(1<<uint(i)) != 0 {
-			f(RealReg(i), rs.vrs[i])
+	for i, vr := range rs {
+		if vr != VRegInvalid {
+			f(RealReg(i), vr)
 		}
 	}
 }