diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index 291ca91c61..abfdfc1b24 100644
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -963,7 +963,10 @@ func (s *regAllocState) regalloc(f *Func) {
 			}
 
 			// Save the starting state for use by merge edges.
-			var regList []startReg
+			// We append to a stack allocated variable that we'll
+			// later copy into s.startRegs in one fell swoop, to save
+			// on allocations.
+			regList := make([]startReg, 0, 32)
 			for r := register(0); r < s.numRegs; r++ {
 				v := s.regs[r].v
 				if v == nil {
@@ -976,7 +979,8 @@ func (s *regAllocState) regalloc(f *Func) {
 				}
 				regList = append(regList, startReg{r, v, s.regs[r].c, s.values[v.ID].uses.pos})
 			}
-			s.startRegs[b.ID] = regList
+			s.startRegs[b.ID] = make([]startReg, len(regList))
+			copy(s.startRegs[b.ID], regList)
 
 			if s.f.pass.debug > regDebug {
 				fmt.Printf("after phis\n")