https://sourceforge.net/p/sdcc/bugs/4154/

Index: src/mcs51/ralloc.c
===================================================================
--- src/mcs51/ralloc.c	(revision 17011)
+++ src/mcs51/ralloc.c	(working copy)
@@ -641,6 +641,7 @@
 spillThis (symbol * sym)
 {
   int i;
+  bool ownsRegs = bitVectBitValue (_G.regAssigned, sym->key);
   /* if this is rematerializable or has a spillLocation
      we are okay, else we need to create a spillLocation
      for it */
@@ -658,7 +659,8 @@
     {
       if (sym->regs[i])
         {
-          freeReg (sym->regs[i]);
+          if (ownsRegs)
+            freeReg (sym->regs[i]);
           sym->regs[i] = NULL;
         }
     }