code review 92760044: runtime: use duff zero and copy to initialize memory

src/cmd/5g/peep.c

 // Inferno utils/5c/peep.c
 // http://code.google.com/p/inferno-os/source/browse/utils/5c/peep.c
 //
 //      Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
 //      Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
 //      Portions Copyright © 1997-1999 Vita Nuova Limited
 //      Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
 //      Portions Copyright © 2004,2006 Bruce Ellis
 //      Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
 //      Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
@@ skipping 1140 matching lines @@
 
                 if(s != nil) {
                         if(copysub(&p->to, v, s, 1))
                                 return 1;
                         return 0;
                 }
                 if(copyau(&p->to, v))
                         return 4;
                 return 3;
         case ADUFFZERO:
+                // R0 is zero, used by DUFFZERO, cannot be substituted.
+                // R1 is ptr to memory, used and set, cannot be substituted.
                 if(v->type == D_REG) {
                         if(v->reg == REGALLOC_R0)
                                 return 1;
                         if(v->reg == REGALLOC_R0+1)
                                 return 2;
                 }
                 return 0;
         case ADUFFCOPY:
+                // R0 is scratch, set by DUFFCOPY, cannot be substituted.
+                // R1, R2 areptr to src, dst, used and set, cannot be substituted.
                 if(v->type == D_REG) {
                         if(v->reg == REGALLOC_R0)
                                 return 3;
                         if(v->reg == REGALLOC_R0+1 || v->reg == REGALLOC_R0+2)
                                 return 2;
                 }
                 return 0;
 ························
         case ATEXT:     /* funny */
                 if(v->type == D_REG)
@@ skipping 431 matching lines @@
         return regtyp(a) && a->reg == REGSP;
 }
 
 int
 smallindir(Addr *a, Addr *reg)
 {
         return reg->type == D_REG && a->type == D_OREG &&
                 a->reg == reg->reg &&
                 0 <= a->offset && a->offset < 4096;
 }