Allow symbols to be used for 12-bit displacements.

The parse_mem_bx function now returns a function to call to add an
action to the action list to handle the evaluation of the
displacement. This allows us to delay adding said action until
after we have emitted the actions for the instruction encodings
themselves.

Code like this should now work:

int x = 24
| st r1, x(sp)

dynasm/dasm_s390x.h

@@ -234,8 +234,10 @@ void dasm_put(Dst_DECL, int start, ...)
       case DASM_IMM16:
       case DASM_IMM32:
       case DASM_DISP20:
-      case DASM_DISP12:
         fprintf(stderr, "not implemented\n");
+      case DASM_DISP12:
+        CK((n>>12) == 0, RANGE_I);
+        b[pos++] = n;
 	break;
       }
     }
@@ -296,7 +298,7 @@ int dasm_link(Dst_DECL, size_t *szp)
         case DASM_IMM32:
         case DASM_DISP20:
         case DASM_DISP12:
-          fprintf(stderr, "not implemented\n");
+          pos++;
 	  break;
 	}
       }
@@ -364,9 +366,11 @@ int dasm_encode(Dst_DECL, void *buffer)
         case DASM_IMM16:
         case DASM_IMM32:
         case DASM_DISP20:
-        case DASM_DISP12:
           fprintf(stderr, "not implemented\n");
           break;
+        case DASM_DISP12:
+          cp[-1] |= n&0xfff;
+	  break;
 	default: *cp++ = ins; break;
 	}
       }

dynasm/dasm_s390x.lua

@@ -316,6 +316,8 @@ end
 
 -- Parse memory operand of the form d(x, b) where 0 <= d < 4096 and b and x
 -- are GPRs.
+-- If the fourth return value is not-nil then it needs to be called to
+-- insert an action.
 -- Encoded as: xbddd
 local function parse_mem_bx(arg)
   local d, x, b = split_memop(arg)
@@ -326,11 +328,10 @@ local function parse_mem_bx(arg)
     end
     return dval, x, b, nil
   end
-  -- TODO: handle d being a symbol.
+  if match(d, "^[rf]1?[0-9]?") then
-  -- Action is currently the final return value (the caller needs to add it
+    werror("expected immediate operand, got register")
-  -- to the action list at a later point).
+  end
-  werror("parse_mem_bx: not implemented")
+  return 0, x, b, function() waction("DISP12", nil, d) end
-  return nil
 end
 
 -- Parse memory operand of the form d(b) where -(2^20)/2 <= d < (2^20)/2 and
@@ -1018,7 +1019,7 @@ local function parse_template(params, template, nparams, pos)
       op2 = op2 + shl(b, 12) + d
       wputhw(op1); wputhw(op2);
       if a then
-        werror("disp12 actions not yet implemented")
+        a()
       end
     elseif p == "k" then
 
@@ -1034,7 +1035,7 @@ local function parse_template(params, template, nparams, pos)
       op2 = op2 + shl(b, 12) + d
       wputhw(op1); wputhw(op2);
       if a then
-        werror("disp12 actions not yet implemented")
+        a()
       end
     elseif p == "z" then
       op2 = op2 + parse_gpr(params[1])
@@ -1046,8 +1047,8 @@ end
 function op_template(params, template, nparams)
   if not params then return template:gsub("%x%x%x%x%x%x%x%x", "") end
   -- Limit number of section buffer positions used by a single dasm_put().
-  -- A single opcode needs a maximum of 3 positions.
+  -- A single opcode needs a maximum of 5 positions.
-  if secpos+3 > maxsecpos then wflush() end
+  if secpos+5 > maxsecpos then wflush() end
   local lpos, apos, spos = #actlist, #actargs, secpos
   local ok, err
   for t in gmatch(template, "[^|]+") do