MOOCodeSequences.txt

This file documents, in very terse fashion, the opcode sequences emitted by the
MOO compiler and accepted by the MOO decompiler.  It is an invaluable reference
while reading the code in either `code_gen.c' or `decompile.c'.

NOTE: Since MOO database files contain suspended tasks, and since the file
      representation of those tasks includes both source code and a current PC
      for each frame on the stack, IT IS CRITICALLY IMPORTANT that these code
      sequences not change between releases of the server.  Otherwise, when the
      code for a frame was recompiled at server start-up, its associated saved
      PC value might no longer be valid, leading to almost certain catastrophe.
      If the database file format were at some point changed to contain the
      bytecodes (and not the source code) for suspended task frames, then this
      restriction could (at least one release later) be relaxed.

stmt:
	  {[ELSE]IF ( expr ) stmts}+ [ELSE stmts] ENDIF

		<expr>		; Once for each arm
		IF / EIF next	;
		<stmts>		;
		JUMP done	;
	    next:		;

		<stmts>		; If there's an ELSE part

	    done:

	| FOR id IN ( expr ) stmts ENDFOR

		<expr>
		NUM 1
	    top:
		FOR_LIST id done
		<stmts>
		JUMP top
	    done:

	| FOR id IN [ expr1 .. expr2 ] stmts ENDFOR

		<expr1>
		<expr2>
	    top:
		FOR_RANGE id done
		<stmts>
		JUMP top
	    done:

	| WHILE [id] ( expr ) stmts ENDWHILE

	    top:
		<expr>

		WHILE done		; if there is no ID
		WHILE_ID id done	; if there is an ID

		<stmts>
		JUMP top
	    done:

	| FORK [id] ( expr ) stmts ENDFORK

		<expr>
		FORK / FORK_WITH_ID vector [id]

		{vector: <stmts>}

	| expr ;

		<expr>
		POP

	| RETURN ;

		RETURN0

	| RETURN expr ;

		<expr>
		RETURN

    	| TRY stmts_b {EXCEPT [id_i] ( codes_i ) stmts_i}+ ENDTRY

		<codes_1>
		PUSH_LABEL handler_1
		...
		<codes_N>
		PUSH_LABEL handler_N
		TRY_EXCEPT N
		<stmts_b>
		END_EXCEPT done
		...
	    handler_i:
		PUT id_i	; if <id_i> is supplied
		POP
		<stmts_i>
		JUMP done	; all but last handler
		...
	    done:

    	| TRY stmts_b FINALLY stmts_h ENDTRY

		TRY_FINALLY handler
		<stmts_b>
		END_FINALLY
	    handler:
		<stmts_h>
		CONTINUE

	| BREAK [id];
	| CONTINUE [id];

		EXIT <stack-level> <label>	; if there is no ID
		EXIT_ID <stack-level> <label>	; if there is an ID
	;

expr:
	  NUMBER
	| STRING
	| # NUMBER
	| # - NUMBER
	| ERROR

		NUM n		; if NUMBER and IN_OPTIM_NUM_RANGE(n)

		IMM v		; otherwise

	| id

		PUSH id

	| expr1 && expr2
	| expr1 || expr2

		<expr1>
		AND / OR done
		<expr2>
	    done:

	| - expr
	| ! expr

		<expr>
		UNARY_MINUS / NOT

	| $ id
	| expr1 . id
	| expr1 . ( expr2 )

		<expr1>
		<expr2>
		GET_PROP

	| expr1 == expr2
	| expr1 != expr2
	| expr1 < expr2
	| expr1 <= expr2
	| expr1 > expr2
	| expr1 >= expr2
	| expr1 IN expr2
	| expr1 + expr2
	| expr1 - expr2
	| expr1 * expr2
	| expr1 / expr2
	| expr1 % expr2
	| expr1 [ expr2 ]

		<expr1>
		<expr2>
		EQ / NE / LT / LE / GT / GE / IN / ADD / MINUS / MULT / DIV
		   / MOD / REF

	| expr1 [ expr2 .. expr3 ]

		<expr1>
		<expr2>
		<expr3>
		RANGE_REF

	| { arglist }

		<arglist>

	| id ( arglist )

		<arglist>
		BI_FUNC_CALL fn

	| expr1 : id ( arglist )
	| expr1 : ( expr2 ) ( arglist )

		<expr1>
		<expr2>
		<arglist>
		CALL_VERB

	| expr1 ? expr2 | expr3

		<expr1>
		IF_QUES else
		<expr2>
		JUMP done
	    else:
		<expr3>
	    done:
		
	| {id | expr_l1 . ( expr_l2 )}
			{[ expr_i ]}*
			{[ expr_r1 .. expr_r2 ]}? = expr_r

		PUSH id		; if id and indexed/subranged

		<expr_l1>	; if expr.expr
		<expr_l2>	;
		PUSH_GET_PROP	; if expr.expr and indexed/subranged

		...		; if indexed
		<expr_i>	;
		PUSH_REF	; if indexed/subranged further
		...		;

		<expr_r1>	; if subranged
		<expr_r2>	;

		<expr_r>

		PUT_TEMP	; if indexed/subranged

		RANGESET	; if subranged

		...		; if indexed
		INDEXSET	;
		...		;

		PUT		; if id

		PUT_PROP	; if expr.expr

		POP		; if indexed/subranged
		PUSH_TEMP	;

	| { scatter } = expr_r

		/* Example:
		 *    {a, ?b, ?c = expr_c, @d, ?e = expr_e, f} = expr_r
		 */

		<expr_r>
		SCATTER 6, 2, 4: a/0, b/1, c/default_c, d/0, e/default_e,
			^  ^  ^	 f/0, done
			|  |  |  ^
			|  |  |  |
			|  |  |  +-- list of id/label pairs + done label
			|  |  +----- 1-index of `@' argument (or # of args)
			|  +-------- number of required arguments
			+----------- number of arguments
	    default_c:
		<expr_c>
		PUT c
		POP
	    default_e:
		<expr_e>
		PUT e
		POP
	    done:

	| ` expr ! codes [=> expr_d] '

		<codes>
		PUSH_LABEL handler
		CATCH
		<expr>
		END_CATCH done
	    handler:
		NUM 1		; if <expr_d> is omitted
		REF		;

		POP		; if <expr_d> is supplied
		<expr_d>	;
	    done:

	;

codes:
	  ANY

		NUM 0

	| ne_arglist

		<ne_arglist>

arglist:
	  /* NOTHING */

		MAKE_EMPTY_LIST

        | ne_arglist

		<ne_arglist>

	;

ne_arglist:
	expr

		<expr>
		MAKE_SINGLETON_LIST

	| @ expr

		<expr>
		CHECK_LIST_FOR_SPLICE

	| ne_arglist , expr

		<ne_arglist>
		<expr>
		LIST_ADD_TAIL

	| ne_arglist , @ expr

		<ne_arglist>
		<expr>
		LIST_APPEND

	;