patch to goto optional semicolon changing opcodes

[Patrick Donnelly <batrick@...>]

Sorry about vague subject. I'll try to clarify:

I'm using luac to confirm that the code formatted by [1] does not
change the behavior of a program. I've noticed that the optional
semicolon that terminates a goto or break statement [inside an if
block (I don't know why)] will influence the bytecode generation. If
there is a semicolon, an extra unreachable JMP is added:

$ luac -l -p - <<< "for i = 1, 2 do if true then break end end"

main <stdin:0,0> (9 instructions at 0x24a20d0)
0+ params, 5 slots, 1 upvalue, 4 locals, 2 constants, 0 functions
        1       [1]     LOADK           0 -1    ; 1
        2       [1]     LOADK           1 -2    ; 2
        3       [1]     LOADK           2 -1    ; 1
        4       [1]     FORPREP         0 3     ; to 8
        5       [1]     LOADBOOL        4 1 0
        6       [1]     TEST            4 1
        7       [1]     JMP             0 1     ; to 9
        8       [1]     FORLOOP         0 -4    ; to 5
        9       [1]     RETURN          0 1
$ luac -l -p - <<< "for i = 1, 2 do if true then break; end end"

main <stdin:0,0> (10 instructions at 0xca50d0)
0+ params, 5 slots, 1 upvalue, 4 locals, 2 constants, 0 functions
        1       [1]     LOADK           0 -1    ; 1
        2       [1]     LOADK           1 -2    ; 2
        3       [1]     LOADK           2 -1    ; 1
        4       [1]     FORPREP         0 4     ; to 9
        5       [1]     LOADBOOL        4 1 0
        6       [1]     TEST            4 1
        7       [1]     JMP             0 2     ; to 10
        8       [1]     JMP             0 0     ; to 9
        9       [1]     FORLOOP         0 -5    ; to 5
        10      [1]     RETURN          0 1


The patch to fix this is simple, similar to removing the optional
semicolon for the return statement.

Index: lua-5.2.0/src/lparser.c
===================================================================
--- lua-5.2.0/src/lparser.c     (revision 47)
+++ lua-5.2.0/src/lparser.c     (working copy)
@@ -1191,6 +1191,7 @@
     luaX_next(ls);  /* skip break */
     label = luaS_new(ls->L, "break");
   }
+  testnext(ls, ';');  /* skip optional semicolon */
   g = newlabelentry(ls, &ls->dyd->gt, label, line, pc);
   findlabel(ls, g);  /* close it if label already defined */
 }


I don't really consider this a bug (obviously) but hopefully Lua 5.2.1
can include this.


[1] http://lua-users.org/lists/lua-l/2012-04/msg01082.html

-- 
- Patrick Donnelly