Re: Logic bug

Ryota Hirose.

2011/1/31 Gavin Wraith <gavin@wra1th.plus.com>

Some code plus disassembly (5.2(alpha)):

arrow = "=>"
--[[
1 SETTABUP 0 -1 -2 ; _ENV "arrow" "=>"
--]]
print("(7 or true) and (8 or true)",arrow,(7 or true) and (8 or true))
--[[
2 GETTABUP 0 0 -3 ; _ENV "print"
3 LOADK 1 -4 ; "(7 or true) and (8 or true)"
4 GETTABUP 2 0 -1 ; _ENV "arrow"
5 LOADK 3 -5 ; 7
6 TEST 3 0 1
7 JMP 0 ; to 8
8 LOADK 3 -6 ; 8
9 TEST 3 0 1
10 JMP 1 ; to 12
11 LOADBOOL 3 1 0
12 CALL 0 4 1 ; correct result: 8
--]]
print("(7 or true) and (true or 8)", arrow,(7 or true) and (true or 8))
--[[
13 GETTABUP 0 0 -3 ; _ENV "print"
14 LOADK 1 -7 ; "(7 or true) and (true or 8)"
15 GETTABUP 2 0 -1 ; _ENV "arrow"
16 LOADK 3 -5 ; 7
17 TEST 3 0 1
18 JMP 5 ; to 24
19 JMP 3 ; to 23
20 LOADK 3 -6 ; 8
21 JMP 2 ; to 24
22 LOADBOOL 3 0 1
23 LOADBOOL 3 1 0
24 CALL 0 4 1
25 RETURN 0 1 ; bug! incorrect result: 7
--]]

It looks to me as if the JMPs in lines 18 and 19 each overshoot by 1.
I am also puzzled as to why line 20 is unreachable. Surely, without
optimizations, every alternative should appear as a possible path of
execution. I also do not understand how line 22 can get there,
seeing that "false" does not appear in the source text.

The bug appears to start at line 18. I would have expected

18 JMP 6 ; to 25
19 LOADBOOL 3 1 0
20 TEST 3 0 1
21 JMP 2 ; to 24
22 LOADK 3 -6 ; 8
23 JMP 1 ; to 25
24 LOADBOOL 3 1 0
25 CALL 0 4 1
26 RETURN 0 1

but maybe I have not understood the code-planting functions properly.

Somehow lcode.c needs to use the reduction

(anything or true) and expr --> expr

and, dually,

(anything and false|nil) or expr --> expr

so that lines like 5,6,7 could be omitted.

--
Gavin Wraith (gavin@wra1th.plus.com)
Home page: http://www.wra1th.plus.com/