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DEAD code

tim stockton

highlighted code

code is being generated by the compiler for all switch statements when the level of optimization is lower than Level 4, that DOES NOT EXECUTE. i'm not concerned with eliminating it, which is obviously accomplished by increasing the level of optimization, but i need to know why it's there??????

eg. This is an example of the disassembly associated with a switch statement when optimization set at Level 1...

switch (x)
C:0x0013 E508 MOV A,0x08
C:0x0015 120039 LCALL C?CCASE(C:0039)
C:0x0018 00 NOP
C:0x0019 36 ADDC A,@R0
C:0x001A 00 NOP
C:0x001B 00 NOP
C:0x001C 36 ADDC A,@R0
C:0x001D 00 NOP
C:0x001F 00 NOP
C:0x0021 22 RET
{
case 0:
do something
case 1:
do something

.....etc.

default:

}
the code lines C:0x0018 - C:0x0021 never execute. the call to library function C?CCASE determines where to jump and these lines are always skipped. why are they inserted by the compiler????

tim

  • 0

    It is quite common among compilers for the "basic" code generator to just produce code with no thought for efficiency; it is left to the optimiser so sort out the clever stuff - including dead-code removal.

  • 0 in reply to Andrew Neil

    irony Level 1 is dead code elimination....

    this thing inserted dead code when generating every switch statement. does anyone have a better explanation than "compilers produce dead code". i have to account for all unexecuted instructions...

  • 0

    Actually, that is NOT dead code at all. But, it takes a complete example to see the MAGIC behind what's going on. For example, the following function:

    int func (unsigned char x)
{
switch (x)
  {
  case 0:
    return (1);
  case 1:
    return (1);
  case 2:
    return (3);
  case 6:
    return (3);
  case 9:
    return (3);
  case 12:
    return (3);
  case 16:
    return (3);
  case 27:
    return (3);
  case 28:
    return (3);
  case 39:
    return (3);
  case 45:
    return (3);
  default:
    return (5);
  }
}

    Generates the following code when compiled:

                 ; FUNCTION _func (BEGIN)
                                           ; SOURCE LINE # 1
;---- Variable 'x' assigned to Register 'R7' ----
                                           ; SOURCE LINE # 2
                                           ; SOURCE LINE # 3
0000 EF                MOV     A,R7
0001 120000      E     LCALL   ?C?CCASE
0004 0000        R     DW      ?C0002
0006 00                DB      00H
0007 0000        R     DW      ?C0004
0009 01                DB      01H
000A 0000        R     DW      ?C0012
000C 02                DB      02H
000D 0000        R     DW      ?C0012
000F 06                DB      06H
0010 0000        R     DW      ?C0012
0012 09                DB      09H
0013 0000        R     DW      ?C0012
0015 0C                DB      0CH
0016 0000        R     DW      ?C0012
0018 10                DB      010H
0019 0000        R     DW      ?C0012
001B 1B                DB      01BH
001C 0000        R     DW      ?C0012
001E 1C                DB      01CH
001F 0000        R     DW      ?C0012
0021 27                DB      027H
0022 0000        R     DW      ?C0012
0024 2D                DB      02DH
0025 0000              DW      00H
0027 0000        R     DW      ?C0014
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 5
0029         ?C0002:
                                           ; SOURCE LINE # 6
0029 8000              SJMP    ?C0015
                                           ; SOURCE LINE # 7
002B         ?C0004:
                                           ; SOURCE LINE # 8
002B         ?C0015:
002B 7E00              MOV     R6,#00H
002D 7F01              MOV     R7,#01H
002F 22                RET     
                                           ; SOURCE LINE # 9
0030         ?C0005:
                                           ; SOURCE LINE # 10
0030 800E              SJMP    ?C0023
                                           ; SOURCE LINE # 11
0032         ?C0006:
                                           ; SOURCE LINE # 12
0032         ?C0016:
0032 800C              SJMP    ?C0023
                                           ; SOURCE LINE # 13
0034         ?C0007:
                                           ; SOURCE LINE # 14
0034         ?C0017:
0034 800A              SJMP    ?C0023
                                           ; SOURCE LINE # 15
0036         ?C0008:
                                           ; SOURCE LINE # 16
0036         ?C0018:
0036 8008              SJMP    ?C0023
                                           ; SOURCE LINE # 17
0038         ?C0009:
                                           ; SOURCE LINE # 18
0038         ?C0019:
0038 8006              SJMP    ?C0023
                                           ; SOURCE LINE # 19
003A         ?C0010:
                                           ; SOURCE LINE # 20
003A         ?C0020:
003A 8004              SJMP    ?C0023
                                           ; SOURCE LINE # 21
003C         ?C0011:
                                           ; SOURCE LINE # 22
003C         ?C0021:
003C 8002              SJMP    ?C0023
                                           ; SOURCE LINE # 23
003E         ?C0012:
                                           ; SOURCE LINE # 24
003E         ?C0022:
003E 8000              SJMP    ?C0023
                                           ; SOURCE LINE # 25
0040         ?C0013:
                                           ; SOURCE LINE # 26
0040         ?C0023:
0040 7E00              MOV     R6,#00H
0042 7F03              MOV     R7,#03H
0044 22                RET     
                                           ; SOURCE LINE # 27
0045         ?C0014:
                                           ; SOURCE LINE # 28
0045 7E00              MOV     R6,#00H
0047 7F05              MOV     R7,#05H
                                           ; SOURCE LINE # 29
                                           ; SOURCE LINE # 30
0049         ?C0003:
0049 22                RET     
             ; FUNCTION _func (END)

    As you'll notice, the DW and DB following the call to the ?C?CCASE routine are defines for the address of the case and the number of the case. So, for example:

    0004 0000        R     DW      ?C0002
0006 00                DB      00H

    defines that ?C0002 is the address of the code to execute for case 00H. And,

    0007 0000        R     DW      ?C0004
0009 01                DB      01H

    defines ?C0004 is the address of the code to execute for case 01H.

    The ?C?CCASE routine uses this jump table to get to the code to execute for each case statement.

    Jon

  • 0 in reply to Jon Ward

    this is plainly obvious when the file is translated using the #pragma src directive.

    thank you jon