Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!bfmny0!tneff
From: tneff@bfmny0.UUCP (Tom Neff)
Newsgroups: comp.sys.intel
Subject: Re: PLM vs. C for 80286/80386
Keywords: PLM C
Message-ID: <14386@bfmny0.UUCP>
Date: 7 Jun 89 15:28:45 GMT
References: <598@philtis.UUCP> <14381@bfmny0.UUCP> <1765@auspex.auspex.com>
Reply-To: tneff@bfmny0.UUCP (Tom Neff)
Organization: ^
Lines: 104

In article <1765@auspex.auspex.com> guy@auspex.auspex.com (Guy Harris) writes:
> [ I had said: ]
>>For instance if you want a CASE driven state machine in a critical
>>interrupt handler, PL/M implements DO CASE as a jump indirect indexed
>>off a short array of offsets, e.g. JMP CASETBL[BX]. C on the other hand
>>does 'switch' as a full if-then/elseif-then/elseif-then... series.
>
>I sincerely hope Intel's C compiler does not do so for *all* "switch"
>statements; any competent C compiler can and will turn a "dense"
>"switch" statement - i.e., one where the cases form a reasonably tight
>range - into an indexded jump.  (The definition of "dense" varies from
>compiler to compiler.)

Not wanting to make rash statements, I retested it.  I'm right.  Here's
the proof (about 70 lines so hit N if you believe me <grin>):

iC-86  COMPILER   JUNK                              05/27/89 11:23:32  PAGE   1

DOS 3.30 (038-N) iC-86 COMPILER V4.0, COMPILATION OF MODULE JUNK
OBJECT MODULE PLACED IN JUNK.OBJ
COMPILER INVOKED BY: Z:\L86\IC86.EXE JUNK.C OPTIMIZE(3) CODE PAGEWIDTH(80)

 line level  incl

    1             main()
    2             {
    3     1           int x;
    4     1
    5     1           switch (x) {
    6     2               case 0: foo(x);
    7     2                       break;
    8     2               case 1: fee(x);
    9     2                       break;
   10     2               case 2: fie(x);
   11     2                       break;
   12     2           }
   13     1       }

iC-86  COMPILER   JUNK                              05/27/89 11:23:32  PAGE   2
                  ASSEMBLY LISTING OF OBJECT CODE

                                                   ; STATEMENT # 2
                           main      PROC NEAR
             0000  56              PUSH    SI
             0001  57              PUSH    DI
             0002  55              PUSH    BP
             0003  8BEC            MOV     BP,SP
             0005  51              PUSH    CX
                                                   ; STATEMENT # 5
             0006  8B5EFE          MOV     BX,[BP].x
             0009  EB19            JMP     @3
                           @4:
                                                   ; STATEMENT # 6
             000B  FF76FE          PUSH    [BP].x  ; 1
             000E  E80000          CALL    foo
             0011  EB0E            JMP     @7
                                                   ; STATEMENT # 7
                           @5:
                                                   ; STATEMENT # 8
             0013  FF76FE          PUSH    [BP].x  ; 1
             0016  E80000          CALL    fee
             0019  EB06            JMP     @7
                                                   ; STATEMENT # 9
                           @6:
                                                   ; STATEMENT # 10
             001B  FF76FE          PUSH    [BP].x  ; 1
             001E  E80000          CALL    fie
                           @7:
             0021  59              POP     CX      ; 1
                                                   ; STATEMENT # 11
             0022  EB0F            JMP     @2
                                                   ; STATEMENT # 12
                           @3:
             0024  83FB00          CMP     BX,0H
             0027  74E2            JZ      @4
             0029  83FB01          CMP     BX,1H
             002C  74E5            JZ      @5
             002E  83FB02          CMP     BX,2H
             0031  74E8            JZ      @6
                           @2:
                                                   ; STATEMENT # 13
             0033  8BE5            MOV     SP,BP
             0035  5D              POP     BP
             0036  5F              POP     DI
             0037  5E              POP     SI
             0038  C3              RET
                           main      ENDP

MODULE INFORMATION:

     CODE AREA SIZE               = 0039H     57D
     CONSTANT AREA SIZE           = 0000H      0D
     DATA AREA SIZE               = 0000H      0D
     MAXIMUM STACK SIZE           = 000CH     12D

iC-86  COMPILER   JUNK                              05/27/89 11:23:32  PAGE   3
                  ASSEMBLY LISTING OF OBJECT CODE

iC-86 COMPILATION COMPLETE.      0 WARNINGS,     0 ERRORS
-- 
You may redistribute this article only to those who may freely do likewise.
--
Tom Neff				UUCP:     ...!uunet!bfmny0!tneff
    "Truisms aren't everything."	Internet: tneff@bfmny0.UU.NET