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Mail Archives: djgpp/1996/10/21/05:46:12

Date: Mon, 21 Oct 1996 11:24:34 GMT
From: kvhk AT ivs4 DOT barco DOT com (Koen Van Herck)
Message-Id: <9610211124.AA19264@ivs4.>
To: bug-gcc AT prep DOT ai DOT mit DOT edu
Subject: Optimization bug in gcc2.7.2.1 ?
Cc: djgpp AT delorie DOT com

Hello,

I think I have found a bug in gcc 2.7.2.1. The simple program generates
incorrect output when it is compiled with -O2. 

I have reported this bug for gcc 2.7.2, but I was told it would be
fixed in gcc 2.7.2.1. As I have just tested, this seems not true.
I have used gcc 2.7.2.1 which comes with djgpp v2.1.

I have found that the bug did not appear with gcc 2.6.3 (DJGPP
V1.?)

I have included the assembler listings. It seems that the error is
caused by the assignment at 0x0070 in bug.lis.

When comparing the two listings (with -O1 and -O2), it seems that the
-O1 version is more efficient than the -O2. Maybe this is also a
bug. -O2 should be better (or at least as good) as -O1, shouldn't it ?
But since I'm not a compiler or assembler specialist, I could be
completely wrong in this. I would be very glad hearing your opinion,
though. 

If you need more information, please ask.


Best regards,

Koen Van Herck.



System description
------------------
PC running MS-DOS 6.22 & 4dos 5.0
Intel 486DX2-66

DJGPP V2.1, which includes
GCC 2.7.2.1

Test program - bug.c
--------------------
#include <stdio.h>

void main(void)
{
    static int i, j, k, l, m;

    i = 12;
    j = 34;
    k = 56;
    l = 78;
    
    printf("j=%d\n", j);
    
    m = i * j % 179 * k % 179;
    i = j;
    j = k;
    
    printf("&i=%x\n", (void *)&i); 
    k = i;
    
    printf("k=%d\n", k);
}

Description of the bug
----------------------
> gcc -v
Reading specs from d:/djgpp/lib\specs
gcc version 2.7.2.1

> gcc -O2 -g -Wa,-alhdn -o bug bug.c > bug.lis
> gcc -O1 -g -Wa,-alhdn -o nobug bug.c > nobug.lis

> nobug
j=34
&i=8810
k=34

This is the expected output. (k==j==34)

> bug
j=34
&i=8810
k=56

This is an error. (k==56 != j==34)

Here follow the 2 generated assembler listings:

Assembler output nobug.lis
--------------------------
   1              		.file	"bug.c"
   2              	gcc2_compiled.:
   3              	___gnu_compiled_c:
   4              	.text
  18              	.lcomm _i.2,4
  19              	.lcomm _j.3,4
  20              	.lcomm _k.4,4
  21              	.lcomm _l.5,4
  22              	.lcomm _m.6,4
  23              	LC0:
  24 0000 6A3D2564 		.ascii "j=%d\12\0"
  24      0A00
  25              	LC1:
  26 0006 26693D25 		.ascii "&i=%x\12\0"
  26      780A00
  27              	LC2:
  28 000d 6B3D2564 		.ascii "k=%d\12\0"
  28      0A00
  29 0013 90       		.align 2
  31              	.globl _main
  32              	_main:
  34 0014 55       		pushl %ebp
  35 0015 89E5     		movl %esp,%ebp
  36 0017 E8E4FFFF 		call ___main
  36      FF
   1:bug.c         **** #include <stdio.h>
   2:bug.c         **** 
   3:bug.c         **** void main(void)
   4:bug.c         **** {
   5:bug.c         ****     static int i, j, k, l, m;
   6:bug.c         **** 
   7:bug.c         ****     i = 12;
  44 001c C705B000 		movl $12,_i.2
  44      00000C00 
  44      0000
   8:bug.c         ****     j = 34;
  46 0026 C705B400 		movl $34,_j.3
  46      00002200 
  46      0000
   9:bug.c         ****     k = 56;
  48 0030 C705B800 		movl $56,_k.4
  48      00003800 
  48      0000
  10:bug.c         ****     l = 78;
  50 003a C705BC00 		movl $78,_l.5
  50      00004E00 
  50      0000
  11:bug.c         ****     
  12:bug.c         ****     printf("j=%d\n", j);
  52 0044 6A22     		pushl $34
  53 0046 68000000 		pushl $LC0
  53      00
  54 004b E8B0FFFF 		call _printf
  54      FF
  13:bug.c         ****     
  14:bug.c         ****     m = i * j % 179 * k % 179;
  56 0050 A1B00000 		movl _i.2,%eax
  56      00
  57 0055 0FAF05B4 		imull _j.3,%eax
  57      000000
  58 005c B9B30000 		movl $179,%ecx
  58      00
  59 0061 99       		cltd
  60 0062 F7F9     		idivl %ecx
  61 0064 89D0     		movl %edx,%eax
  62 0066 0FAF05B8 		imull _k.4,%eax
  62      000000
  63 006d 99       		cltd
  64 006e F7F9     		idivl %ecx
  65 0070 8915C000 		movl %edx,_m.6
  65      0000
  15:bug.c         ****     i = j;
  67 0076 A1B40000 		movl _j.3,%eax
  67      00
  68 007b A3B00000 		movl %eax,_i.2
  68      00
  16:bug.c         ****     j = k;
  70 0080 A1B80000 		movl _k.4,%eax
  70      00
  71 0085 A3B40000 		movl %eax,_j.3
  71      00
  17:bug.c         ****     
  18:bug.c         ****     printf("&i=%x\n", (void *)&i); 
  73 008a 68B00000 		pushl $_i.2
  73      00
  74 008f 68060000 		pushl $LC1
  74      00
  75 0094 E867FFFF 		call _printf
  75      FF
  19:bug.c         ****     k = i;
  77 0099 A1B00000 		movl _i.2,%eax
  77      00
  78 009e A3B80000 		movl %eax,_k.4
  78      00
  20:bug.c         ****     
  21:bug.c         ****     printf("k=%d\n", k);
  80 00a3 50       		pushl %eax
  81 00a4 680D0000 		pushl $LC2
  81      00
  82 00a9 E852FFFF 		call _printf
  82      FF
  22:bug.c         **** }
  84 00ae C9       		leave
  85 00af C3       		ret


Assembler output bug.lis
------------------------
   1              		.file	"bug.c"
   2              	gcc2_compiled.:
   3              	___gnu_compiled_c:
   4              	.text
  18              	.lcomm _i.2,4
  19              	.lcomm _j.3,4
  20              	.lcomm _k.4,4
  21              	.lcomm _l.5,4
  22              	.lcomm _m.6,4
  23              	LC0:
  24 0000 6A3D2564 		.ascii "j=%d\12\0"
  24      0A00
  25              	LC1:
  26 0006 26693D25 		.ascii "&i=%x\12\0"
  26      780A00
  27              	LC2:
  28 000d 6B3D2564 		.ascii "k=%d\12\0"
  28      0A00
  29 0013 90       		.align 2
  31              	.globl _main
  32              	_main:
  34 0014 55       		pushl %ebp
  35 0015 89E5     		movl %esp,%ebp
  36 0017 56       		pushl %esi
  37 0018 53       		pushl %ebx
  38 0019 E8E2FFFF 		call ___main
  38      FF
   1:bug.c         **** #include <stdio.h>
   2:bug.c         **** 
   3:bug.c         **** void main(void)
   4:bug.c         **** {
   5:bug.c         ****     static int i, j, k, l, m;
   6:bug.c         **** 
   7:bug.c         ****     i = 12;
  46 001e C705BC00 		movl $12,_i.2
  46      00000C00 
  46      0000
   8:bug.c         ****     j = 34;
  48 0028 C705C000 		movl $34,_j.3
  48      00002200 
  48      0000
   9:bug.c         ****     k = 56;
  50 0032 C705C400 		movl $56,_k.4
  50      00003800 
  50      0000
  10:bug.c         ****     l = 78;
  52 003c C705C800 		movl $78,_l.5
  52      00004E00 
  52      0000
  11:bug.c         ****     
  12:bug.c         ****     printf("j=%d\n", j);
  54 0046 6A22     		pushl $34
  55 0048 68000000 		pushl $LC0
  55      00
  56 004d E8AEFFFF 		call _printf
  56      FF
  13:bug.c         ****     
  14:bug.c         ****     m = i * j % 179 * k % 179;
  58 0052 8B0DC000 		movl _j.3,%ecx
  58      0000
  59 0058 0FAF0DBC 		imull _i.2,%ecx
  59      000000
  60 005f 89C8     		movl %ecx,%eax
  61 0061 BBB30000 		movl $179,%ebx
  61      00
  62 0066 99       		cltd
  63 0067 F7FB     		idivl %ebx
  64 0069 89D1     		movl %edx,%ecx
  65 006b A1C40000 		movl _k.4,%eax
  65      00
  66 0070 A3C00000 		movl %eax,_j.3
  66      00
  67 0075 0FAFC8   		imull %eax,%ecx
  68 0078 89C8     		movl %ecx,%eax
  69 007a 99       		cltd
  70 007b F7FB     		idivl %ebx
  71 007d 89D6     		movl %edx,%esi
  72 007f 8935CC00 		movl %esi,_m.6
  72      0000
  15:bug.c         ****     i = j;
  74 0085 8B1DC000 		movl _j.3,%ebx
  74      0000
  75 008b 891DBC00 		movl %ebx,_i.2
  75      0000
  16:bug.c         ****     j = k;
  17:bug.c         ****     
  18:bug.c         ****     printf("&i=%x\n", (void *)&i); 
  77 0091 68BC0000 		pushl $_i.2
  77      00
  78 0096 68060000 		pushl $LC1
  78      00
  79 009b E860FFFF 		call _printf
  79      FF
  19:bug.c         ****     k = i;
  81 00a0 A1BC0000 		movl _i.2,%eax
  81      00
  82 00a5 A3C40000 		movl %eax,_k.4
  82      00
  20:bug.c         ****     
  21:bug.c         ****     printf("k=%d\n", k);
  84 00aa 50       		pushl %eax
  85 00ab 680D0000 		pushl $LC2
  85      00
  86 00b0 E84BFFFF 		call _printf
  86      FF
  22:bug.c         **** }
  89 00b5 8D65F8   		leal -8(%ebp),%esp
  90 00b8 5B       		popl %ebx
  91 00b9 5E       		popl %esi
  92 00ba C9       		leave
  93 00bb C3       		ret



----
Koen Van Herck
Electronic Design Engineer
E-mail: Koen DOT VanHerck AT barco DOT com

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