[Bug c/43774] New: option -O2 generates wrong assembly code

[Bug c/43774] New: option -O2 generates wrong assembly code

[dirtysalt1987 at gmail dot com]

compiling the following code with -O2, the program will core dump.
I check the assembly code output, it seems the 'strlen' function call is
replaced by the 'builtin strlen' funciton and will read the first four byte on
a invalid memory page.
And if i replace the mmap with malloc and run under the Valgrind[3.5.0],the
Valgrind also reports 'Invalid read of size 4'.

Ps:How to workaround this piece of code???I think there are two ways
a.mmap 4 bytes or more to make sure the strlen will not read the invalid memory
b.use the gcc option '-fno-builtin-strlen' to make sure the 'strlen' is not
replaced.
But I'm not sure is there a more elegant way to workaroud this??

====================================================================
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/mman.h>
typedef struct _x_t
{
    int offset; //'strlen' is replaced iff. this field exists.
    char data[0];
}x_t;

int main()
{   
    //make a 4K memory page.
    char *buff=mmap(NULL,4096,PROT_WRITE | PROT_READ ,MAP_PRIVATE |
MAP_ANONYMOUS,0,0);
    char *buffer = buff+4096-11;
    strcpy(buffer,"0123456789");  
    x_t *x=(x_t*)buffer;
    printf("%d\n",strlen(x->data)); //read a invalid page.
    munmap(buff,4096);
    return 0;
}


-- 
           Summary: option -O2 generates wrong assembly code
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dirtysalt1987 at gmail dot com
 GCC build triplet: GNU/linux 2.6.9 Intel Xeon
  GCC host triplet: GNU/linux 2.6.9 Intel Xeon
GCC target triplet: GNU/linux 2.6.9 Intel Xeon


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774

[Bug c/43774] option -O2 generates wrong assembly code

[jakub at gcc dot gnu dot org]

------- Comment #1 from jakub at gcc dot gnu dot org  2010-04-17 06:55 -------
You should make the struct packed, because otherwise you are accessing it
unaligned.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774

[Bug c/43774] option -O2 generates wrong assembly code

[dirtysalt1987 at gmail dot com]

------- Comment #2 from dirtysalt1987 at gmail dot com  2010-04-17 08:11 -------
(In reply to comment #1)
> You should make the struct packed, because otherwise you are accessing it
> unaligned.
>

I think the main problem coming from that GCC replace the 'strlen' function
with the following code.

.L2:
        movl    (%ecx), %eax //$ecx is the beginning of string..
        addl    $4, %ecx //seems to access the content in 4 bytes each time.
        leal    -16843009(%eax), %edx
        notl    %eax
        andl    %eax, %edx
        andl    $-2139062144, %edx
        je      .L2

But it seems the piece of code can only be used when the valid memory size is
4*N,otherwise it will make a 'Invalid read'...

So under what sitaution, GCC will replace 'strlen' call with the above code. 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774

[Bug c/43774] option -O2 generates wrong assembly code

[dirtysalt1987 at gmail dot com]

------- Comment #3 from dirtysalt1987 at gmail dot com  2010-04-17 09:45 -------
(In reply to comment #1)
> You should make the struct packed, because otherwise you are accessing it
> unaligned.
> 

If I recompile it with -O0, the assembly code call 'strlen' function.And I use
Valgrind to run it,there is no 'Invalid read'


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774

[Bug c/43774] option -O2 generates wrong assembly code

[schwab at linux-m68k dot org]

------- Comment #4 from schwab at linux-m68k dot org  2010-04-17 09:49 -------
The compiler can assume that *x is correctly aligned, so this is not a bug.


-- 

schwab at linux-m68k dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774

[Bug c/43774] option -O2 generates wrong assembly code

[dirtysalt1987 at gmail dot com]

------- Comment #5 from dirtysalt1987 at gmail dot com  2010-04-17 10:28 -------
(In reply to comment #1)
> You should make the struct packed, because otherwise you are accessing it
> unaligned.
> 

3ku for your reply.Your reply is much more useful than Axxx's reply.3ku very
much:).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774

[Bug c/43774] option -O2 generates wrong assembly code

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2010-04-19 01:08 -------
Just a littke more: alignof (struct _x_t) is 4 therefor so is the alignof(_x_t
::data).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774

[Bug c/43774] option -O2 generates wrong assembly code

[dirtysalt1987 at gmail dot com]

------- Comment #7 from dirtysalt1987 at gmail dot com  2010-04-19 02:25 -------
(In reply to comment #6)
> Just a littke more: alignof (struct _x_t) is 4 therefor so is the alignof(_x_t
> ::data).
> 
3ku very much,with your explanation I think I can understand it..3ku very much.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43774