the code below behaves differently at -O1 and -O2.
#include <stdio.h>
static void swapcopy_int( int * target, char * source, int num_bytes )
{
int * to = target ;
int * from = (int *) source ;
for(int count=0; count < num_bytes/sizeof(int); count++ ){
int t2 ;
int t1 = *from++ ;
*(((char *)&t2)+3) = *(((char *)&t1) ) ;
*(((char *)&t2)+2) = *(((char *)&t1)+1) ;
*(((char *)&t2)+1) = *(((char *)&t1)+2) ;
*(((char *)&t2) ) = *(((char *)&t1)+3) ;
*to++ = t2 ;
}
}
int main()
{
int num_rows = 0;
int num_cols = 0;
num_rows = 0x10000000;
num_cols = 0x20000000;
swapcopy_int ( &num_rows, (char *)&num_rows, sizeof( int ) );
swapcopy_int ( &num_cols, (char *)&num_cols, sizeof( int ) );
printf (" rows = %d cols = %d \n", num_rows, num_cols );
return 0;
}
$ g++ -O1 xx.c
$ ./a.out
the target is 0xbf8e3acc (10000000) the source is 0xbf8e3acc (10000000) bytes =
4
the target is 0xbf8e3ac8 (20000000) the source is 0xbf8e3ac8 (20000000) bytes =
4
rows = -1740992833 cols = -1740992833
^
These are wrong !!!!
$ g++ -O2 xx.c
$ ./a.out
the target is 0xbfe7985c (10000000) the source is 0xbfe7985c (10000000) bytes =
4
the target is 0xbfe79858 (20000000) the source is 0xbfe79858 (20000000) bytes =
4
rows = 16 cols = 32
^
These are right
$ g++ --version
g++ (GCC) 4.4.1
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
--
Summary: Code does not work at -O1
Product: gcc
Version: 4.4.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: bdavis at gcc dot gnu dot org
GCC host triplet: i686/linux/gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43240
