Hi,

there has been a open problem with all GCC 3.x on Linux x86 for quite a
long time. If you run configure the double order is not determined
correctly. So you still have to set

#define DOUBLE_WORD_ORDER 1

manually in the automatically created config.h. Otherwise Squeak will
produce later a lot of float errors and break down quickly.

Responsible for the detection of the double order is the
AC_C_DOUBLE_ORDER macro used in the configure script. This macro is
defined in acinclude.m4. It tries to detect the double order using the
following test program:

main()
      {
      double d = 1.0;
      return *(int*)&d == 0;
      }

If you compile it with "gcc -g -O2" as it is normal within configure
scripts on Linux, the program does not yield the desired results when
using GCC 3.x. This is because of the "-O2" optimizing option, which
seems to have a different behavior since GCC 3.0. The reason seems to be
a mix of options that -O2 stands for. If you, for example, provide
"-fno-strict-aliasing" or "-fno-schedule-insns2" in addition to "-O2",
then the result is the originally intended. But only providing the two
complementary options without "-O2" is not enough to reproduce the
strange behavior that "-O2" alone leads to.

However, the problem follows from the double*-to-int*-cast, because a
pointer to an 8-byte-value is cast to a 4-byte-value. Such a cast has
actually no defined behavior. So the result can change at different
optimization levels.

By means of an union struct one can perform the double order test in a
consistent way. You just have to replace the test program from above by
the following in the AC_C_DOUBLE_ORDER macro definition in acinclude.m4:


main()
      {
      union
           {
           double d;
           int i;
           } u;
      u.d = 1.0;
      return u.i == 0;
      }


I have tested it and it seems to work fine on Linux 2.4.20 and Solaris
2.8 with GCC 2.95.3 and GCC 3.2. Does it work generally or could there
appear any problems? If not, it would be nice, if Ian could incorporate
this change into the current UNIX Squeak source distros.

Lars