Optimization Problem With Data Conversion

[Ian Lance Taylor]

Raymond Lu <songyulu@hdfgroup.org> writes:

> Our HDF5 software has been having some data conversion problem with
> GCC's optimization for a few years.   One example is to convert data 
>>From short to int.  You can find the program at
>
>  ftp://ftp.hdfgroup.uiuc.edu/pub/outgoing/slu/tmp/ctest.c
>
> When I use "gcc -O2" or "gcc -O3" to compile it, I get wrong values
> after the conversion.  When I use "gcc -O0" or "gcc -O1", the values
> are correct.

          /* This line causes the trouble. */
	  *((int*)d) = (int)(*((short*)s));

This is an aliasing violation.  Read the documentation for the
-fstrict-aliasing option.

Ian