Nathan Summers writes: > On Thu, Aug 6, 2009 at 3:00 PM, Nicolas Robidoux wrote > > > > Is the use of such tricks discouraged for GEGL (because gfloats could > > be doubles at some point?) > > > > We can take the absolute value by setting the sign bit to zero: > > // Example 13.24 > > float f; > > *(int*)&f &= 0x7FFFFFFF; // set sign bit to zero > > Doesn't this violate pointer aliasing rules? Indeed. Consequence of your observation: The next edition of Agner Fog's online optimization manuals http://www.agner.org/optimize/#manuals will contain an updated version of the above example: union { float f; int i; } u; u.i &= 0x7FFFFFFF; // set sign bit to zero and the following caveat: In general, it is faster to access a floating point variable as an integer if it is stored in memory, but not if it is a register variable. The union forces the variable to be stored in memory, at least temporarily. Using the methods in the above examples will therefore be a disadvantage if other nearby parts of the code could benefit from using registers for the same variables. Using type casting of pointers instead of unions in the above examples may not work on compilers that rely on the strict aliasing rule of standard C, specifying that pointers of different types cannot point to the same object, except for char pointers. Nicolas Robidoux Laurentian University _______________________________________________ Gegl-developer mailing list Gegl-developer@xxxxxxxxxxxxxxxxxxxxxx https://lists.XCF.Berkeley.EDU/mailman/listinfo/gegl-developer
