> > The only time the switch makes a difference is when the program is already > incorrect. I really doubt Arch is going to enable a flag slowing down all > programs to make invalid programs behave > *differently* (not necessary as they were intended to behave, just > *differently*). > > GCC is correctly noticing a situation that would result in undefined > behaviour, > and optimizing based on the assumption that it never happens. The solution > is to write valid code not relying on undefined behaviour - accessing beyond > the end of an array is undefined behaviour regardless of whether there's > more allocated memory. > > C99 has this feature as a flexible-length array member using `foo array[];` > and > that might be valid C++11 but I'm not sure and I don't feel like digging > through the standard. Using `foo array[0]` will also work because it's a GNU > extension, but keep in mind that you've left the land of standard C++. > > Compilers are going to get better and better at optimizing away code that's > not needed if the program is assumed to be correct. I recommend using > another language if you don't want to worry about incorrect code that seems > to work now breaking from future optimizations.
I hear you. I, however, disagree when you qualify the old C trick pattern as incorrect. My A B toy structs are a simplification of what I have seen in the AMD ADL SDK and was causing the loop problem with: http://code.google.com/p/overdrive5/ What is debatable, IMO, is how widespread the usage of this particular pattern is. Personally, I am not using it but I knew it for having seen it in the past a couple of times. My expectations of compiler optimization is that they should be transparent and respect the intent of the program. This optimization switch must probably be doing a lot of good stuff but this particular manifestation is, IMO, a stunning reinterpretation of an explicit request to iterate n times. ________________________________ CONFIDENTIALITY : This e-mail and any attachments are confidential and may be privileged. If you are not a named recipient, please notify the sender immediately and do not disclose the contents to another person, use it for any purpose or store or copy the information in any medium.
