Agner`s CPU blog

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More Parallels
Author: Agner Fog Date: 2010-01-23 02:28
I have always thought that it is bad programming practice to make software that relies on specific CPU brands and models. Any program that makes assumptions based on the CPU model number is likely to be obsolete as soon as a new processor appears on the market.

Unfortunately, this practice is more widespread than I thought. Here are some examples:

  1. According to CNET news, Skype has made a deal with Intel to limit the functionality on non-Intel computers, alleging that contemporary AMD processors were not powerful enough. See CNET News and Slashdot News. This limitation has later been removed.
     
  2. 64-bit Windows will only run on processors with known vendor names. This is a serious barrier for new companies to enter the x86 processor market and it is an obstackle to emulation. Same problem in FreeBSD. In fact, VIA had to make a feature to change the vendor string in their processors in order to run 64-bit Windows. See analysis by Geoff Chappell. Incidentally, this feature made it possible for me to make my CPUID manipulation program.
     
  3. An Intel employee is making a separate branch for the Intel Atom processor in a certain function in the Gnu Libc library (see libc-alpha archive). Intel also have a separate branch for the Atom processor in their IPP library. If this practice spreads then the libraries will soon be bloated with many separate branches for different brands and models of microprocessors. In my opinion, the branching should be based on certain performance characteristics, not on specific processor models. If necessary, add CPUID bits for specific performance parameters, or let the software test which branch is fastest.
     
Thank you to Yuhong Bao for sending me the first two examples.
 
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