Re: Brain-scratcher: New example of the delay
- Posted by bug Jun 15, 2009
- 1044 views
See http://en.wikipedia.org/wiki/Self-modifying_code in particular the last sentence of this:
Interaction of cache and self-modifying code
In some cases short sections of self-modifying code executes more slowly on modern processors. This is because a modern processor will usually try to keep blocks of code in its cache memory. Each time the program rewrites a part of itself, the rewritten part must be loaded into the cache again, which results in a slight delay, if the modified codelet shares the same cache line with the modifying code, as is the case when the modified memory address is located within a few bytes to the one of the modifying code.
The cache invalidation issue on modern processors usually means that self-modifying code would still be faster only when the modification will occur rarely, such as in the case of a state switching inside an inner loop.
Most modern processors load the machine code before they execute it, which means that if an instruction that is too near the instruction pointer is modified, the processor will not notice, but instead execute the code as it was before it was modified. See Prefetch Input Queue (PIQ). PC processors have to handle self-modifying code correctly for backwards compatibility reasons but they are far from efficient at doing so[citation needed].
