I was told that running programs generate probability data used to optimize repeated instructions.
For example, if an “if-then-else” control structure has been evaluated TRUE 8/10 times, then the next time the “if-then-else” statement is being evaluated, there is an 80% chance the condition will be TRUE. This statistic is used to prompt hardware to load the appropriate data into the registers assuming the outcome will be TRUE. The intent is to speed up the process. If the statement does evaluate to TRUE, data is already loaded to the appropriate registers. If the statement evaluates to FALSE, then the other data is loaded in and simply written over what was decided “more likely”.
I have a hard time understanding how the probability calculations don’t out-weigh the performance cost of decisions it’s trying to improve. Is this something that really happens? Does it happen at a hardware level? Is there a name for this?
I can seem to find any information about the topic.
I was told that running programs generate probability data used to optimize repeated instructions.
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This is done. It’s called branch prediction. The cost is non-trivial, but it’s handled by dedicated hardware, so the cost is almost entirely in terms of extra circuitry — it doesn’t affect the time taken to execute the code.
That means the real cost would be one of lost opportunity — i.e., if there was some other way of designing a CPU that used that amount of circuitry for some other purpose and gained more from doing so. My immediate guess is that the answer is usually no — branch prediction is generally quite effective in terms of return on investment.