comparision and addition of two integers : in detail

I would like to know how many machine cycles does it take to compare two integers and 开发者_如何学Pythonhow many if I add that and which one is easier?

basically i m looking to see which one is more expensive generally ?? Also I need an answer from c, c++, java perspective ....

helps is appreciated thanks!!

The answer is yes. And no. And maybe.

There are machines that can compare two values in their spare time between cycles, and others that need several cycles. On the old PDP8 you first had to negate one operand, do an add, and then test the result to do a compare.

But other machines can compare much faster than add, because no register needs to be modified.

But on still other machines the operations take the same time, but it takes several cycles for the result of the compare to make it to a place where one can test it, so, if you can use those cycles the compare is almost free, but fairly expensive if you have no other operations to shove into those cycles.

The simple answer is one cycle, both operations are equally easy.

A totally generic answer is difficult to give, since processor architectures are amazingly complex when you get down into the details.

All modern processors are pipelined. That is, there are no instructions where the operands go in on cycle c, and the result is available on cycle c+1. Instead, the instruction is broken down into multiple steps.

The instructions are read into the front end of the processor, which decodes the instruction. This may include breaking it down into multiple micro-ops. The operands are then read into registers, and then the execution units handle the actual operation. Eventually the answer is returned back to a register.

The instructions go through one pipeline stage each cycle, and modern CPUs have 10-20 pipeline stages. So it could be upto 20 processor cycles to add or compare two numbers. However, once one instruction has been through one stage of the pipeline, another instruction can be read into that stage. The ideal is that each clock cycle, one instruction goes into the front end, while one set of results comes out the other.

There is massive complexity involved in getting all this to work. If you want to do a+b+c, you need to add a+b before you can add c. So a lot of the work in the front end of the processor involves scheduling. Modern processors employ out-of-order execution, so that the processor will examine the incoming instructions, and re-order them such that it does a+b, then gets on with some other work, and then does result+c once the result is available.

Which all brings us back to the original question of which is easier. Because usually, if you're comparing two integers, it is to make a decision on what to do next. Which means you won't know your next instruction until you've got the result of the last one. Because the instructions are pipelined, this means you can lose 20 clock cycles of work if you wait.

So modern CPUs have a branch predictor which makes a guess what the result will be, and continues executing the instructions. If it guesses wrong, the pipeline has to be thrown out, and work restarted on the other branch. The branch predictor helps enormously, but still, if the comparison is a decision point in the code, that is for more difficult for the CPU to deal with than the addition.

Comparison is done via subtraction, which is almost the same as addition, except that the carry and subtrahend are complemented, so a - b - c becomes a + ~b + ~c. This is already accounted for in the CPU and basically takes the same amount of time either way.