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Articles: CPU
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### Infinite Replay Looping

In the previous example we discussed the situation when a command from an independent chain fits into the “hole” between the dependent commands. Unfortunately the scheduler of contemporary NetBurst processors is not “smart enough” to track down the entire chain of dependent commands sent to replay. In other words, the scheduler always considers the operation to be performed correctly. Now let’s take a closer look at the situation when a command from the same dependency chain fits into the “hole”. This matter is very important for the proper performance estimates, because the indirect dependency chains can be very frequently found in programs.

As the simplest example here I would like to offer you a Total Sum calculation, when the result of every next operation depends on the result of the previous one directly.

for (int I = 0; I< count; I++)
sum+=array[I];

Well, let’s analyze how the replay works with the help of this example of a long data dependency chain (see Pic.5 below).

ADD R1, R2 //1 – R1 = R1+R2
ADD R1, R2 //2 – R1 = R1+R2
ADD R1, R2 //3 – R1 = R1+R2
ADD R1, R2 //4 – R1 = R1+R2
ADD R1, R2 //5 – R1 = R1+R2
ADD R1, R2 //6 – R1 = R1+R2
ADD R1, R2 //7 – R1 = R1+R2
ADD R1, R2 //8 – R1 = R1+R2
ADD R1, R2 //9 – R1 = R1+R2
………

Just like in the previous example (Pic.4b) we will take the L1 load latency equal to 2 clock cycles, and the ADD latency – to 1 clock cycle. X is not in L1 cache (L1 miss), but is found in L2 cache.

Pic.5

So, we see the following picture: the scheduler doesn’t know what commands have been executed correctly, but it continues sending new commands from the dependency chain for execution inserting them into “holes” between commands sent to replay. Since not all the previous commands have been executed correctly, the commands in the “holes” will also be executed incorrectly and will have to get back to replay. All commands of this chain will keep rotating in the replay loop one by one like links of the chain circled around a rod. If we create a time diagram representing the Dispatch stage at each given moment of time, it will look as follows:

Pc.6: The chain of commands getting into the replay system.

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