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Back-End: Instruction Scheduling, Memory Access Instructions, and Clusters in Software Insert PDF417 in Software Back-End: Instruction Scheduling, Memory Access Instructions, and Clusters bar code for visual C#.net




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Back-End: Instruction Scheduling, Memory Access Instructions, and Clusters generate, create none none on none projectsbarcodeean barcodetype c# in the inte none none ger unit where it was computed to the input of the integer unit (the same one or another) that will use it as an operand, bypassing the store in, say, the physical register le at commit time. Although conceptually this is as simple as forwarding in the ve-stage pipeline of 2, the scale at which it must be done makes it challenging. The number of required forwarding buses is the product of the number of stages that can be the producers of the forwarding (it can be more than one per functional unit if the latter implements several opcodes with different latencies) and the issue width.

Trying to improve performance by increasing the number of functional units and the issue width is translated into a quadratic increase in the number of wires as well as an increase in their overall length. In multigigahertz processors, this might imply that forwarding cannot be done in a single cycle. In the last section of this chapter, we shall introduce clustered processors, which are a possible answer to the challenge.

. EAN8 5.2 Memory-Accessing Instructions Instruction none none s transmitting data to and from registers, namely the load and store instructions, present challenges that are not raised by arithmetic logical instructions. Load instructions are of particular importance in that many subsequent instructions are dependent on their outcome. An important feature that distinguishes memory-accessing instructions from the other instruction types is that they require two computing stages in the back-end, namely address computation and memory hierarchy access.

The presence of these two separate stages led to irresolvable RAW dependencies in the simple ve-stage pipelines of 2. In the rst design that we presented, a bubble had to be inserted between a load and a subsequent instruction, depending on the value of the load; in the second design a bubble needed to be introduced when the RAW dependency was between an arithmetic logical instruction and the address generation step of a load store instruction. In superscalar implementations, the address generation is either done in one of the integer units (cf.

the Alpha 21164 in Section 3.1.3) or done in a separate pipeline that is interfaced directly with the data cache (the most common alternative in current designs; see, for example, the AGU in the Intel P6 microarchitecture of Section 3.

3.3). In in-order processors, like the Alpha 21164, RAW dependencies between a load and a subsequent instruction decoded in the same cycle will delay the dependent instruction, and therefore all following instructions, by 1 cycle.

As mentioned in 3, the check for such an occurrence is done in the last stage of the front-end, using a scoreboard technique that keeps track of all instructions in ight. In out-of-order processors, in a similar fashion, the wakeup of instructions depending on the load is delayed by the latency of the data cache access. A number of compiler techniques have been devised to alleviate the load dependency delays, and the reader is referred to compiler technology books for more information.

In contrast with the simple pipelines of 2, where we assumed that the Mem stage always took 1 cycle, real implementations dealing with caches need to. 5.2 Memory-Accessing Instructions guess the none for none latency of the load operation. As we saw in 3 in the case of the in-order Alpha, the guess was the common case, namely, a rst-level data cache hit. In case of a miss, instructions dependent on the load cannot be issued, and if one of them has already been issued, it is aborted.

We also saw in the rst section of this chapter that the wakeup stage in out-of-order processors was speculative because of the possibility of cache misses. A consequence was that instructions had to remain in their windows or reservation stations until it was certain that they were no longer subject to speculation (i.e.

, committed or aborted due to branch misprediction), because they might be woken up again if they were dependent on a load operation that resulted in a cache miss. As already mentioned in Section 3.3.

3 when giving an overview of the Intel P6 microarchitecture, out-of-order processors add another opportunity for optimization, namely load speculation. There are two circumstances wherein a load cannot proceed without speculation: (i) the operands needed to form the memory address are not yet available, and (ii) the contents of the memory location that will be addressed are still to be updated. Speculation on the load address is mostly used in conjunction with data prefetching.

We will return to the topic of address prediction when we discuss prefetching in 6. In this section, we present some techniques for the second type of speculation, generally called memory dependence prediction because the dependencies are between load and preceding store instructions. 5.

2.1 Store Instructions and the Store Buffer Let us rst elaborate on the way store instructions are handled. There are two possible situations once the memory address has been generated (whether address translation via a TLB is needed or not is irrelevant to this discussion): either the result to be stored is known, or it still has to be computed by a preceding instruction.

However, even in the rst case, the result cannot be stored in the data cache until it is known that the store instruction will be committed. Therefore, store results and, naturally, their associated store addresses need to be stored in a store buffer. The store buffer is organized as a circular queue with allocation of an entry at decode time and removal at commit time.

Because the store result might be written in the cache later than the time at which it is known (because instructions are committed in order), status bits for each entry in the store buffer will indicate whether: r The entry is available (bit AV). r The store has been woken up and the store address has been computed and entered in the store buffer, but the result is not yet available in the store buffer (state AD). r The store address and the result are in the store buffer, but the store instruction has not been committed (state RE).

r The store instruction has been committed (state CO). When the store instruction has been committed (head of the ROB and head of the store buffer), the result will be written to the cache as soon as possible, and.
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