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VLIW/EPIC Processors in Software Use pdf417 in Software VLIW/EPIC Processors

3.5 VLIW/EPIC Processors use software pdf-417 2d barcode printer todevelop pdf417 on software Scan GS1 BarCodes predication. Predicated in Software pdf417 structions are tagged with the name of a predicate, (i.e.

, a Boolean value), typically generated by the result of the computation of the conditional expression following an if statement. If the predicate is true the instruction is executed otherwise it is not. The simplest form of a predicated instruction is a conditional move, such as the one found for example in the ISA of the DEC Alpha.

As an illustration, the sequence of two instructions corresponding to the high-level statement if A = = 0 then B = C with the values of A, B, and C being respectively in registers R1, R2, R3 would be translated into an ISA without conditional moves as. bnez move R1,Label / bran ch if R1 = 0 / R2, R3 / R2 R3 /. Label: add R4, R5, R2 With the availability of a conditional move, the rst two instructions can be replaced by a single instruction as in. cmovz R2,R3,R1 / R2 R3 Software PDF 417 if R1 = 0 / add R4, R5, R2 / no need for label /. Predication can be very us eful to avoid branches in the case of if then else statements. An if-conversion is shown for the diamond example of Figure 3.8, corresponding to the high-level language statement If (conditional expression) then S1 else S2.

On the left is the usual code: The conditional expression is evaluated. It is then tested: if true, the branch is taken and the sequence of statements S1 is executed; if false, the sequence of statements S2 is executed. Note that we need an unconditional branch after the last statement in S2.

With predication the conditional expression is computed and yields a predicate. All statements in S1 and S2 are predicated with the name of the predicate. The statements in S1 are to be executed if the predicate is true, and those in S2 are to be executed if the predicate is false.

Note that the value of the predicate is needed only when the results have to be committed, so if, for example, S1 is a single statement with long latency, it can be started before the value of the predicate is known. Also, statements in S1 and S2 can be executed concurrently. The example shows that two transfer-of-control instructions, one of which needs to be predicted, have been avoided.

As shown in this example, predication is quite attractive if the number of instructions in S1 and S2 is rather small and the branch is highly unpredictable. However, predication has some drawbacks. First, there must be resources, namely, registers, dedicated to storing predicates, because several branches can be performed concurrently.

Granted, predicated registers are only 1 bit wide, and this will not much affect the amount of processor state. However, the names of these registers are the tags used for the predicated instructions. In the example in Figure 3.

8, the value of the predicate would be stored in a register and the name of the register would be part of the instructions in S1 and S2. Thus, each predicated instruction needs a eld as wide as the logarithm of the number of predicate. If cond. expr. then S1 else S2 p = cond. expr. T F S1.p S1 S2 S2.~p p = cond. expr. Superscalar Processors Figure 3.8. Example of if-conversion: (a) usual code, (b) predicated code. registers. This might incr ease substantially the length of instructions and/or result in awkward instruction formats. Second, predicated instructions that will not commit still use resources until the last pipeline stage.

There might be means to abort the instructions as soon as it is discovered that the predicate is false, but it would add complexity to the design, something that EPIC designers would frown upon in view of their rst argument listed. In any case, the predicated instructions must be fetched from the I-cache, thus potentially decreasing the bandwidth from the latter. Whether predication enhances performance or not is still a subject of debate.

. Control Speculation In add pdf417 2d barcode for None ition to predication, EPIC designers advocate the use of control speculation. This means hoisting a basic block of instructions above a branch. These instructions become speculative.

Of course, speculative instructions cannot be committed until the branch condition is known. The advantages of this speculation are that resources can be better utilized in portions of programs that exhibit low ILP, the heights of long dependence chains can be lowered, and instructions with long latency can be started earlier, as was the case for predication. In addition, however, to the fact that results of speculative instructions must be buffered until the branch condition has been resolved, another problem that may arise is that of exceptions.

An exception raised by a speculative instruction, or a predicated instruction, should not be handled if the speculative instruction is not to be executed. A way to avoid this problem is to append a 1-bit tag, often called a poison bit, to the result register of a speculative instruction that has raised an exception. The poison bit percolates through to result registers of instructions that use a register with a poison bit as a source operand.

If the speculation was correct, then the exception must be handled (and hence a mechanism must be provided to let the hardware know where the exception occurred); otherwise it can be ignored. Finally, the prediction of the latency of load operations is quite dif cult. We already have seen that in both in-order and out-of-order superscalars, some guessing and subsequent repair work was needed.

The same will be true for EPIC processors. Statically managing the cache hierarchy by software prefetching (cf. Section 6.

2.1), and specifying in which member of the cache hierarchy a particular datum should be stored, are attempts at solving this dif cult problem..

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