Limitations of ILP

The Hardware Model An ideal processor is one where all constraints on ILP are removed. The only limits on ILP in such a processor are those imposed by the actual data flows through either registers or memory. The assumptions made for an ideal or perfect processor are as follows: 1.Register renaming —There are an infinite number of virtual registers available, and hence all WAW and WAR hazards are avoided and an unbounded number of instructions can begin execution simultaneously. 2.Branch prediction —Branch prediction is perfect. All conditional branches are predicted exactly. 3.Jump prediction —All jumps (including jump register used for return and computed jumps) are perfectly predicted. When combined with perfect branch prediction, this is equivalent to having a processor with perfect speculation and an unbounded buffer of instructions available for execution. 4.Memory address alias analysis —All memory addresses are known exactly, and a load can be moved before a store provided that the addresses are not identical. Note that this implements perfect address alias analysis. 5.Perfect caches —All memory accesses take 1 clock cycle. In practice, superscalar processors will typically consume large amounts of ILP hiding cache misses, making these results highly optimistic. To measure the available parallelism, a set of programs was compiled and optimized with the standard MIPS optimizing compilers. The programs were instrumented and executed to produce a trace of the instruction and data references. Every instruction in the trace is then scheduled as early as possible, limited only by the data dependences. Since a trace is used, perfect branch prediction and perfect alias analysis are easy to do. With these mechanisms, instructions may be scheduled much earlier than they would otherwise, moving across large numbers of instructions on which they are not data dependent, including branches, since branches are perfectly predicted. The effects of various assumptions are given before looking at some ambitious but realizable processors. Limitations on the Window Size and Maximum Issue Count          To build a processor that even comes close to perfect branch prediction and perfect alias analysis requires extensive dynamic analysis, since static compile time schemes cannot be perfect. Of course, most realistic dynamic schemes will not be perfect, but the use of dynamic schemes will provide the ability to uncover parallelism that cannot be analyzed by static compile time analysis. Thus, a dynamic processor might be able to more closely match the amount of parallelism uncovered by our ideal processor.

The Effects of Realistic Branch and Jump Prediction

       Our ideal processor assumes that branches can be perfectly predicted: The outcome of any branch in the program is known before the first instruction is executed! Of course, no real processor can ever achieve this.

        We assume a separate predictor is used for jumps. Jump predictors are important primarily with the most accurate branch predictors, since the branch frequency is higher and the accuracy of the branch predictors dominates.

The five levels of branch prediction

1.Perfect —All branches and jumps are perfectly predicted at the start of execution.

2.Tournament-based branch predictor —The prediction scheme uses a correlating 2-bit predictor and a noncorrelating 2-bit predictor together with a selector, hich chooses the best predictor for each branch.

The Effects of Finite Registers

Our ideal processor eliminates all name dependences among register references using an infinite set of virtual registers. To date, the IBM Power5 has provided the largest numbers of virtual registers: 88 additional floating-point and 88 additional integer registers, in addition to the 64 registers available in the base architecture. All 240 registers are shared by two threads when executing in multithreading mode, and all are available to a single thread when in single-thread mode.

The Effects of Imperfect Alias Analysis

Our optimal model assumes that it can perfectly analyze all memory dependences, as well as eliminate all register name dependences. Of course, perfect alias analysis is not possible in practice: The analysis cannot be perfect at compile time, and it requires a potentially unbounded number of comparisons at run time (since the number of simultaneous memory references is unconstrained).

The three models are

1. Global/stack perfect—This model does perfect predictions for global and stack references and assumes all heap references conflict. This model represents an idealized version of the best compiler-based analysis schemes currently in production. Recent and ongoing research on alias analysis for pointers should improve the handling of pointers to the heap in the future.

2. Inspection—This model examines the accesses to see if they can be determined not to interfere at compile time. For example, if an access uses R10 as a base register with an offset of 20, then another access that uses R10 as a base register with an offset of 100 cannot interfere, assuming R10 could not have changed. In addition, addresses based on registers that point to different allocation areas (such as the global area and the stack area) are assumed never to alias. This analysis is similar to that performed by many existing commercial compilers, though newer compilers can do better, at least for looporiented programs.

3. None—All memory references are assumed to conflict. As you might expect, for the FORTRAN programs (where no heap references exist), there is no difference between perfect and global/stack perfect analysis.