Superscalar Execution

In the traditional processor pipeline model under ideal circumstances one new instruction enters the processor’s and one instruction completes execution each cycle. Thus, for the best case the processor can have an average execution rate of one clock per instruction. A superscalar processor allows multiple unrelated instructions to start on the same clock cycle on separate hardware units or pipelines. Under ideal conditions a superscalar processors could have an average clocks per instruction (CPI) be less one, meaning your 2GHz processor might be able to execute “billions and billions of instructions per second” as the famed astronomer Carl Sagan might like to say.

Different processors have different constraints on which instructions can be issued together for superscalar execution. A processor such as the Intel Atom can issue two instructions per cycle. However, the compiler needs to take care in the instructions it selects and the order of those instructions because the Atom processor has restrictions on which instructions can start at the same time. For example the Intel Atom processor can only start one older X87 floating point instruction per cycle, so the compiler should avoid back-to-back X87 floating point instructions. The Intel Haswell-based processors have four units for basic arithmetic/logic operations and three units for load/store operations and the Intel Skylake-based processor can issue up to 6 uops per cycle. Given these detailed differences selecting and ordering the instructions is probably best left to the compiler.

Like the pipeline, superscalar execution is attempting to have more instructions processed concurrently. Techniques that allow the pipelined processor work well also apply to superscalar processor. The caches need to quickly provide instructions and data to feed to the processor otherwise it is waiting. The processor needs to have good branch prediction and the code should have longer sequences of unrelated instructions.

For a computationally intensive loop in user-code a loop-unrolling optimization might provide a larger sequence of instructions giving the processor more opportunities to issue multiple instructions in a single clock cycle. An example of manual loop unrolling can be seen in the BLAS (Basic Linear Algebra Subprogram) saxpy function. The saxpy function multiples the n elements in the vector sx by the scalar sa and adds each element to the matching element in the sy vector. With n elements in each vector the naive implementation of the loop would be something like:

for (i = 0; i < n; i += 1) {
    sy[i] += sa * sx[i];

Manual loop unrolling for four iterations might convert the above loop into a loop to handle the cases of small vectors and vectors that are not evenly divisible by the unrolling factor followed by the unrolled loop:

m = n % 4;
for (i = 0; i < m; ++i) {
    sy[i] += sa * sx[i];
if (n>=4) {
    for (i = m; i < n; i += 4) {
        sy[i] += sa * sx[i];
        sy[i + 1] += sa * sx[i + 1];
        sy[i + 2] += sa * sx[i + 2];
        sy[i + 3] += sa * sx[i + 3];

Whether this type of optimization is profitable depends heavily on the size of the vectors being used. If all the vectors are just a few elements, the additional overhead to check the size of the vectors and set things up may eliminate the benefit of the loop unwinding. You can use tools such as Linux perf event tool identify computationally intensive areas of code that might benefit from such optimizations.

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