Introduction to Superscalar Execution

Tom Kelliher, CS 240

Feb. 14, 2000

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Outline

1. Introduction to superscalar pipelining

2. Data dependencies.

3. Out of order execution.

4. Multiple Instruction Issue

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More superscalar execution and other advanced pipelining techniques.

Introduction to Superscalar Pipelining

1. Historical Progression of IPC: < 1, = 1, > 1. The entire pipeline must be widened.

   Challenges: small register files, multiple-branch predictions, multiple line fetches from caches.

2. Range of parallelism: coarse- to fine-grained.

3. Superscalar techniques address ILP. Let's parallelize a sequential binary.

4. What's the upper bound on IPC? It depends.

   Text processing: low, mostly.

   Image processing, multimedia: high.

   Median operation on an image example:

   medianImage(image dest, image src)
   {
      for each pixel, p, in src
         p in dest = medianPixel(p in src);
   }
   
   medianPixel(pixel p)
   {
      find the <= 8 neighboring pixels of p;
      compute and return the median value;
   }
   

   Challenges: exposing potential ILP to the compiler.

   Example. Parallelize the following:

   sum = 0;
   
   for (i = 0; i < last; ++i)
      sum += array[i];
   

5. Compiler techniques: loop unrolling, invariant code migration, strength reduction, etc.

Types of Data Dependencies

1. RAR. Not a problem at all.

2. RAW. A ``true'' dependency.

3. WAR. A ``false'' dependency.

4. WAW. Another ``false'' dependency.

      r1 = r2 + r3
      r4 = r1 + r5
      r1 = r6 + r7
      r8 = r1 + r4

Rename the previous example where the Register Alias Table (RAT) is initially:

r1  -> p12     r2  ->  p6     r3  ->  p9     r4  -> p15
r5  -> p1      r6  -> p10     r7  ->  p8     r8  -> p14

Free List: p5, p11, p13, p4.

Out of Order execution

1. What is it?

2. In-order completion.

3. How is it done?

Multiple Instruction Issue

1. In-order execution case.

   Structural hazard stalls.

2. Out of order execution case.

   Only stall if no free list entries.