Compile-time Loop Splitting for Distributed Memory ... - Stanford AI Lab

time of the loop.
Unfortunately, the data needed by a processor is often located on more than one
processing element so that no loop invariants exist for optimization. However, because
arrays are often both accessed and distributed in segments of contiguous array cells,
intervals of a loop access data from a single processor and have their own invariants.
Thus, each such interval has its own invariants. By dividing the loop into these intervals,
the code transformations can still be performed, albeit on a smaller scale.
A compiler can isolate these intervals by performing a loop transformation called loop
splitting. Loop splitting divides a loop into subloops, which in entirety have the same effect
as the single loop. These subloops can then be reduced in computation.
In the context of distributed memory multiprocessors, this thesis explores the
improvement of array references allowed by the loop splitting transformation. More
specifically, this paper examines program speedup resulting from loop splitting, the code
transformations code hoisting and strength reduction, and the subsequent compiler
optimizations.
1.1 Overview
Section 2 describes array management in distributed memory multiprocessors. This topic
includes partitioning of task and data as well as alignment for minimal execution time.
Then, the method and complexity of array reference expressions are presented to illustrate
the problem this thesis attempts to ameliorate.
Section 3 provides an overview of loop splitting. First, the relevant loop
transformations (general loop splitting and peeling) and compiler optimizations (code
hoisting and strength reduction) are presented. Next, these elements are brought together
by describing the loop splitting transformation for compiler optimizations. Then, to
prepare for Section 4, this section presents the loop splitting framework for optimizing
array reference expressions on a distributed memory multiprocessor.
Section 4, the crux of this thesis, describes in detail the loop splitting study. This
includes the methodology and performance results of several experiments. The results are
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