EECS 583 – Homework 2
Winter 2003
Assigned: Monday, January 27, 2003
Due: Monday, February 10, 2003
The goal for this homework is to implement a heuristic hyperblock formation algorithm
that performs selective if-conversion for inner-most loops. On the surface, this requires a
large amount of code to accomplish this task. However, the bulk of the code will be
provided to you (tail duplication, if-conversion). Your main task is to first implement a
simple control flow optimization, backedge coalescing, to convert any multi-backedge
loop into a single backedge loop. Second, implement a heuristic block selection
algorithm to “intelligently” choose the set of basic blocks that will be if-converted. You
are free to define any heuristic that you wish and are encouraged to be creative here.
Please try to avoid just blindly using the BSV algorithm discussed in class.
Your algorithm should operate on an inner-most loop, where you wish to perform
selective if-conversion of the loop body. You should create at most 1 hyperblock for the
loop, so you do not need to worry about making other HBs out of basic blocks that you
do not choose for your primary HB. The algorithm should be applied to all the inner
most loops in the program.
Your algorithm should consist of the following 5 steps:
Loop detection – Using the code from HW1, identify all the loops in a
Procedure. Mark those that are innermost for subsequent processing.
Backedge coalescing – For each inner-most with multiple backedges, combine
all backedges into a single backedge using the technique described in class.
Block selection – Identify the subset of blocks within the loop that you wish to
if-convert using whatever means you wish. The goal is to produce the fastest
code possible as compared with the original basic block implementation.
Tail duplication – Replicate the set of basic blocks and adjust the control flow
edges such that the selected blocks contain no side entrances. This code will be
provided to you, see elcor/src/Control/el_tail_duplication.cpp
If-conversion – Eliminate all the control flow among the selected basic blocks
by using predicated execution. This code will be provided to you, see elcor/src/
Control/el_if_converter.cpp
Block Selection
This will obviously require come thought. You should start with the lecture on
Hyperblocks that talks about the 2 previous techniques, block selection value and path
selection. Think about the cost/benefits of if-conversion. On the benefit side, you
eliminate branches, mispredictions and allow for more instruction overlap. On the
negative side, you may increase resource utilization, dependence height, and possibly
