Compiler algorithms on if-conversion, speculative predicates assignment and predicated code optimizations

Abstract

Instruction level parallelism has become more and more important in today's microprocessor design. These microprocessors have multiple function units, which can execute more than one instruction at the same machine cycle to enhance the uniprocessor performance. Since the function units are usually pipelined in such microprocessors, branch misprediction penalty tremendously degrades the CPU performance. In order to reduce the branch misprediction penalty, predicated operation has been introduced in such microprocessor design as one of the new architectural features, which allows compilers to remove branches from programs.

The compiler converts programs with conditional branch to predicated code on these microprocessors with predicate operation support by using comparison instructions to set up Boolean predicates corresponding to branch conditions. Instructions previously guarded by a branch are guarded by a predicate. Predicates guard execution by either executing or nullifying the instruction according to the predicate's value.

In this report we present an algorithm to convert a control flow graph into predicated code. The algorithm shown in this report not only minimizes the number of predicates used for basic blocks in the control flow, but also moves the predicate assignments as early as possible to relax dependence constrains introduced by the if-conversion for the later phases of compiler such as the instruction scheduler and register allocator. The algorithm doesn't simply assign the predicate associated with a basic block to the instructions in the basic block. It assigns the predicate associated with the “farthest” dominator basic block to an instruction, to which the instruction can be speculatively moved without introducing compensation code in the instruction scheduling.

An optimization algorithm for if-converted code is discussed in the report as well. The algorithm is specially developed for predicated code optimization, which is different from the traditional optimizations. Common subexpression hoisting and sinking employs flow analysis, that extends the traditional common subexpression elimination technique. This algorithm requires traditional flow analysis information such as dominate and post-dominate relationship. Actually, the algorithm can be generalized on predicated code after if-conversion, which is particularly beneficial for single assignment DAG representation of the predicated code.