Compiler-controlled memory

Abstract

Optimizations aimed at reducing the impact of memory operations on execution speed have long concentrated on improving cache performance. These efforts achieve a. reasonable level of success. The primary limit on the compiler's ability to improve memory behavior is its imperfect knowledge about the run-time behavior of the program. The compiler cannot completely predict runtime access patterns.There is an exception to this rule. During the register allocation phase, the compiler often must insert substantial amounts of spill code ; that is, instructions that move values from registers to memory and back again. Because the compiler itself inserts these memory instructions, it has more knowledge about them than other memory operations in the program.Spill-code operations are disjoint from the memory manipulations required by the semantics of the program being compiled, and, indeed, the two can interfere in the cache. This paper proposes a hardware solution to the problem of increased spill costs---a small compiler-controlled memory (CCM) to hold spilled values. This small random-access memory can (and should) be placed in a distinct address space from the main memory hierarchy. The compiler can target spill instructions to use the CCM, moving most compiler-inserted memory traffic out of the pathway to main memory and eliminating any impact that those spill instructions would have on the state of the main memory hierarchy. Such memories already exist on some DSP microprocessors. Our techniques can be applied directly on those chips.This paper presents two compiler-based methods to exploit such a memory, along with experimental results showing that speedups from using CCM may be sizable. It shows that using the register allocation's coloring paradigm to assign spilled values to memory can greatly reduce the amount of memory required by a program.