Register Allocation

Because of the central role that register allocation plays, both in speeding up the code and in making other optimizations useful, it is one of the most important - if not the most important - of the optimizations. [这语法……] Register allocation algorithms today are based on a technique called graph coloring. The basic idea behind graph coloring is to construct a graph representing the possible candidates for allocation to a register and then to use the graph to allocate registers.

Roughly speaking, the problem is how to use a limited set of colors so that no two adjacent nodes in a dependency graph have the same color. The emphasis in the approach is to achieve 100% register allocation of active variables. The problem of coloring a graph in general can take exponential time as a function of the size of the graph (NP-complete). There are heuristic algorithms, however, that work well in practice, yielding close allocations that run in near-linear time.

Graph coloring works best when there are at least 16 (and preferably more) general-purpose registers available for global allocation for integer variables and additional registers for floating point. Unfortunately, graph coloring does not work very well when the number of registers is small because the heuristic algorithms for coloring the graph are likely to fail.

Register allocation is much more effective for stack-allocated objects than for global variables, and register allocation is essentially impossible for heap-allocated objects because they are accessed with pointers. Global variables and some stack variables are impossible to allocate because they are aliased - there are multiple ways to refer to the address of a variable, making it illegal to put it into a register.
(Most heap variables are effectively aliased for today's compiler technology.)

For example, consider the following code sequence, where & returns the address of a variable and * dereferences a pointer. The variable a could not be register allocated across the assignment to *p without generating incorrect code. Aliasing causes a substantial problem because it is often difficult or impossible to decide what objects a pointer may refer to.

所以，提高访问内存的速度很重要啊。

六级/考研单词: allocate, graph, construct, adjacent, invariable, yield, globe, float, stack, multiple, illicit, heap, assign, seldom