Register allocation

In compiler optimization, register allocation is the process of assigning a large number of target program variables onto a small number of CPU registers. In compiler optimization, register allocation is the process of assigning a large number of target program variables onto a small number of CPU registers. Register allocation can happen over a basic block (local register allocation), over a whole function/procedure (global register allocation), or across function boundaries traversed via call-graph (interprocedural register allocation). When done per function/procedure the calling convention may require insertion of save/restore around each call-site. In many programming languages, the programmer may use any number of variables. The computer can quickly read and write registers in the CPU, so the computer program runs faster when more variables can be in the CPU's registers. Also, sometimes code accessing registers is more compact, so the code is smaller, and can be fetched faster if it uses registers rather than memory. However, the number of registers is limited. Therefore, when the compiler is translating code to machine-language, it must decide how to allocate variables to the limited number of registers in the CPU. Not all variables are in use (or 'live') at the same time, so, over the lifetime of a program, a given register may be used to hold different variables. However, two variables in use at the same time cannot be assigned to the same register without corrupting one of the variables. If there are not enough registers to hold all the variables, some variables may be moved to and from RAM. This process is called 'spilling' the registers. Over the lifetime of a program, a variable can be both spilled and stored in registers: this variable is then considered as 'split'. Accessing RAM is significantly slower than accessing registers and so a compiled program runs slower. Therefore, an optimizing compiler aims to assign as many variables to registers as possible. A high 'Register pressure' is a technical term that means that more spills and reloads are needed; it is defined by Braun et al. as 'the number of simultaneously live variables at an instruction'. In addition, some computer designs cache frequently-accessed registers. So, programs can be further optimized by assigning the same register to a source and destination of a move instruction whenever possible. This is especially important if the compiler is using an intermediate representation such as static single-assignment form (SSA). In particular, when SSA is not fully optimized it can artificially generate additional move instructions.