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Concept
Register renaming
Summary
In computer architecture, register renaming is a technique that abstracts logical registers from physical registers.
Every logical register has a set of physical registers associated with it.
When a machine language instruction refers to a particular logical register, the processor transposes this name to one specific physical register on the fly.
The physical registers are opaque and cannot be referenced directly but only via the canonical names.
This technique is used to eliminate false data dependencies arising from the reuse of registers by successive instructions that do not have any real data dependencies between them.
The elimination of these false data dependencies reveals more instruction-level parallelism in an instruction stream, which can be exploited by various and complementary techniques such as superscalar and out-of-order execution for better performance.
In a register machine, programs are composed of instructions which operate on values.
The instructions must name these values in order to distinguish them from one another.
A typical instruction might say: “add and and put the result in ”.
In this instruction, , and are the names of storage locations.
In order to have a compact instruction encoding, most processor instruction sets have a small set of special locations which can be referred to by special names: registers.
For example, the x86 instruction set architecture has 8 integer registers, x86-64 has 16, many RISCs have 32, and IA-64 has 128.
In smaller processors, the names of these locations correspond directly to elements of a .
Different instructions may take different amounts of time; for example, a processor may be able to execute hundreds of instructions while a single load from the main memory is in progress.
Shorter instructions executed while the load is outstanding will finish first, thus the instructions are finishing out of the original program order.
Out-of-order execution has been used in most recent high-performance CPUs to achieve some of their speed gains.
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