Trace processors: exploiting hierarchy and speculation
Eric Rotenberg, James E. Smith · 1999
In high-performance processors, increasing the number of instructions fetched and executed in parallel is becoming increasingly complex, and the peak bandwidth is often underutilized due to control and data dependences. A trace processor 1) efficiently sequences through programs in large units, called traces, and allocates trace-sized units of work to distributed processing elements (PEs), and 2) uses aggressive speculation to par-tially alleviate the effects of control and data dependences. A trace is a dynamic sequence of instructions, typically 16 to 32 instructions in length, which embeds any number of taken or not-taken branch instructions. The hierarchical, trace-based approach to increas-ing parallelism overcomes basic inefficiencies of managing fetch and execution resources on an individual instruction basis. This thesis shows the trace processor is a good microarchitecture for implementing wide-issue machines. Three key points support this conclusion. 1. Trace processors perform better than wide-issue superscalar counterparts because they deliver high instruction throughput without significantly increasing cycle time. The underlying reason: trace processor cycle time is more sensitive to individual PE com-