A Tale of Two Processors: Revisiting the RISC-CISC Debate

Abstract

The contentious debates between RISC and CISC have died down, and a CISC ISA, the x86 continues to be popular. Nowadays, processors with CISC-ISAs translate the CISC instructions into RISC style micro-operations (eg: uops of Intel and ROPS of AMD). The use of the uops (or ROPS) allows the use of RISC-style execution cores, and use of various micro-architectural techniques that can be easily implemented in RISC cores. This can easily allow CISC processors to approach RISC performance. However, CISC ISAs do have the additional burden of translating instructions to micro-operations. In a 1991 study between VAX and MIPS, Bhandarkar and Clark showed that after canceling out the code size advantage of CISC and the CPI advantage of RISC, the MIPS processor had an average 2.7x advantage over the studied CISC processor (VAX). A 1997 study on Alpha 21064 and the Intel Pentium Pro still showed 5% to 200% advantage for RISC for various SPEC CPU95 programs. A decade later and after introduction of interesting techniques such as fusion of micro-operations in the x86, we set off to compare a recent RISC and a recent CISC processor, the IBM POWER5+ and the Intel Woodcrest. We find that the SPEC CPU2006 programs are divided between those showing an advantage on POWER5+ or Woodcrest, narrowing down the 2.7x advantage to nearly 1.0. Our study points to the fact that if aggressive micro-architectural techniques for ILP and high performance can be carefully applied, a CISC ISA can be implemented to yield similar performance as RISC processors. Another interesting observation is that approximately 40% of all work done on the Woodcrest is wasteful execution in the mispredicted path.