Wolves in CISC Clothing

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Soft Boost to a Heavy Metal’s 2nd Language

When Intel and HP designed the Itanium’s IA-64 instruction set architecture nearly a decade ago, they included an x86 compatibility mode in which the processor would directly execute x86 software using a subset of the IA-64 visible processor state to represent the x86 visible processor state. Special instructions were provided to switch between native and x86 modes [15]. But the x86 performance of even the second generation Itanium 2 is far behind that of contemporary x86 processors; the product of incredible competition driven progress over the last five years. Based on various claims from Intel, HP, and anecdotal accounts, it appears that an Itanium 2 running in compatibility mode executes x86 code at approximately 20 – 30% of native performance. Since current IPF and x86 processors have similar native integer performance no sane person would buy IPF hardware if a large fraction of their computing workload was accomplished by x86 based software.

If the Itanium’s x86 mode was left to quietly fulfill its apparent original purpose of running PC style personal productivity applications on IA-64 based workstations, thus avoiding the need for a separate PC, the existing performance would probably be fine. Unfortunately for Intel, two things have focused unwanted attention on the poor x86 performance of the Itanium. The first is the runaway success of its own Xeon x86 server and workstation processors. To some extent the Itanium family (IPF) competes with Xeon as well as the RISC based systems it is targeted at. It is easier to sell IPF to Xeon users if only key, performance sensitive applications need be replaced or supplemented with IA-64 equivalents. But what fraction of a Xeon user’s overall x86 software must realistically be replaced with IA-64 equivalents in a switch to IPF based systems depends to a large extent on Itanium’s x86 performance. The second is the introduction of AMD’s Opteron, a 64 bit server processor that executes existing x86 software at very high performance levels.


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