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To: Joe NYC who wrote (158788)2/14/2002 3:22:48 PM
From: Tenchusatsu  Read Replies (2) of 186894
 
Joe,

[WBMW] and speculated execution and predication will give enormously high branch prediction rates.

Actually, this is somewhat misleading. Predication doesn't improve the branch prediction rate. Instead, it helps to reduce the number of branches in the code in the first place. The fewer branches you have, the fewer branch stalls due to mispredicts.

As for speculation, I don't see how that relates at all to branch prediction. Speculation is basically prefetching done in the assembly code.

<I think McKinley has more execution and address calculation blocks than Merced. Are these explicitly addressed by the compiler, that is, does the compiler know how many there are>

Itanium is not a completely static architecture, unlike most versions of VLIW. This is a common misconception.

In this case, assembly instructions will still be arranged into bundles of three. The fetch unit takes each bundle, and the decode unit (or whatever) unpacks the instructions in each bundle. Then all of the unpacked instructions get sent to the various execution units.

The bundling of instructions helps to resolve some of the dependencies between instructions in a block of code. That simplifies the job of the front-end part of the Itanium pipeline. The back-end part (the execution units) resolves the rest of the dependencies and is actually able to execute instructions out-of-order, to an extent. That's how additional execution units in McKinley can help.

Tenchusatsu
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