What's SMT in this context?

recursivecaveat 10 hours ago | parent [-]

Simultaneous Multi-Threading (hyper-threading as Intel calls it). I'm not a cpu guy, but I think the ALU used for subtraction would be a more valuable resource to leave available to the other thread than whatever implements a xor. Hence you prefer to use the xor for zeroing and conserve the ALU for other threads to use.

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tliltocatl 10 hours ago | parent | next [-]

I don't think that's how it works.

- Normally ALU implements all "light" operations (i. e. add/sub/and/or/xor) in a single block, separating them would result in far more interconnect overhead. Often, CPUs have specialized adder-only units for address generation, but never a xor-specialized block.

- All CPUs that implement hyper-threading also optimize a XOR EAX,EAX into MOV EAX,ZERO/SET FLAGS (where ZERO is an invisible zero register just like on Itanium and RISCs). This helps register renaming and eliminates a spurious dependency.

- The XOR trick is about as old as 8086 if not older.

	▲	Symmetry 3 hours ago \| parent [-]
		Right. Keeping down the number of slots the scheduler and bypass network need to worry about is an important design pressure.

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fredoralive 10 hours ago | parent | prev | next [-]

By the time you get to a CPU complex enough to be to have SMT it is likely to detect these “clear register” patterns and special case them.

XOR would also be handled by the ALU, the L is for logic.

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IshKebab 8 hours ago | parent | prev [-]

Most CPU use the same ALU for xor and sub.