Agner`s CPU blog

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Jump prefetch?
Author: csdt Date: 2017-01-27 10:21
Branch predictors is currently a hard part of a CPU.
I propose to help it a bit in the same way we can help the cache: tell it in advance where we want to jump.

I think this can be done by a couple of instructions:
- chose the destination from 2 immediates based on a condition
- unconditionally set the destination of a jump from a register
- unconditionally jump to the previously set destination

This can help basic control flow if the program is not limited by the instruction bandwidth with the second instruction.
And it can also help indirect calls (virtual call through a vtable for instance).

For safety reason, this "special register" should be saved and restored during context switches.
For security reason, the "special register" should be reset after each jump whatever instruction is used for the jump, and a signal should be send if the third instruction is executed while the "special register" is not in a valid state.

What do you think of this ?

 
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