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Jump prefetch?
Author: csdt Date: 2017-01-30 09:53
Agner wrote:
csdt wrote:
The point of this solution is to replace conditional jumps by unconditional jumps to predefined targets.
It is not very clear what you mean here. It is possible, of course, to put the jump target address in a register and then do an indirect jump to the register value. This doesn't solve the problem in current hardware designs because the value of the target register is not known at the time the next instruction has to be fetched into the pipeline. You would need to store the target address several clock cycles in advance and then have a mechanism for signalling to the instruction fetcher at what point it would have to fetch from the new address. The compiler then has to place the switch-target instruction at the right position in the instruction sequence. The distance from the switch-target instruction to the point of divergence has to fit the length of the pipeline, which depends on the specific hardware implementation. This may be possible, but I am not aware of any attempt to construct such a machinery.
The idea is not to help the prediction, but to "override" the prediction for a single given jump instruction. So there is no failed prediction in the same way as you cannot "mispredict" an unconditional jump to a fixed target.
Let me give you a few examples (pseudo-asm syntax).

The 3 instructions are:
- set_jmp target
- set_jmp cond target0 target1 (set target to "target0" if "cond" is 0 and to "target1" otherwise)
- jmp2target

Here is a simple loop:

(prolog)
loop:
%r01 = sub.i32 %r01 1
set_jmp %r01 loop_end loop
(loop body)
jmp2target
loop_end:
(epilog)

So basically, the jump is always taken, and you know exactly the address of the jump in advance.
Of course, you need to know when load the instructions of the target address, but this can be handled by the branch predictor like a regular unconditional jump to a fixed address.

The same applies to if conditions:

%01 = (compute the condition)
set_jmp %r01 if_end if_then
(some unrelated computation)
jmp2target
if_then:
(conditional computation)
if_end:
(unconditional computation)

Of course, if you set the target too early, you need to wait, so you need to execute the "set_jmp" early enough, but the same problem arises with data prefetching anyway.

 
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