Branch prediction hints in an ISA

Hey I was just thinking. Does anyone know of any Instruction Sets
that have an extra bit in the branch instruction to be used as a
"hint" for the branch prediction logic?


It seems to me that in some cases a compiler would know which path a
branch would likely take and could give a hint to the processor's
pipeline using an extra bit in the branch instruction (i.e. 1 =
usually branches, 0 = usually doesn't branch).


If the compiler could make good enough predictions, you might be able
to remove all the branch prediction logic from (and cache) from the
processor (and replace it with something else) and have similar
performance.




Just curious.


Thanks,
tim
[Yes, it's been done many times. IA-64, for example, has it. I gather
that static branch prediction is OK, but it's no substitute for dynamic
branch prediction. -John]

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