Re: Branch prediction hints in an ISA

timgleason@my-deja.com wrote:
> 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?


Some do. Most will predict a forward branch as not taken and a
backward branch as taken if they don't have any branch prediction
information. IA64 has a rather more complex scheme (no surprise!).


But according to the Alpha optimisation guide, if you know
which way a branch is likely to go it is almost always worth
moving the less taken branch to somewhere completely different.
This reduces the working set. If you move it to the end of
the function then it will be a forward branch, and so will be
predicted not taken.


--
Erik Corry erik@arbat.com Ceterum censeo, Microsoftem esse delendam!
    Interviewer: "Real programmers use cat as their editor."
    Bill Joy: "That's right! There you go! It is too much trouble to say ed,
                                  because cat's smaller and only needs two pages of memory."

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