Re: Branch prediction hints in an ISA

Erik Corry <erik@arbat.com> writes:
>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.


Absolutely; with most modern machines, one wants to minimize the
number of taken branches, whether the branch is predicted or not.


However, there are occasions where you cannot follow this piece of
advice. One such sitation is where a basic block ends with a
conditional branch to its own start; putting a basic block *after*
itself is pretty tough :-) Therefore having a mechanism that tells the
branch predictor that a branch should be predicted to be taken is
still useful.


Zoltan Somogyi <zs@cs.mu.OZ.AU> http://www.cs.mu.oz.au/~zs/
Department of Computer Science and Software Engineering, Univ. of Melbourne

Return to the comp.compilers page.
Search the comp.compilers archives again.