* Tom Herbert <[email protected]> wrote:
> [....] gcc turns these switch statements into jump tables (not function
> tables
> which is what Ingo's example code was using). [...]
So to the extent this still matters, on most x86 microarchitectures that count,
jump tables and function call tables (i.e. virtual functions that C++ uses) are
generally optimized by the same branch predictor hardware mechanism. Indirect
jumps (jump tables) and indirect calls (function pointer tables) are very
similar
conceptually. That is why posted the indirect calls test code.
( The only branching variant that will perform badly even on the latest uarchs
are
indirect returns: to modify the return address on the stack. )
So my narrow performance point stands, if any sort of indirect jump is used.
They
should be avoided if possible, because it's pretty hard for the hardware to get
it
right.
As Linus noticed, data lookup tables are the intelligent solution: if you
manage
to offload the logic into arithmetics and not affect the control flow then
that's
a big win. The inherent branching will be hidden by executing on massively
parallel arithmetics units which effectively execute everything fed to them in
a
single cycle.
In any case, when submitting such patches, please get into the habit of looking
at
and posting perf stat output - it will give us a good idea about the quality of
an
implementation.
Thanks,
Ingo
