On Tue, Feb 17, 2015 at 10:06:07AM +0000, David Laight wrote:
> From: Greg KH
> > > + for (i = 0 ; i < urb->actual_length ; i += 2) {
> > > + tty_flag = TTY_NORMAL;
> > > +
> > > + if (unlikely(data[i+0] & UART_LSR_BRK_ERROR_BITS)) {
> >
> > Never use unlikely() unless you can prove that it actually matters if
> > you use it. Hint, it's almost impossible to prove, so don't use it, the
> > compiler and processor look-ahead is almost smarter than we are.
>
> That just isn't true.
>
> The compiler cannot know the actual control flow - so cannot correctly
> arrange the code so that the branches are statically predicted
> correctly for the required path (usually the most common path).
>
> There are a lot of places where a few extra clocks for a mispredicted
> branch don't really matter, and even in very hot paths where it does
> matter it can be quite difficult to get the compiler to optimise the
> branches 'correctly' - you can need to add asm comments in order to
> generate non-empty code blocks.
>
> In addition unlikely() is also a note to the human reader.
>
> I did a lot of work adding likely/unlikely to some code in order
> to minimise the 'worst case' code path. I got there, but some
> parts were initially non-intuitive.
Yes, but remember that old patch that Andi did to actually check to see
if unlikely/likely mattered and was placed correctly? Turns out that
90% of the usages were wrong. So humans are horrible at using these
markings, so I will not accept them unless you can _prove_ it matters in
the code.
For a urb callback, that's not an issue at all, the usb callback is so
slow that you will almost never make a difference, sorry.
So again, don't do it in driver code unless you can prove it.
thanks,
greg k-h
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