RE: [PATCH V6 03/10] USB: f81232: implement RX bulk-in ep

From: David Laight
Date: Tue Feb 17 2015 - 05:06:47 EST

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.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at