Messages in this thread |  | | From | David Laight <> | Subject | RE: [PATCH V6 03/10] USB: f81232: implement RX bulk-in ep | Date | Tue, 17 Feb 2015 10:06:07 +0000 |
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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.
david
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