Re: [PATCH 0/4] jump label patches

[Richard Henderson <rth@redhat.com>]

On 10/05/2009 10:39 PM, Roland McGrath wrote:
> Of course, a first important point is what the actual compiled code
> sequences look like.  I'm hoping Richard (who implemented the compiler
> feature for us) can help us with making sure our expectations jibe with the
> code we'll really get.  There's no benefit in optimizing our asm not to
> introduce a jump into the hot path if the compiler actually generates the
> tracing path first and gives the hot path a "jmp" around it anyway.

At present, the asm goto extension gives no prediction weights to any 
path.  I had hoped that the -freorder-blocks pass (enabled with -O2) 
would automatically place the relevant fallthrough blocks immediately 
after the asm goto.  It did happen for small test cases, but a message 
from Jason downthread indicates that it doesn't always happen.

> 	if (__builtin_expect(0,0)) do_trace: __attribute__((cold)) { ... }

An attribute cold on a label is something that I've suggested, but have 
not yet implemented.  I think that might be the easiest way to add 
prediction weights to an asm goto.


r~