lkml.org 
[lkml]   [2012]   [Feb]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs
From
Date
On Wed, 2012-02-22 at 15:56 +0100, Ingo Molnar wrote:

> > Because it really just looks like a stronger "unlikely()" and
> > fundamentally it really isn't. [...]
>
> Well, the fact is that right now it *is* a stronger unlikely()
> on architectures that have jump-labels and it's mapped to
> unlikely() on others.
>

Has gcc been fix to make it truly an unlikely case and remove the "jmp;
jmp" problem of before? I'm still using gcc 4.6.0 which has the
following code for a tracepoint (example is the
trace_sched_migrate_task().

5b4a: e9 00 00 00 00 jmpq 5b4f <set_task_cpu+0x5e>

The above is the jump label that turns into a nop at boot up.

5b4f: eb 19 jmp 5b6a <set_task_cpu+0x79>

Here we jump over some of the trace code (this is the fast path)

5b51: 49 8b 7d 08 mov 0x8(%r13),%rdi
5b55: 44 89 e2 mov %r12d,%edx
5b58: 48 89 de mov %rbx,%rsi
5b5b: 41 ff 55 00 callq *0x0(%r13)
5b5f: 49 83 c5 10 add $0x10,%r13
5b63: 49 83 7d 00 00 cmpq $0x0,0x0(%r13)
5b68: eb 41 jmp 5bab <set_task_cpu+0xba>

Below is the continuation of the fast path.

5b6a: 48 8b 43 08 mov 0x8(%rbx),%rax
5b6e: 44 39 60 18 cmp %r12d,0x18(%rax)
5b72: 74 0c je 5b80 <set_task_cpu+0x8f>


Again, I'm using gcc 4.6.0 and maybe it has been fixed.

-- Steve




\
 
 \ /
  Last update: 2012-02-22 16:15    [from the cache]
©2003-2011 Jasper Spaans