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SubjectRe: [PATCH 0/4] jump label patches
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~


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