Jeremy Fitzhardinge wrote:
Ingo Molnar wrote:
And once we accept the static markers, we might as well make them as
cheap as possible.
Sure, so long as you take "as cheap as possible" to mean cheap in both
implementation complexity as well as runtime cost.
I don't have any specific objections to any of the stuff that Mathieu is
working on, but it does worry me that each time a problem is addressed
it ends up being an even more subtle piece of code. I just haven't seen
enough concrete justification to make me feel comfortable with it all.
It seems to me that a relatively simple implementation which allows the
desired tracing/marking functionality is the first step. If that proves
to cause a significant performance deficit then enabled then we can work
out how to address it in due course. But doing it all at once before
merging anything seems like overkill, particularly when we're talking
about specifics of gcc's codegen patterns, disassembling code fragments,
etc.
I really feel that the latest information that has come up has indicated
that things are really not what they should be. They are in line, have
a substantial probe cost, and we're messing around with how to jump
around them.
That's not the problem.
I maintain what I said before: a call instruction (which defaults to a
NOP), and then extract the state based on debugging info or assembler
annotations.
As far as patchable static jumps, I can see the utility of them, but I
don't think this project is one of them. However, I believe the right
way to do them is via compiler support.
-hpa
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