I think that's -why- it's not written until commit, unless that
changed recently. The serialization support makes instructions wait
after decode, I think, until the serializing instruction commits.
Gabe
Quoting Ali Saidi <sa...@umich.edu>:
That actually isn't required. Since the update only happens at commit
the instruction is non-speculative at that point in time.
Ali
On
Mon, 30 May 2011 19:12:50 -0700, Gabe Black wrote: You want to mark
them nonspeculative too. Generally speaking, you can't undo a
speculative update to a miscreg.
Gabe
On 05/30/11 16:23, Ali Saidi
wrote:
Korey is pretty much correct. The miscellaneous registers are
read at any time, but they're only written at commit. So if you're
updating the state in some way that effects future instructions the code
that does the update must be marked IsSerializeAfter which prevents any
future instructions from issuing until the current one commits.
Ali
On May 29, 2011, at 11:49 PM, Korey Sewell wrote:
If you don't mind
my asking, how is this made safe? Is it some form of speculation?
Misc. registers refer to non-speculative system state, so any
instruction that is going to change these is typically marked with a
Serializing or IsNonSpeculative flag.
The CPU Models can then treat
these type of instructions appropriately. For the O3, I think it will
wait to the instruction reaches the head of the ROB to eventually
execute and commit it.
Can you track the path of that instruction in
O3 to confirm that? --
- Korey
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