Hello again. I'm currently working on x86 support in O3, and one of
the problems I've run into is that the stupd (store, update base)
microop expects to update the base register in completeAcc. I'm pretty
sure and Kevin has confirmed that completeAcc is never called on stores
in O3, I think partially because the instruction is considered executed
when the store gets sent to the load/store queue. Please fogive me Kevin
if I mangle the facts here. That might be because the store is
considered the memory system's problem at that point, and generally
nothing is done in store's completeAcc function anyway. This isn't true,
though, in either ARM or x86 when they try to update the base register
there.
One possible solution might be to call completeAcc right away if
there's no fault in translation. At that point, assuming there isn't
some sort of machine check fault because you stored to something you
shouldn't, the instruction really is at the end of it's life and can be
considered executed. This would, I think, also address the problem we
had with ARM where the updated base register would hang around until the
store completed instead of being immediately available for subsequent
instructions, and eliminate the need for splitting the operation into
multiple microops. There would be a performance hit (hopefully a small
one) from calling completeAcc all the time even when most of the time it
doesn't do anything. That may or may not matter.
Any thoughts? Any corrections, Kevin?
Gabe
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