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Re: Scheduling and inserting NOPs


Hi!

In my earlier message I explained what people do today for processors
which need NOP instructions to be inserted.

Nobody is working on implementing a better approach.

I had a small discussion with Ian, which showed that my problem is not understood. I try to summarize my previous emails:


I have a processor with a 6 stages pipeline. Every stage is used for exactly one cycle. This means:
1) there can never be a hardware structural hazard (that's good)
2) scheduling is not needed (even better)


So what's the problem? The problem is that there are 2 stage which can read registers and there is no bypass/result-forwarding from the register-write-back to the first register-read-stage.
The following program is correct on my processor (pseudo-code):
A0 := &a
A0 := &b
NOP
A1 := 1;
A2 := (A0) + A1 ; contents of memory at label a plus 1
A3 := (A0) + A1 ; contents of memory at label b plus 1


There has to be a number of instructions (e.g. NOPs) between writing the address-register A0 and reading it for indirect memory- access. To solve this I need scheduling. I implemented the scheduler specification including its bypasses. The code is scheduled correctly, as I can see in the verbose output. The missing/empty cycles are visible - but empty.

Now I need a pass to insert the missing NOPs. The approach Ian mentioned with the DFA-simulator does not work, because the simulator detects structural hardware hazards only (see http://www.nabble.com/Re %3A-Question-of-the-DFA-scheduler-p603486.html), which can not occur in my processor. Or did the DFA change?

NOP-insertion could be done in the hook TARGET_SCHED_REORDER with a little bit of book-keeping the cycles, but in this hook no code can be added.

In the thread above the ia64 port is mentioned, I have to check that. But isn't there a simple solution to insert NOPs?

Boris


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