Re: Dependences for call-preserved regs on exposed pipeline target?

[Maxim Kuvyrkov <maxim at codesourcery dot com>]

On 27/11/2012, at 4:34 AM, Greg McGary wrote:

> On 11/25/12 23:33, Maxim Kuvyrkov wrote:
>> You essentially need a fix-up pass just before the end of compilation (machine-dependent reorg, if memory serves me right) to space instructions consuming values from CPRs from the CALL_INSNS that set those CPRs.  I.e., for the 99% of compilation you don't care about this restriction, it's only the very last VLIW bundling and delay slot passes that need to know about it.
>> 
>> You, probably, want to make the 2nd scheduler pass run as machine-dependent reorg (as ia64 does) and enable an additional constraint (through scheduling bypass) for the scheduler DFA to space CALL_INSNs from their consumers for at least for 2 cycles.  One challenge here is that scheduler operates on basic blocks, and it is difficult to track dependencies across basic block boundaries.  To workaround basic-block scope of the scheduler you could emit dummy instructions at the beginning of basic blocks that have predecessors that end with CALL_INSNs.  These dummy instructions would set the appropriate registers (probably just assign the register to itself), and you will have a bypass (see define_bypass) between these dummy instructions and consumers to guarantee the 2-cycle delay.
> 
> Thanks for the advice.  We're already on the same page--I have most of what you
> recommend: I only schedule once from machine_dependent_reorg, after splitting
> loads/stores, calls/branches into "init" and "fini" phases bound at fixed clock
> offsets by record_delay_slot_pair().  I already have a fixup pass to handle
> inter-EBB hazards.  (The selective scheduler would handle interblock
> automatically, but I had trouble with it initially with split load/stores.  I want
> to revisit that.)  Regarding CPRs, I strongly desire to avoid kludgy fixups for
> schedules created with an incomplete dependence graph when the generic scheduler
> can do the job perfectly with a complete dependence graph.


I wonder if "kludgy fixups" refers to the dummy-instruction solution I mentioned above.  The complete dependence graph is a myth.  You cannot have a complete dependence graph for a function -- scheduler works on DAG regions (and I doubt it will ever support anything more complex), so you would have to do something to account for inter-region dependencies anyway.

It is simpler to have a unified solution that would handle both inter- and intra-region dependencies, rather than implementing two different approaches.

--
Maxim Kuvyrkov
CodeSourcery / Mentor Graphics