Re: PING^1 [PATCH v3] sched: Change no_real_insns_p to no_real_nondebug_insns_p [PR108273]

[Alexander Monakov <amonakov@ispras.ru>]

On Thu, 9 Nov 2023, Maxim Kuvyrkov wrote:

> Hi Kewen,
> 
> Below are my comments.  I don't want to override Alexander's review, and if
> the patch looks good to him, it's fine to ignore my concerns.
> 
> My main concern is that this adds a new entity -- forceful skipping of
> DEBUG_INSN-only basic blocks -- to the scheduler for a somewhat minor change
> in behavior.  Unlike NOTEs and LABELs, DEBUG_INSNs are INSNS, and there is
> already quite a bit of logic in the scheduler to skip them _as part of normal
> operation_.

I agree with the concern. I hoped that solving the problem by skipping the BB
like the (bit-rotted) debug code needs to would be a minor surgery. As things
look now, it may be better to remove the non-working sched_block debug counter
entirely and implement a good solution for the problem at hand.

> 
> Would you please consider 2 ideas below.
> 
> #1:
> After a brief look, I'm guessing this part is causing the problem:
> haifa-sched.cc <http://haifa-sched.cc/>:schedule_block():
> === [1]
>   /* Loop until all the insns in BB are scheduled.  */
>   while ((*current_sched_info->schedule_more_p) ())
>     {
>       perform_replacements_new_cycle ();
>       do
> 	{
> 	  start_clock_var = clock_var;
> 
> 	  clock_var++;
> 
> 	  advance_one_cycle ();

As I understand, we have spurious calls to advance_one_cycle on basic block
boundaries, which don't model the hardware (the CPU doesn't see BB boundaries)
and cause divergence when passing through a debug-only BB which would not be
present at all without -g.

Since EBBs and regions may not have jump targets in the middle, advancing
a cycle on BB boundaries does not seem well motivated. Can we remove it?

Can we teach haifa-sched to emit RTX NOTEs with hashes of DFA states on BB
boundaries with -fcompare-debug is enabled? It should make the problem
readily detectable by -fcompare-debug even when scheduling did not diverge.

Alexander