On Thu, 2020-04-23 at 10:40 +0000, xuemaosheng wrote:
> Product: GCC
> Component: rtl-optimization
> Version: 7.3.0
>
>
> If we add the flag DO_PREDICATION in scheduling ebb, the compiler will try to
> predicate the insn when the producer insn has been issued,
> and put the consumer insn into suitable queue. For example as shown in
> schedule verbose dump file:
>
> ;; | insn | prio |
> ......................
> ;; | 387 | 27 | t1=a5==0 AGS0|AGS1
> ;; | 388 | 27 | pc={(t1!=0)?L184:pc} PCU
> ;; | 459 | 26 | ev10=[a0+0xbc0] AGL0|AGL1
> ......................
>
>
> ;; 1--> + 387 t1=a5==0 :AGS0|AGS1
>
> deferring rescan insn with uid = 459.
> ;; dependencies resolved: insn + 459 predicated
> ;; Ready-->Q: insn + 459: queued for 2 cycles (change queue
> index)
> ;; tick updated: insn + 459 into queue with cost=2
>
> insn 387 is a test insn, insn 388 is a jump insn, insn 459 is a load insn.
> After predicating, insn 459 convert into this form: [!t1] ev10 = [a0+0xbc0]
> and put insn 459 into queue[3]. INSN_TICK (459) = 3;
>
> After issuing jump insn 388, the compiler will try to resotre pattern in insn
> 459 as shown in the following dump files.
>
> ;; Ready list after ready_sort: + 388:94:prio=27
> ;; Ready list (t = 1): + 388:94:prio=27
> [1;1]:388
> ;; 1--> + 388 pc={(t1!=0)?L184:pc} :PCU
> restoring pattern for insn 459
> deferring rescan insn with uid = 459.
>
> However, the INSN_TICK of insn 459 doesn't calculate again.
> Actually, after restoring pattern, the insn can issue more earlier.
> If we recalculate the INSN_TICK of insn 459, we will get INSN_TICK (459) = 2,
> then the load insn 459 can issue at clock t = 2 instead of clock t = 3.
>
> So, can we add the following code to recalculate the INSN_TICK in function
> restore pattern?
>
> restore_pattern (dep_t dep, bool immediately)
> {
> rtx_insn *next = DEP_CON (dep);
> int tick = INSN_TICK (next);
>
> .........................
>
> if (DEP_TYPE (dep) == REG_DEP_CONTROL)
> {
> if (sched_verbose >= 5)
> fprintf (sched_dump, "restoring pattern for insn %d\n",
> INSN_UID (next));
> haifa_change_pattern (next, ORIG_PAT (next));
> + update_insn_after_change (next);
> + if ((TODO_SPEC (next) & (HARD_DEP | DEP_POSTPONED)) == 0)
> + {
> + fix_tick_ready (next);
> + tick = INSN_TICK (next);
> + }
> }
> ...........................
>
> I found the similar code in function apply_replacement (dep_t dep, bool
> immediately).
>
> I think this patch can improve instruction parallelism.
It's a possibility -- it'd need to be tested of course.
I don't think anyone is actively working on improving predication support, so
it's possible that you're going to find more issues. Also note you were looking
at gcc-7.3.0. Work like this should be done on the trunk -- that's what you'll
need to test against and produce patches against.
jeff
