Re: [PATCH V2] Disable sched1 in functions that call setjmp

[Qing Zhao <qing.zhao@oracle.com>]

> On Dec 22, 2022, at 12:56 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
> 
> 
> On Thu, 22 Dec 2022, Jose E. Marchesi via Gcc-patches wrote:
> 
>> The first instruction scheduler pass reorders instructions in the TRY
>> block in a way `b=true' gets executed before the call to the function
>> `f'.  This optimization is wrong, because `main' calls setjmp and `f'
>> is known to call longjmp.
>> 
>> As discussed in BZ 57067, the root cause for this is the fact that
>> setjmp is not properly modeled in RTL, and therefore the backend
>> passes have no normalized way to handle this situation.
>> 
>> As Alexander Monakov noted in the BZ, many RTL passes refuse to touch
>> functions that call setjmp.  This includes for example gcse,
>> store_motion and cprop.  This patch adds the sched1 pass to that list.
>> 
>> Note that the other instruction scheduling passes are still allowed to
>> run on these functions, since they reorder instructions within basic
>> blocks, and therefore they cannot cross function calls.
>> 
>> This doesn't fix the fundamental issue, but at least assures that
>> sched1 wont perform invalid transformation in correct C programs.
> 
> I think scheduling across calls in the pre-RA scheduler is simply an oversight,
> we do not look at dataflow information and with 50% chance risk extending
> lifetime of a pseudoregister across a call, causing higher register pressure at
> the point of the call, and potentially an extra spill.

I am a little confused, you mean pre-RA scheduler does not look at the data flow
 information at all when scheduling insns across calls currently?

Qing


> 
> Therefore I would suggest to indeed solve the root cause, with (untested):
> 
> diff --git a/gcc/sched-deps.cc b/gcc/sched-deps.cc
> index 948aa0c3b..343fe2bfa 100644
> --- a/gcc/sched-deps.cc
> +++ b/gcc/sched-deps.cc
> @@ -3688,7 +3688,13 @@ deps_analyze_insn (class deps_desc *deps, rtx_insn *insn)
> 
>       CANT_MOVE (insn) = 1;
> 
> -      if (find_reg_note (insn, REG_SETJMP, NULL))
> +      if (!reload_completed)
> +       {
> +         /* Do not schedule across calls, this is prone to extending lifetime
> +            of a pseudo and causing extra spill later on.  */
> +         reg_pending_barrier = MOVE_BARRIER;
> +       }
> +      else if (find_reg_note (insn, REG_SETJMP, NULL))
>         {
>           /* This is setjmp.  Assume that all registers, not just
>              hard registers, may be clobbered by this call.  */
> 
> Alexander