Alexander Monakov via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> 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.
>
> 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. */
+1 for trying this FWIW. There's still plenty of time to try an
alternative solution if there are unexpected performance problems.
Richard
