> 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.
Let me see if Alexander's patch fixes the issue at hand (it must) and
will also do some regression testing.
