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. */
Alexander
