On Fri, Sep 05, 2025 at 09:37:42AM +0200, Richard Biener wrote:
> Yes. I'll note that while we fixed some issues with code motion across
> possible exit points we do not have good means of testing for invalid
> code motion of UB invoking stmts. Guessing from the ones that actually
> caused traps the problem is likely bigger for UB causing stmts (I for
> example fixed PRE for trap(), but not for general UB - we'd have to
> avoid PREing all variable shifts or any signed arithmetic).
>
> For what you describe returns_twice would work given it adds an
> incoming edge, making the call first in a BB. This prevents any
> code motion upwards, it doesn't require us to classify stmts into
> those we can speculate and those we can't. On RTL a BARRIER
> might do the trick?
BARRIER in RTL is like __builtin_unreachable (), so the observable
checkpoint certainly can't be represented like that.
Anyway, my point is that one call call std::observable_checkpoint ()
or exit (0) in any external function (unless say some standard describes
what the function does), so we really should be creating any non-builtin
call we know nothing about like that.
Jakub
