On Fri, Sep 05, 2025 at 08:51:10AM +0100, Iain Sandoe wrote:
> > 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.
>
> A volatile read would work, I guess - but the issue then is where to read
> from - the intent of the paper is to generate no code, i.e. that the function
> is only there for it’s effect as a checkpoint.
I think a volatile read is not sufficient for preventing time travel.
I think a CALL_INSN is safe (exactly because it could have done exit (0)
or std::observable_checkpoint () inside of it where the compiler doesn't see
it) but in RTL has the major disadvantage (even if optimized away during
final) that RA clobbers all call clobbered registers there.
I wonder if we treat a volatile asm similarly (especially with "memory"
clobber), because it actually might contain a call, though obviously RA
doesn't add the cost of clobbering call clobbered registers there (if one
calls from inline asm, one needs to save/restore those registers or use
a different ABI with no call clobbered registers or only the ones specified
in clobbers of the inline asm). And inline asm can expand to nothing or
almost nothing (the latter if it has inputs/outputs, but this one doesn't
need any, just "memory").
Jakub
