Re: [PATCH] gimple, internal-fn: Add IFN_TRAP and use it for __builtin_unreachable [PR106099]

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jul 27, 2022 at 10:09:34AM +0000, Richard Biener wrote:
> > We chose to sanitize not just explicit user __builtin_unreachable calls,
> > but also the internally generated __builtin_unreachable calls (with the
> > one exception of fall through to end of C++ function returning non-void,
> > which had before a separate sanitizer) and we've been doing it since 2013
> > when ubsan was added.
> > Even for the internally generated unreachable calls like those from
> > devirtualization or other reasons like ivcanon/unrolling, having the
> > possibility to get some runtime diagnostics or trap can be useful over
> > just falling through to random following code.
> 
> So at least for the unrolling use the intent is to have the
> unreachable () fully elided by later passes.  Honza can correct me
> if I'm wrong.  Using __builtin_trap from the start until sanopt
> may prevent some of that from happening, keeping dead conditions
> live, no?

That is true.
I guess changing the sanopt gate back and building __builtin_unreachable
only in the ivcanon/unrolling case is possible too.

Without or with this patch then, the advantage of the patch would be that
we wouldn't need to recompute vops if we replace unreachables with traps
during the sanopt pass.
> 
> > Previously we'd always emit __builtin_unreachable, then perhaps in some
> > cases could e.g. optimize it away (say if there is a guarding condition
> > around the implicitly added unreachable turning the condition into VRP
> > info and optimizing the conditional away), otherwise the sanopt pass
> > would turn those __builtin_unreachable calls into __builtin_trap.
> > With the recent changes, we don't run the sanopt pass when only
> > doing -fsanitize=unreachable (or -funrechable-traps) though, so we need
> > to emit the trap/__ubsan_handle_unreachable/__builtin_unreachable right
> > away.
> 
> Why did the recent changes not just replace __builtin_unreachable
> at RTL expansion time?  Was the intent really to force the paths
> to be kept live?  I can see that for user or frontend generated
> unreachables but not so much for some of the middle-end ones.

It is easier on GIMPLE and perhaps allows e.g. sharing the data for
__ubsan_handle_unreachable calls.  sanopt pass is quite late anyway.

	Jakub