Re: [PATCH 3/3] Fix ubsan tests by disabling of an optimization.

[Jakub Jelinek <jakub at redhat dot com>]

On Thu, Jul 09, 2015 at 09:34:25AM -0600, Jeff Law wrote:
> On 07/09/2015 08:13 AM, Jakub Jelinek wrote:
> >On Thu, Jul 09, 2015 at 03:56:35PM +0200, mliska wrote:
> >>---
> >>  gcc/testsuite/g++.dg/ubsan/vptr-1.C | 2 +-
> >>  gcc/testsuite/g++.dg/ubsan/vptr-2.C | 2 +-
> >>  gcc/testsuite/g++.dg/ubsan/vptr-3.C | 2 +-
> >>  3 files changed, 3 insertions(+), 3 deletions(-)
> >
> >I'd actually think it would be better to give up on the
> >UBSAN_* internal calls in tail merging.  Those internal pass
> >arguments based on their gimple_location, so tail merging breaks
> >them.
> So I think the larger question here is should differences in gimple
> locations prevent tail merging?  I'd tend to think not, which then begs the

Generally no.

> question, are the UBSAN calls special enough to warrant an exception?

ASAN internal calls too I suppose.  And yes, I believe they are special
enough to warrant an exception.  The speciality is in them being lowered
later on into a call that is passed as one argument a structure containing
file:line into derived from that location, and for those calls that info is
very important (by using -fsanitize=address or -fsanitize=undefined
the user already says that he doesn't care that much about generated code
quality, the extra overhead is already there).  Another option would be for
-fsanitize=address or undefined etc. to disable various optimization options
(it does already disable some like non-null optimizations, because it is
essential, but I believe there is no reason not to perform tail merging
even with those options, as long as there are none of the problematic
internal calls involved, or if the locus is the same.  One could consider
gimple_location as yet another parameter to those internal calls, not
emitted directly just to avoid wasting compiler memory.

	Jakub