Re: [PATCH 2/2] tree-optimization/109123 - run -Wuse-afer-free only early

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Mar 15, 2023 at 10:49:19AM +0000, Richard Biener via Gcc-patches wrote:
> The following switches the -Wuse-after-free diagnostics from emitted
> during the late access warning passes to the early access warning
> passes to make sure we run before passes performing code motion run
> which are the source of a lot of false positives on use-after-free
> not involving memory operations.
> 
> The patch also fixes issues in c-c++-common/Wuse-after-free-6.c and
> g++.dg/warn/Wuse-after-free3.C.
> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu (without 1/2
> sofar, but its testcase XFAILed).
> 
> OK?
> 
> Thanks,
> Richard.
> 
> 	PR tree-optimization/109123
> 	* gimple-ssa-warn-access.cc (pass_waccess::warn_invalid_pointer):
> 	Do not emit -Wuse-after-free late.
> 	(pass_waccess::check_call): Always check call pointer uses.
> 
> 	* gcc.dg/Wuse-after-free-pr109123.c: New testcase.
> 	* c-c++-common/Wuse-after-free-6.c: Un-XFAIL case.
> 	* g++.dg/warn/Wuse-after-free3.C: Remove expected duplicate
> 	diagnostic.

I guess this is this is related to the never ending debate of what to
do with the middle end late warnings, whether we should kill them
altogether, move into analyzer framework and/or run early but perform
IPA analysis for them.

Doing the diagnostics in the early waccess pass is certainly fine
no matter what, it will emit there more accurate diagnostics.
A big question is whether to avoid diagnosing it late as well (other option
is just make sure we don't diagnose same statements twice, once in early
waccess and once in late waccess, which can be solved with warning
suppressions (unless it is already properly suppressed)), especially for
GCC 13.
I see we have two copies of early waccess (so, we need to get the
suppression right even if the warning is just moved to early waccess),
one is very shortly after building ssa, so before early inlining,
then there is another early one for -O+ non-Og shortly after IPA,
and the late one is immediately before expansion.

So, the question is if the ccp after IPA is sufficient to uncover
the most of the use after free issues we want to diagnose, if yes,
then perhaps your patch is ok but we need to decide what to do about
-Og, whether we shouldn't add
      NEXT_PASS (pass_warn_access, /*early=*/true);
into -Og pass queue after 
      NEXT_PASS (pass_object_sizes);
or so.

I think even the pre-IPA optimizations can do some harm for use after free
false positives, but hopefully not as much as the post IPA optimizations.

	Jakub