Re: [PATCH][RFC] Sanitize equals and hash functions in hash-tables.

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, May 21, 2019 at 12:07 AM Jeff Law <law@redhat.com> wrote:
>
> On 5/13/19 1:41 AM, Martin Liška wrote:
> > On 11/8/18 9:56 AM, Martin Liška wrote:
> >> On 11/7/18 11:23 PM, Jeff Law wrote:
> >>> On 10/30/18 6:28 AM, Martin Liška wrote:
> >>>> On 10/30/18 11:03 AM, Jakub Jelinek wrote:
> >>>>> On Mon, Oct 29, 2018 at 04:14:21PM +0100, Martin Liška wrote:
> >>>>>> +hashtab_chk_error ()
> >>>>>> +{
> >>>>>> +  fprintf (stderr, "hash table checking failed: "
> >>>>>> +           "equal operator returns true for a pair "
> >>>>>> +           "of values with a different hash value");
> >>>>> BTW, either use internal_error here, or at least if using fprintf
> >>>>> terminate with \n, in your recent mail I saw:
> >>>>> ...different hash valueduring RTL pass: vartrack
> >>>>>                     ^^^^^^
> >>>> Sure, fixed in attached patch.
> >>>>
> >>>> Martin
> >>>>
> >>>>>> +  gcc_unreachable ();
> >>>>>> +}
> >>>>>   Jakub
> >>>>>
> >>>> 0001-Sanitize-equals-and-hash-functions-in-hash-tables.patch
> >>>>
> >>>> From 0d9c979c845580a98767b83c099053d36eb49bb9 Mon Sep 17 00:00:00 2001
> >>>> From: marxin <mliska@suse.cz>
> >>>> Date: Mon, 29 Oct 2018 09:38:21 +0100
> >>>> Subject: [PATCH] Sanitize equals and hash functions in hash-tables.
> >>>>
> >>>> ---
> >>>>  gcc/hash-table.h | 40 +++++++++++++++++++++++++++++++++++++++-
> >>>>  1 file changed, 39 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/gcc/hash-table.h b/gcc/hash-table.h
> >>>> index bd83345c7b8..694eedfc4be 100644
> >>>> --- a/gcc/hash-table.h
> >>>> +++ b/gcc/hash-table.h
> >>>> @@ -503,6 +503,7 @@ private:
> >>>>
> >>>>    value_type *alloc_entries (size_t n CXX_MEM_STAT_INFO) const;
> >>>>    value_type *find_empty_slot_for_expand (hashval_t);
> >>>> +  void verify (const compare_type &comparable, hashval_t hash);
> >>>>    bool too_empty_p (unsigned int);
> >>>>    void expand ();
> >>>>    static bool is_deleted (value_type &v)
> >>>> @@ -882,8 +883,12 @@ hash_table<Descriptor, Allocator>
> >>>>    if (insert == INSERT && m_size * 3 <= m_n_elements * 4)
> >>>>      expand ();
> >>>>
> >>>> -  m_searches++;
> >>>> +#if ENABLE_EXTRA_CHECKING
> >>>> +    if (insert == INSERT)
> >>>> +      verify (comparable, hash);
> >>>> +#endif
> >>>>
> >>>> +  m_searches++;
> >>>>    value_type *first_deleted_slot = NULL;
> >>>>    hashval_t index = hash_table_mod1 (hash, m_size_prime_index);
> >>>>    hashval_t hash2 = hash_table_mod2 (hash, m_size_prime_index);
> >>>> @@ -930,6 +935,39 @@ hash_table<Descriptor, Allocator>
> >>>>    return &m_entries[index];
> >>>>  }
> >>>>
> >>>> +#if ENABLE_EXTRA_CHECKING
> >>>> +
> >>>> +/* Report a hash table checking error.  */
> >>>> +
> >>>> +ATTRIBUTE_NORETURN ATTRIBUTE_COLD
> >>>> +static void
> >>>> +hashtab_chk_error ()
> >>>> +{
> >>>> +  fprintf (stderr, "hash table checking failed: "
> >>>> +     "equal operator returns true for a pair "
> >>>> +     "of values with a different hash value\n");
> >>>> +  gcc_unreachable ();
> >>>> +}
> >>> I think an internal_error here is probably still better than a simple
> >>> fprintf, even if the fprintf is terminated with a \n :-)
> >> Fully agree with that, but I see a lot of build errors when using internal_error.
> >>
> >>> The question then becomes can we bootstrap with this stuff enabled and
> >>> if not, are we likely to soon?  It'd be a shame to put it into
> >>> EXTRA_CHECKING, but then not be able to really use EXTRA_CHECKING
> >>> because we've got too many bugs to fix.
> >> Unfortunately it's blocked with these 2 PRs:
> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87845
> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87847
> > Hi.
> >
> > I've just added one more PR:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90450
> >
> > I'm sending updated version of the patch that provides a disablement for the 3 PRs
> > with a new function disable_sanitize_eq_and_hash.
> >
> > With that I can bootstrap and finish tests. However, I've done that with a patch
> > limits maximal number of checks:
> So rather than call the disable_sanitize_eq_and_hash, can you have its
> state set up when you instantiate the object?  It's not a huge deal,
> just thinking about loud.
>
>
>
> So how do we want to go forward, particularly the EXTRA_EXTRA checking
> issue :-)

There is at least one PR where we have a table where elements _in_ the
table are never compared against each other but always against another
object (I guess that's usual even), but the setup is in a way that the
comparison function only works with those.  With the patch we verify
hashing/comparison for something that is never used.

So - wouldn't it be more "correct" to only verify comparison/hashing
at lookup time, using the object from the lookup and verify that against
all other elements?

Richard.

>
> Jeff