[PATCH][RFC] Poison bitmap_head->obstack

Martin Sebor msebor@gmail.com
Sat Dec 8 18:19:00 GMT 2018

On 12/7/18 1:10 AM, Richard Biener wrote:
> On Thu, 6 Dec 2018, Martin Sebor wrote:
>> On 12/5/18 7:58 AM, Richard Biener wrote:
>>> On Wed, 5 Dec 2018, Jeff Law wrote:
>>>> On 12/4/18 6:16 AM, Richard Biener wrote:
>>>>> This tries to make bugs like that in PR88317 harder to create by
>>>>> introducing a bitmap_release function that can be used as
>>>>> pendant to bitmap_initialize for non-allocated bitmap heads.
>>>>> The function makes sure to poison the bitmaps obstack member
>>>>> so the obstack the bitmap was initialized with can be safely
>>>>> released.
>>>>> The patch also adds a default constructor to bitmap_head
>>>>> doing the same, but for C++ reason initializes to a
>>>>> all-zero bitmap_obstack rather than 0xdeadbeef because
>>>>> the latter isn't possible in constexpr context (it is
>>>>> by using unions but then things start to look even more ugly).
>>>>> The stage1 compiler might end up with a few extra runtime
>>>>> initializers but constexpr makes sure they'll vanish for
>>>>> later stages.
>>>>> I had to paper over that you-may-not-use-memset-to-zero classes
>>>>> with non-trivial constructors warning in two places and I
>>>>> had to teach gengtype about CONSTEXPR (probably did so in
>>>>> an awkward way - suggestions and pointers into gengtype
>>>>> appreciated).
>>>>> Bootstrapped (with host GCC 4.8) on x86_64-unknown-linux-gnu,
>>>>> testing in progress.
>>>>> The LRA issue seems to be rare enough (on x86_64...) that
>>>>> I didn't trip over it sofar.
>>>>> Comments?  Do we want this?  Not sure how we can easily
>>>>> discover all bitmap_clear () users that should really
>>>>> use bitmap_release (suggestion for a better name appreciated
>>>>> as well - I thought about bitmap_uninitialize)
>>>>> Richard.
>>>>> 2018-12-04  Richard Biener  <rguenther@suse.de>
>>>>> 	* bitmap.c (bitmap_head::crashme): Define.
>>>>> 	* bitmap.h (bitmap_head): Add constexpr default constructor
>>>>> 	poisoning the obstack member.
>>>>> 	(bitmap_head::crashme): Declare.
>>>>> 	(bitmap_release): New function clearing a bitmap and poisoning
>>>>> 	the obstack member.
>>>>> 	* gengtype.c (main): Make it recognize CONSTEXPR.
>>>>> 	* lra-constraints.c (lra_inheritance): Use bitmap_release
>>>>> 	instead of bitmap_clear.
>>>>> 	* ira.c (ira): Work around warning.
>>>>> 	* regrename.c (create_new_chain): Likewise.
>>>> I don't see enough complexity in here to be concerning -- so if it makes
>>>> it harder to make mistakes, then I'm for it.
>>> Any comment about the -Wclass-memaccess workaround sprinkling around two
>>> (void *) conversions?  I didn't dig deep enough to look for a more
>>> appropriate solution, also because there were some issues with older
>>> host compilers and workarounds we installed elsewhere...
>> Using '*head = du_head ();' is the solution I like to encourage
>> to zero out typed objects.  When the memory isn't initialized
>> and the type has a user-defined ctor, bypassing it is undefined
>> even if the ctor is a no-op (the ctor starts the lifetime of
>> the object).  In that case, placement new is the appropriate
>> way to bring the object to life and value-initialize it:
>>    new (head) du_head ();
>> If that's not good enough or portable enough to ancient compilers
>> then I would suggest adding a comment to explain the intent of
>> the cast.
> Yes, I know.  But we have workarounds like the following and I
> didn't want to open up another hole just for this debugging feature
>    /* Versions of GCC before 4.4 sometimes leave certain objects
>       uninitialized when value initialized, though if the type has
>       user defined default ctor, that ctor is invoked.  As a workaround
>       perform clearing first and then the value initialization, which
>       fixes the case when value initialization doesn't initialize due to
>       the bugs and should initialize to all zeros, but still allows
>       vectors for types with user defined default ctor that initializes
>       some or all elements to non-zero.  If T has no user defined
>       default ctor and some non-static data members have user defined
>       default ctors that initialize to non-zero the workaround will
>       still not work properly; in that case we just need to provide
>       user defined default ctor.  */
>    memset (dst, '\0', sizeof (T) * n);
> #endif
>    for ( ; n; ++dst, --n)
>      ::new (static_cast<void*>(dst)) T ();

Why not then introduce a couple of helpers to do it without
duplicating the workaround each time we find ourselves faced
with this problem?  Something like this perhaps:

   template <class T>
   inline void value_init (void *p, size_t n)
     for (T *q = static_cast<T*> (p); n; --n, ++q)
       ::new (q) T();
      memset (p, 0, n * sizeof (T));

   template <class T>
   inline void value_assign (T *p, size_t n)
     for ( ; n; --n, ++p)
       *p = T ();
      memset (p, 0, n * sizeof (T));

That way the code will be clean and portable at the same time,
and the workarounds easy to find and remove if we choose once
the broken compilers are no longer supported.


> Richard.
>> Martin
>>> Otherwise yes, it makes it harder to do mistakes.  I'll probably
>>> use bitmap_head::crashme instead of 0xdeadbeef in bitmap_release.
>>> And of course we'd need to hunt down users of bitmap_clear that
>>> should be bitmap_release instead...
>>> Thanks,
>>> Richard.

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