Re: [Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[Andrew Pinski]

On Fri, Jan 16, 2009 at 10:37 AM, rguenth at gcc dot gnu dot org
 wrote:
>
>
> --- Comment #6 from rguenth at gcc dot gnu dot org  2009-01-16 18:37 
> ---
> Guaranteed by 7.18.1.4.
>


"These types are optional."

:)

[Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[rguenth at gcc dot gnu dot org]

--- Comment #6 from rguenth at gcc dot gnu dot org  2009-01-16 18:37 ---
Guaranteed by 7.18.1.4.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38835

[Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[pinskia at gmail dot com]

--- Comment #5 from pinskia at gmail dot com  2009-01-16 18:33 ---
Subject: Re:  field-insensitive PTA causes libstdc++ miscompiles

On Fri, Jan 16, 2009 at 10:30 AM, rguenth at gcc dot gnu dot org
 wrote:
>
> (void *)((intptr_t)&iptr + (intptr_t)p - (intptr_t)&iptr)
>
> <==>
>
> (void *)(intptr_t)p
>
> which is guaranteed by the std

No that is not guaranteed because of:
If the result cannot be represented in the integer type, the behavior
is undefined. The result need not be in the range of values of any
integer
type.

:).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38835

Re: [Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[Andrew Pinski]

On Fri, Jan 16, 2009 at 10:30 AM, rguenth at gcc dot gnu dot org
 wrote:
>
> (void *)((intptr_t)&iptr + (intptr_t)p - (intptr_t)&iptr)
>
> <==>
>
> (void *)(intptr_t)p
>
> which is guaranteed by the std

No that is not guaranteed because of:
If the result cannot be represented in the integer type, the behavior
is undefined. The result need not be in the range of values of any
integer
type.

:).

[Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[rguenth at gcc dot gnu dot org]

--- Comment #4 from rguenth at gcc dot gnu dot org  2009-01-16 18:30 ---
  void *p;
  intptr_t iptr = (intptr_t)p - (intptr_t)&iptr;

of course.  Then this is just

(void *)((intptr_t)&iptr + (intptr_t)p - (intptr_t)&iptr)

<==>

(void *)(intptr_t)p

which is guaranteed by the std


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38835

[Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[rguenther at suse dot de]

--- Comment #3 from rguenther at suse dot de  2009-01-16 18:27 ---
Subject: Re:  field-insensitive PTA causes
 libstdc++ miscompiles

On Fri, 16 Jan 2009, pinskia at gcc dot gnu dot org wrote:

> --- Comment #2 from pinskia at gcc dot gnu dot org  2009-01-16 18:22 
> ---
>   return reinterpret_cast<_Tp*>(reinterpret_cast<_UIntPtrType>(this)
> + _M_diff);
> 
> 
>(const struct _Fwd_list_node_base *) this p+ (long unsigned int) ((const
> struct _Relative_pointer_impl *) this)->_M_diff
> 
> 
> That seems fine.  In fact If we are failing because of aliasing issues, then 
> we
> have a bigger issue really because the code could have been written as:
>   return reinterpret_cast<_Tp*>(reinterpret_cast(this) +
> _M_diff);
> 
> And still be defined so I don't think this is really PR 36227.

No, char * is not allowed to change objects either.  What the above code
does is use the pointer -> intptr -> pointer rule by storing a pointer
as a relative offset to the store, like so:

  void *p;
  intptr_t iptr = (intptr_t)&iptr - (intptr_t)p;

and reconstructing p via

  (void *)((intptr_t)&iptr + iptr);

which is perfectly valid, but we break that because we fold the latter
to (void *)(&iptr + iptr) which is of course invalid.

Richard.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38835

[Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[pinskia at gcc dot gnu dot org]

--- Comment #2 from pinskia at gcc dot gnu dot org  2009-01-16 18:22 ---
  return reinterpret_cast<_Tp*>(reinterpret_cast<_UIntPtrType>(this)
+ _M_diff);


   (const struct _Fwd_list_node_base *) this p+ (long unsigned int) ((const
struct _Relative_pointer_impl *) this)->_M_diff


That seems fine.  In fact If we are failing because of aliasing issues, then we
have a bigger issue really because the code could have been written as:
  return reinterpret_cast<_Tp*>(reinterpret_cast(this) +
_M_diff);

And still be defined so I don't think this is really PR 36227.


-- 

pinskia at gcc dot gnu dot org changed:

   What|Removed |Added

  BugsThisDependsOn|36227   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38835

[Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles

[rguenth at gcc dot gnu dot org]

--- Comment #1 from rguenth at gcc dot gnu dot org  2009-01-16 13:36 ---
Mine.


-- 

rguenth at gcc dot gnu dot org changed:

   What|Removed |Added

 AssignedTo|unassigned at gcc dot gnu   |rguenth at gcc dot gnu dot
   |dot org |org
 Status|UNCONFIRMED |ASSIGNED
 Ever Confirmed|0   |1
   Last reconfirmed|-00-00 00:00:00 |2009-01-16 13:36:04
   date||


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38835