[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183

--- Comment #7 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-31 10:06:27 UTC ---
(In reply to comment #6)
> It depends on the specific values of (a) array end alignment and (b) the number
> of bytes read. As long as the array end + number of bytes read can cross a page
> boundary, you're potentially causing SEGV or other errors.

I don't think this can happen.  The access to the out-of-bounds area only
happens if there are pieces inluded in the last (aligned) vector move.
That vector move will be aligned so it can't cross page-boundary.  As
it contains at least one allocated element the access may not trap.

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #8 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-31 10:51:32 UTC ---
This is a non-bug.  The transformation is ok and will never cause a pagefault.

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[mikpe at it dot uu dot se]

------- Comment #6 from mikpe at it dot uu dot se  2010-05-20 11:05 -------
It depends on the specific values of (a) array end alignment and (b) the number
of bytes read. As long as the array end + number of bytes read can cross a page
boundary, you're potentially causing SEGV or other errors.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[irar at il dot ibm dot com]

------- Comment #5 from irar at il dot ibm dot com  2010-05-20 10:24 -------
Even if we are talking about less than vector size from array boundary? And
that boundary is not (vector) aligned.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[mikpe at it dot uu dot se]

------- Comment #4 from mikpe at it dot uu dot se  2010-05-20 10:18 -------
(In reply to comment #3)
> I am curious what is the problem with that? These elements are not used, they
> are just loaded... 

An out-of-bounds read can result in a SEGV if the memory is unmapped. Worse
things can happen if the memory is "special" (think kernels and MMIO).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[irar at il dot ibm dot com]

------- Comment #3 from irar at il dot ibm dot com  2010-05-20 10:04 -------
I am curious what is the problem with that? These elements are not used, they
are just loaded... 


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[hjl dot tools at gmail dot com]

------- Comment #2 from hjl dot tools at gmail dot com  2010-05-20 08:50 -------
(In reply to comment #1)
> Do you mean that extract_even implementation does something illegal with this
> last element? Misaligned load also accesses elements outside the array, but the
> problem is in extract_even?

Vectorizer generates

vect_var_.12_25 = M*vect_pb.7_23{misalignment: 32};
vect_var_.13_27 = M*vect_pb.7_26{misalignment: 32};

Those may read beyond the end of array. Vectorizer
should check that vect_pb.7_23/vect_pb.7_26 + vector
size < end of array.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183

[Bug tree-optimization/44183] Vectorizer may generate invalid memory access

[irar at il dot ibm dot com]

------- Comment #1 from irar at il dot ibm dot com  2010-05-20 07:13 -------
Do you mean that extract_even implementation does something illegal with this
last element? Misaligned load also accesses elements outside the array, but the
problem is in extract_even?

Other than doing something in the backend, we can reduce the number of vector
iterations in cases that may access elements outside array bounds for specific
targets...


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44183