[Bug tree-optimization/60656] [4.8/4.9 regression] x86 vectorization produces wrong code

["congh at google dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60656

Cong Hou <congh at google dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |congh at google dot com

--- Comment #2 from Cong Hou <congh at google dot com> ---
This bug is caused by an optimization in GCC vectorizer that is not
implemented properly. When a reduction operation is vectorized, the
order of elements in vectors directly used in reduction does not
matter. In some cases the vectorizer may generate less code based on
this fact. GCC assigns a property named "vect_used_by_reduction" to
all vectors participating in reductions. However, vectors that are
indirectly used in reduction also have this property. For example,
consider the following three statements (all operands are vectors):

a = b op1 c;
d = a op2 e;
s1 = s0 op3 d;

Here assume the last statement is a reduction one, then a,b,c,d,e all
have the property "vect_used_by_reduction". However, if op2 is
different from op3, then a's element order can affect the final
result. GCC does not check this.