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From: "bmei at broadcom dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/59651] Vectorizer failing to spot dependence causes incorrect code generation.
Date: Tue, 31 Dec 2013 16:53:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59651

--- Comment #1 from Bingfeng Mei <bmei at broadcom dot com> ---
That is interesting. On x86-64, GCC does say it cannot determine dist vector
between a[3] & a[b] and needs run-time aliasing test. In the end it gives up
due to too few iterations. 

note: === vect_analyze_data_ref_dependences ===
(compute_affine_dependence
  stmt_a: _5 = a[3];
  stmt_b: a[b.0_16] = _7;
(analyze_overlapping_iterations 
  (chrec_a = 3)
  (chrec_b = {3, +, -1}_1)
(analyze_siv_subscript 
)
  (overlap_iterations_a = [0])
  (overlap_iterations_b = [0]))
(Dependence relation cannot be represented by distance vector.) 
)
(compute_affine_dependence
  stmt_a: _5 = a[3];
  stmt_b: _5 = a[3];
(analyze_overlapping_iterations 
  (chrec_a = 3)
  (chrec_b = 3)
  (overlap_iterations_a = [0])
  (overlap_iterations_b = [0]))
)
(compute_affine_dependence
  stmt_a: a[b.0_16] = _7;
  stmt_b: a[b.0_16] = _7;
(analyze_overlapping_iterations 
  (chrec_a = {3, +, -1}_1)
  (chrec_b = {3, +, -1}_1)
  (overlap_iterations_a = [0])
  (overlap_iterations_b = [0]))
)
/projects/firepath_tools1_scratch/bmei/trunk/gcc/testsuite/gcc.dg/torture/pr52943.c:13:7:
note: versioning for alias required: bad dist vector for a[3] and a[b.0_16]
/projects/firepath_tools1_scratch/bmei/trunk/gcc/testsuite/gcc.dg/torture/pr52943.c:13:7:
note: mark for run-time aliasing test between a[3] and a[b.0_16]