[Bug tree-optimization/24262] [4.1 Regression] ICE: verify_ssa failed with -O -msse2 -ftree-vectorize

["dorit at il dot ibm dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #2 from dorit at il dot ibm dot com  2005-10-12 07:23 -------
There are two problems here:

1) This is the data-reference structure created (using the same testcase but
with floats instead of doubles):
        Created dr for A[D.1705_7]
          base_address: &A
          offset from base address: (<unnamed type>) (D.1703_5 * 8)
          constant offset from base address: 0
          base_object: A
          step: 4B
          misalignment from base:
          memtag: A
We then use the information in the fields above to calculate the first address
accessed, as follows:
        base_off.31_18 = D.1703_5 * 8;
        vect_pA.32_19 = &A + base_off.31_18;
but the problem is that we can't use the expression "D.1703_5 * 8" from field
"offset from base address" as is, because D.1703_5 depends on other
computations that should take place first. I think we had this problem in the
past but it had been solved. I wonder why it reappeared - maybe this is related
to the second problem:

2) This loop shouldn't be vectorized in the first place, because the vectorizer
can currently handle only consecutive accesses, but this loop accesses
locations {0,1,4,5,8,9}. For some reason (scalar evolution bug? dataref
analysis bug?) we conclude that the step of the access is simply 4B (i.e.
consecutive accesses).
(Indeed for doubles, pairs of iterations can be vectorized ({0,1} {4,5} and
{8,9}) with a proper stride between the vectorized iterations, but the
vectorizer can't do something like that now).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24262