[PATCH] PR30975 - remove wrong assert (fix vectorizer ICE)

[Dorit Nuzman <DORIT at il dot ibm dot com>]

Hi,

The assert on which the testcase in this PR ICEs is plain wrong - I don't
know what I was thinking when I put it there in the first place. The patch
below simply removes the assert. Here's the long explanation why:

The situation in question is a loop with induction and with
multiple-data-sizes. More specifically - vectorizing an induction using a
vector-size VS that cannot fit VF elements (VF = vectorization factor).
This can happen, for example, if the induction is of type int, and there is
another operation in the loop of type char, for which the VF=16; We need to
generate 4 vector operations in order to compute 16 integer results when we
vectorize the induction (i.e. ncopies = 4). This is the code that is
generated:

      vector int vi;
loop:

[1]   vi_0 = phi < {0,1,2,3}, vi_1 >
      ...
[2]   vi_2 = vi_0 + {4,4,4,4}
[3]   vi_3 = vi_2 + {4,4,4,4}
[4]   vi_4 = vi_3 + {4,4,4,4}
      ...
[5]   vi_1 = vi_0 + {16,16,16,16}

When we vectorize a stmt that uses the induction variable - we call
'vect_get_vec_def_for_operand' to create the above def-use cycle that
computes the vector induction, and it returns the first vector induction
def (vi_0). For the rest of the copies, we call
'vect_get_vec_def_for_stmt_copy' (to get vi_2, vi_3, and vi_4). The ICE
occurs in 'vect_get_vec_def_for_stmt_copy' where we (wrongly) assert that
the previous vector-stmt is a phi-node. This is only true for the first
"copy" - i.e. stmt [2]. It is not true for the rest of the copies (stmts
[3],[4]).

What bothered me is how come none of the induction testcases caught this.
Turns out the testcase that was supposed to test this functionality -
vect-iv-8.c - was accidentally missing the loop-update - so instead of an
induction, we have an invariant variablbe... Also, the offending assert is
always true for the first copy-stmt (and only for it), so when ncopies = 2
(which is the case in vect-iv-8.c) we don't ICE (cause in that case we have
only one copy stmt)...

Bootsrapped on powerpc-linux and i386-linux.
Tested on the vectorizer testcases on these platforms.

OK for mainline?

thanks,
dorit

:ADDPATCH vectorizer (non-algorithmic):

       * tree-vect-trasnform.c (vect_get_vec_def_for_stmt_copy): Remove
        wrong assert.

        * gcc.dg/vect/vect-iv-8.c: Fix to include an induction. Xfail.
        * gcc.dg/vect/vect-iv-8a.c: New (same as above, but signed).
        * gcc.dg/vect/pr30795.c: New.

(See attached file: feb18.pr30795.txt)

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