[Bug tree-optimization/111648] [14 Regression] Wrong code at -O2/3 on x86_64-linux-gnu since r14-3243-ga7dba4a1c05

["prathamesh3492 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111648

--- Comment #3 from prathamesh3492 at gcc dot gnu.org ---
Created attachment 56037
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56037&action=edit
Untested fix

The issue is that when a1 is a multiple of vector length, we end up creating
following encoding in result: { base_elem, arg[0], arg[1], ... } where arg is
chosen input vector, which is incorrect.

For above case, vectorizer pass creates VEC_PERM_EXPR<arg0, arg, sel> where:
arg0: { -16, -9, -10, -11 } 
arg1: { -12, -5, -6, -7 } 
sel = { 3, 4, 5, 6 }

arg0, arg1 and sel are encoded with npatterns = 1 and nelts_per_pattern = 3.
Since a1 = 4 and arg_len = 4, it ended up creating the result with
following encoding:
res = { arg0[3], arg1[0], arg1[1] } // npatterns = 1, nelts_per_pattern = 3
    = { -11, -12, -5 }

So for res[4], it used S = (-5) - (-12) = 7
And hence computed it as -5 + 7 = 2.
instead of arg1[2], ie, -6.
which is the difference we see in output at -O0 vs -O2.

The patch tweaks the constratints in valid_mask_for_fold_vec_perm_cst_p to punt
if a1 is a multiple of vector length, so a1 ... ae only selects from stepped
part of the input vector, which seems to fix this issue.
I will run a proper bootstrap+test and post it upstream.