[Bug tree-optimization/118976] [12/13/14/15 regression] Correctness Issue: SVE vectorization results in data corruption when cpu has 128bit vectors but compiled with -mcpu=neoverse-v1 (which is only for 256bit vector cpus)

[avieira at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118976

avieira at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |avieira at gcc dot gnu.org

--- Comment #13 from avieira at gcc dot gnu.org ---
Looked a bit into this and fre4 takes:

_30 = { POLY_INT_CST [4, 4], POLY_INT_CST [5, 4], POLY_INT_CST [6, 4], ... };
...
vect__3.13_46 = ~_30;

and simplifies that ~_30 to a constant:
{ POLY_INT_CST [-4, -4], POLY_INT_CST [-5, -4], POLY_INT_CST [-6, -4], ... }

Which seems wrong to me, as ~(4) = -5, not -4

looking at const_binop in fold-const.cc I see:
    case BIT_NOT_EXPR:
      if (TREE_CODE (arg0) == INTEGER_CST)
        return fold_not_const (arg0, type);
      else if (POLY_INT_CST_P (arg0))
        return wide_int_to_tree (type, -poly_int_cst_value (arg0));
      /* Perform BIT_NOT_EXPR on each element individually.  */
      else if (TREE_CODE (arg0) == VECTOR_CST)
...

the VECTOR_CST just goes over the elements in the VECTOR_CST and calls this
recursively, making the change:
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -1964,7 +1964,7 @@ const_unop (enum tree_code code, tree type, tree arg0)
       if (TREE_CODE (arg0) == INTEGER_CST)
        return fold_not_const (arg0, type);
       else if (POLY_INT_CST_P (arg0))
-       return wide_int_to_tree (type, -poly_int_cst_value (arg0));
+       return wide_int_to_tree (type, ~poly_int_cst_value (arg0));
       /* Perform BIT_NOT_EXPR on each element individually.  */
       else if (TREE_CODE (arg0) == VECTOR_CST)
        {

fixes it for me.

I'm going on holidays tomorrow, so I'll leave this for someone with a bit more
time to pick up. Richard S it seems to have been your change in:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=36fd64086542ed734aded849304723218fa4d6fd
so I'll wait for others to confirm my thinking here, I always get nervous
around bit fiddling ;)