[Bug target/111720] RISC-V: Ugly codegen in RVV

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111720

--- Comment #16 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is that we cannot CSE a VLA typed "load" (whatever that is) to a
constnant.

    char arr[] = {1, 2, 7, 1, 3, 4, 5, 3, 1
    , 0, 1, 2, 4, 4, 9, 9, 1, 2, 7, 1, 3, 4, 5, 3, 
    1, 0, 1, 2, 4, 4, 9, 9};
    char m = 1;

    svint8_t varr = *(svint8_t*)arr;

we don't know what portion of 'arr' this accesses.  The relevant bit in
vn_reference_lookup_3 would be

  /* 3) Assignment from a constant.  We can use folds native encode/interpret
     routines to extract the assigned bits.  */
  else if (known_eq (ref->size, maxsize)
           && is_gimple_reg_type (vr->type)
           && !reverse_storage_order_for_component_p (vr->operands)
           && !contains_storage_order_barrier_p (vr->operands)
           && gimple_assign_single_p (def_stmt)
           && CHAR_BIT == 8
           && BITS_PER_UNIT == 8
           && BYTES_BIG_ENDIAN == WORDS_BIG_ENDIAN
           /* native_encode and native_decode operate on arrays of bytes
              and so fundamentally need a compile-time size and offset.  */
           && maxsize.is_constant (&maxsizei)
           && offset.is_constant (&offseti)
           && (is_gimple_min_invariant (gimple_assign_rhs1 (def_stmt))
               || (TREE_CODE (gimple_assign_rhs1 (def_stmt)) == SSA_NAME
                   && is_gimple_min_invariant (SSA_VAL (gimple_assign_rhs1
(def_

and we fail at maxsize.is_constant (&maxsizei), that's the actual size of
the load.  Maybe there's constraints that are target specific and not
encoded in poly-int that could be used here, but I don't really know.

So yes, pieces of the compiler are defensive about VLA accesses and
they probably have to be.

In particular this part of VN doesn't try to use undefinedness (the access
exceeds the size of 'arr') to limit things - but in the end we'd still
need to construct a VLA typed constant and I have no idea how to do that.

Maybe Richard has an idea.

Note this has nothing to do about whether we have a CLOBBER or not.  You
can "disable" those with -fstack-reuse=none and that doesn't make a
difference.