Mesa (master): nir_opt_deref: ptr_as_array(deref_cast<T*>(x))[0] isn't the same as x[0] if the cast has alignment

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Module: Mesa
Branch: master
Commit: 4b69ae8e1eaaf6591f8e76ec64bf0bc2dcfeb019
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=4b69ae8e1eaaf6591f8e76ec64bf0bc2dcfeb019

Author: Jesse Natalie <jenatali at microsoft.com>
Date:   Sun Apr 11 19:15:35 2021 -0700

nir_opt_deref: ptr_as_array(deref_cast<T*>(x))[0] isn't the same as x[0] if the cast has alignment

This breaks CLOn12's handling of CL CTS test_basic vector_creation for char3 (at least).
Removing this cast causes us to try to load from a deref with no alignment info.

Fixes: 99bb2a4d ("nir/opt_deref: Don't remove casts with alignment information")
Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10165>

---

 src/compiler/nir/nir_deref.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/compiler/nir/nir_deref.c b/src/compiler/nir/nir_deref.c
index 73a6d39a4a4..bc8c4e80b28 100644
--- a/src/compiler/nir/nir_deref.c
+++ b/src/compiler/nir/nir_deref.c
@@ -1123,7 +1123,8 @@ opt_deref_ptr_as_array(nir_builder *b, nir_deref_instr *deref)
    if (nir_src_is_const(deref->arr.index) &&
        nir_src_as_int(deref->arr.index) == 0) {
       /* If it's a ptr_as_array deref with an index of 0, it does nothing
-       * and we can just replace its uses with its parent.
+       * and we can just replace its uses with its parent, unless it has
+       * alignment information.
        *
        * The source of a ptr_as_array deref always has a deref_type of
        * nir_deref_type_array or nir_deref_type_cast.  If it's a cast, it
@@ -1132,6 +1133,7 @@ opt_deref_ptr_as_array(nir_builder *b, nir_deref_instr *deref)
        * opt_deref_cast() above.
        */
       if (parent->deref_type == nir_deref_type_cast &&
+          parent->cast.align_mul == 0 &&
           is_trivial_deref_cast(parent))
          parent = nir_deref_instr_parent(parent);
       nir_ssa_def_rewrite_uses(&deref->dest.ssa,