On 11/6/24 6:02 PM, Richard Biener wrote:
On Wed, Nov 6, 2024 at 12:49 PM Tejas Belagod <tejas.bela...@arm.com> wrote:

Ensure sizeless types don't end up trying to be canonicalised to BIT_FIELD_REFs.

You mean variable-sized?  But don't we know, when there's a constant
array index,
that the size is at least so this indexing is OK?  So what's wrong with a
fixed position, fixed size BIT_FIELD_REF extraction of a VLA object?

Richard.


Ah! The code and comment/description don't match, sorry. This change started out as gating out all canonicalizations of VLA vectors when I had limited understanding of how this worked, but eventually was simplified to gate in only those offsets that were known_le, but missed out fixing the comment/description. So, for eg.

int foo (svint32_t v) { return v[3]; }

canonicalises to a BIT_FIELD_REF <v, 32, 96>

but something like:

int foo (svint32_t v) { return v[4]; }

reduces to a VEC_EXTRACT <>

I'll fix the comment/description.

Thanks,
Tejas.

gcc/ChangeLog:

         * gimple-fold.cc (maybe_canonicalize_mem_ref_addr): Disallow sizeless
         types in BIT_FIELD_REFs.
---
  gcc/gimple-fold.cc | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
index c19dac0dbfd..dd45d9f7348 100644
--- a/gcc/gimple-fold.cc
+++ b/gcc/gimple-fold.cc
@@ -6281,6 +6281,7 @@ maybe_canonicalize_mem_ref_addr (tree *t, bool is_debug = 
false)
        && VECTOR_TYPE_P (TREE_TYPE (TREE_OPERAND (TREE_OPERAND (*t, 0), 0))))
      {
        tree vtype = TREE_TYPE (TREE_OPERAND (TREE_OPERAND (*t, 0), 0));
+      /* BIT_FIELD_REF can only happen on constant-size vectors.  */
        if (VECTOR_TYPE_P (vtype))
         {
           tree low = array_ref_low_bound (*t);
@@ -6294,7 +6295,7 @@ maybe_canonicalize_mem_ref_addr (tree *t, bool is_debug = 
false)
                                          (TYPE_SIZE (TREE_TYPE (*t))));
                   widest_int ext
                     = wi::add (idx, wi::to_widest (TYPE_SIZE (TREE_TYPE 
(*t))));
-                 if (wi::les_p (ext, wi::to_widest (TYPE_SIZE (vtype))))
+                 if (known_le (ext, wi::to_poly_widest (TYPE_SIZE (vtype))))
                     {
                       *t = build3_loc (EXPR_LOCATION (*t), BIT_FIELD_REF,
                                        TREE_TYPE (*t),
--
2.25.1


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