On 11/7/24 5:52 PM, Richard Biener wrote:
On Thu, Nov 7, 2024 at 11:13 AM Tejas Belagod <tejas.bela...@arm.com> wrote:
On 11/7/24 2:36 PM, Richard Biener wrote:
On Thu, Nov 7, 2024 at 8:25 AM Tejas Belagod <tejas.bela...@arm.com> wrote:
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]; }
So this is possibly out-of-bounds?
reduces to a VEC_EXTRACT <>
But if out-of-bounds a VEC_EXTRACT isn't any better than a BIT_FIELD_REF, no?
Someone may have code protecting accesses like so:
/* svcntw () returns num of 32-bit elements in a vec */
if (svcntw () >= 8)
return v[4];
So I didn't error or warn (-Warray-bounds) for this or for that matter
make it UB as it will be spurious. So technically, it may not be OOB access.
Therefore BIT_FIELD_REFs are generated for anything within the range of
a Adv SIMD register and anything beyond is left to be vec_extracted with
SVE instructions.
You still didn't state the technical reason why BIT_FIELD_REF is worse than
.VEC_EXTRACT (which is introduced quite late only btw).
I'm mostly questioning that we have two different canonicalizations that oddly
depend on the constant index. I'd rather always go .VEC_EXTRACT or
always BIT_FIELD_REF (prefer that one) instead of having a mix for VLA vectors.
Sorry, I misunderstood your question. The choice of canonicalization
based on index range wasn't by design - just happened to be a
side-effect of my trying to accommodate VLA poly sizes in place of
constants. When I checked that potentially-out-of-bounds VLA indices
were taking the VEC_EXTRACT route, I didn't think about using
BIT_FIELD_REFs for them too - frankly I didn't know we could even do
that for access outside the minimum vector size.
When I now try to canonicalize all constant VLA indices to
BIT_FIELD_REFs I encounter ICE and gimple verification does not seem to
be happy about potentially accessing outside object size range. If we
have to make BIT_FIELD_REF work for potentially OOB constant VLA
indices, wouldn't this be quite a fundamental assumption we might have
to change?
Thanks,
Tejas.
Richard.
Thanks,
Tejas.
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
