Re: ICE after folding svld1rq to vec_perm_expr duing forwprop

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

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On Thu, 14 Jul 2022 at 17:22, Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Richard Biener <richard.guenther@gmail.com> writes:
> > On Thu, Jul 14, 2022 at 9:55 AM Prathamesh Kulkarni
> > <prathamesh.kulkarni@linaro.org> wrote:
> >>
> >> On Wed, 13 Jul 2022 at 12:22, Richard Biener <richard.guenther@gmail.com> wrote:
> >> >
> >> > On Tue, Jul 12, 2022 at 9:12 PM Prathamesh Kulkarni via Gcc-patches
> >> > <gcc-patches@gcc.gnu.org> wrote:
> >> > >
> >> > > Hi Richard,
> >> > > For the following test:
> >> > >
> >> > > svint32_t f2(int a, int b, int c, int d)
> >> > > {
> >> > >   int32x4_t v = (int32x4_t) {a, b, c, d};
> >> > >   return svld1rq_s32 (svptrue_b8 (), &v[0]);
> >> > > }
> >> > >
> >> > > The compiler emits following ICE with -O3 -mcpu=generic+sve:
> >> > > foo.c: In function ‘f2’:
> >> > > foo.c:4:11: error: non-trivial conversion in ‘view_convert_expr’
> >> > >     4 | svint32_t f2(int a, int b, int c, int d)
> >> > >       |           ^~
> >> > > svint32_t
> >> > > __Int32x4_t
> >> > > _7 = VIEW_CONVERT_EXPR<__Int32x4_t>(_8);
> >> > > during GIMPLE pass: forwprop
> >> > > dump file: foo.c.109t.forwprop2
> >> > > foo.c:4:11: internal compiler error: verify_gimple failed
> >> > > 0xfda04a verify_gimple_in_cfg(function*, bool)
> >> > >         ../../gcc/gcc/tree-cfg.cc:5568
> >> > > 0xe9371f execute_function_todo
> >> > >         ../../gcc/gcc/passes.cc:2091
> >> > > 0xe93ccb execute_todo
> >> > >         ../../gcc/gcc/passes.cc:2145
> >> > >
> >> > > This happens because, after folding svld1rq_s32 to vec_perm_expr, we have:
> >> > >   int32x4_t v;
> >> > >   __Int32x4_t _1;
> >> > >   svint32_t _9;
> >> > >   vector(4) int _11;
> >> > >
> >> > >   <bb 2> :
> >> > >   _1 = {a_3(D), b_4(D), c_5(D), d_6(D)};
> >> > >   v_12 = _1;
> >> > >   _11 = v_12;
> >> > >   _9 = VEC_PERM_EXPR <_11, _11, { 0, 1, 2, 3, ... }>;
> >> > >   return _9;
> >> > >
> >> > > During forwprop, simplify_permutation simplifies vec_perm_expr to
> >> > > view_convert_expr,
> >> > > and the end result becomes:
> >> > >   svint32_t _7;
> >> > >   __Int32x4_t _8;
> >> > >
> >> > > ;;   basic block 2, loop depth 0
> >> > > ;;    pred:       ENTRY
> >> > >   _8 = {a_2(D), b_3(D), c_4(D), d_5(D)};
> >> > >   _7 = VIEW_CONVERT_EXPR<__Int32x4_t>(_8);
> >> > >   return _7;
> >> > > ;;    succ:       EXIT
> >> > >
> >> > > which causes the error duing verify_gimple since VIEW_CONVERT_EXPR
> >> > > has incompatible types (svint32_t, int32x4_t).
> >> > >
> >> > > The attached patch disables simplification of VEC_PERM_EXPR
> >> > > in simplify_permutation, if lhs and rhs have non compatible types,
> >> > > which resolves ICE, but am not sure if it's the correct approach ?
> >> >
> >> > It for sure papers over the issue.  I think the error happens earlier,
> >> > the V_C_E should have been built with the type of the VEC_PERM_EXPR
> >> > which is the type of the LHS.  But then you probably run into the
> >> > different sizes ICE (VLA vs constant size).  I think for this case you
> >> > want a BIT_FIELD_REF instead of a VIEW_CONVERT_EXPR,
> >> > selecting the "low" part of the VLA vector.
> >> Hi Richard,
> >> Sorry I don't quite follow. In this case, we use VEC_PERM_EXPR to
> >> represent dup operation
> >> from fixed width to VLA vector. I am not sure how folding it to
> >> BIT_FIELD_REF will work.
> >> Could you please elaborate ?
> >>
> >> Also, the issue doesn't seem restricted to this case.
> >> The following test case also ICE's during forwprop:
> >> svint32_t foo()
> >> {
> >>   int32x4_t v = (int32x4_t) {1, 2, 3, 4};
> >>   svint32_t v2 = svld1rq_s32 (svptrue_b8 (), &v[0]);
> >>   return v2;
> >> }
> >>
> >> foo2.c: In function ‘foo’:
> >> foo2.c:9:1: error: non-trivial conversion in ‘vector_cst’
> >>     9 | }
> >>       | ^
> >> svint32_t
> >> int32x4_t
> >> v2_4 = { 1, 2, 3, 4 };
> >>
> >> because simplify_permutation folds
> >> VEC_PERM_EXPR< {1, 2, 3, 4}, {1, 2, 3, 4}, {0, 1, 2, 3, ...} >
> >> into:
> >> vector_cst {1, 2, 3, 4}
> >>
> >> and it complains during verify_gimple_assign_single because we don't
> >> support assignment of vector_cst to VLA vector.
> >>
> >> I guess the issue really is that currently, only VEC_PERM_EXPR
> >> supports lhs and rhs
> >> to have vector types with differing lengths, and simplifying it to
> >> other tree codes, like above,
> >> will result in type errors ?
> >
> > That might be the case - Richard should know.
>
> I don't see anything particularly special about VEC_PERM_EXPR here,
> or about the VLA vs. VLS thing.  We would have the same issue trying
> to build a 128-bit vector from 2 64-bit vectors.  And there are other
> tree codes whose input types are/can be different from their output
> types.
>
> So it just seems like a normal type correctness issue: a VEC_PERM_EXPR
> of type T needs to be replaced by something of type T.  Whether T has a
> constant size or a variable size doesn't matter.
>
> > If so your type check
> > is still too late, you should instead recognize that we are permuting
> > a VLA vector and then refuse to go any of the non-VEC_PERM generating
> > paths - that probably means just allowing the code == VEC_PERM_EXPR
> > case and not any of the CTOR/CST/VIEW_CONVERT_EXPR cases?
>
> Yeah.  If we're talking about the match.pd code, I think only:
>
>   (if (sel.series_p (0, 1, 0, 1))
>    { op0; }
>    (if (sel.series_p (0, 1, nelts, 1))
>     { op1; }
>
> need a type compatibility check.  For fold_vec_perm I think
> we should just rearrange:
>
>   gcc_assert (known_eq (TYPE_VECTOR_SUBPARTS (type), nelts)
>               && known_eq (TYPE_VECTOR_SUBPARTS (TREE_TYPE (arg0)), nelts)
>               && known_eq (TYPE_VECTOR_SUBPARTS (TREE_TYPE (arg1)), nelts));
>   if (TREE_TYPE (TREE_TYPE (arg0)) != TREE_TYPE (type)
>       || TREE_TYPE (TREE_TYPE (arg1)) != TREE_TYPE (type))
>     return NULL_TREE;
>
> so that the assert comes after the early-out.
>
> It would be good at some point to relax fold_vec_perm to cases where the
> outputs are a different length from the inputs, since the all-constant
> VEC_PERM_EXPR above could be folded to a VECTOR_CST.
Hi,
For the above case, I think the issue is that simplify_permutation
uses TREE_TYPE (arg0) for res_type,
while it should now use type for lhs.

      /* Shuffle of a constructor.  */
      bool ret = false;
      tree res_type = TREE_TYPE (arg0);
      tree opt = fold_ternary (VEC_PERM_EXPR, res_type, arg0, arg1, op2);

Using, res_type = TREE_TYPE (gimple_get_lhs (stmt)),
resolves the ICE, and emits all constant VEC_PERM_EXPR:

  v2_4 = VEC_PERM_EXPR <{ 1, 2, 3, 4 }, { 1, 2, 3, 4 }, { 0, 1, 2, 3, ... }>;
  return v2_4;

Does the patch look OK to commit after bootstrap+test ?
I will try to address the folding for above VEC_PERM_EXPR in follow-up patch.

Thanks,
Prathamesh
>
> Thanks,
> Richard

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diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
index 69567ab3275..b320ece13f2 100644
--- a/gcc/tree-ssa-forwprop.cc
+++ b/gcc/tree-ssa-forwprop.cc
@@ -2589,7 +2589,7 @@ simplify_permutation (gimple_stmt_iterator *gsi)
 
       /* Shuffle of a constructor.  */
       bool ret = false;
-      tree res_type = TREE_TYPE (arg0);
+      tree res_type = TREE_TYPE (gimple_get_lhs (stmt));
       tree opt = fold_ternary (VEC_PERM_EXPR, res_type, arg0, arg1, op2);
       if (!opt
 	  || (TREE_CODE (opt) != CONSTRUCTOR && TREE_CODE (opt) != VECTOR_CST))