Re: ICE after folding svld1rq to vec_perm_expr duing forwprop

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

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 ?

Thanks,
Prathamesh
>
> >
> > Alternatively, should we allow assignments from fixed-width to SVE
> > vector, so the above
> > VIEW_CONVERT_EXPR would result in dup ?
> >
> > Thanks,
> > Prathamesh